phosphonate Based Ionic Liquids and their Ionogels Synthese ...
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Synthesis, Characterization and Application of Methyl- phosphonate Based Ionic Liquids and their Ionogels Synthese, Charakterisierung und Anwendung von Methyl- phosphonat-basierten ionischen Flüssigkeiten und ihrer Ionogele Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des akademischen Grades eines Doktors der Ingenieurwissenschaften vorgelegt von M.Sc. Swetlana Sachnov aus Charkow (Ukraine) Erlangen 2016
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Transcript of phosphonate Based Ionic Liquids and their Ionogels Synthese ...
Synthesis, Characterization and Application of Methyl-
phosphonate Based Ionic Liquids and their Ionogels
Synthese, Charakterisierung und Anwendung von Methyl-
phosphonat-basierten ionischen Flüssigkeiten und ihrer
Ionogele
Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg
zur Erlangung des akademischen Grades eines Doktors der
Ingenieurwissenschaften
vorgelegt von
M.Sc. Swetlana Sachnov
aus Charkow (Ukraine)
Erlangen 2016
II
Als Dissertation genehmigt von der Technischen Fakultät der
Friedrich-Alexander-Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung: 05.08.2016
Vorsitzender des Promotionsorgans: Prof. Dr. Peter Greil
Gutachter: Prof. Dr. Peter Wasserscheid
Prof. Dr. Jana Zaumseil
III
Die Experimente der vorliegenden Doktorarbeit wurden unter Anleitung
von Prof. Dr. Peter Wasserscheid am Lehrstuhl für Chemische Reaktions-
technik der Friedrich-Alexander-Universität Erlangen-Nürnberg von
Juli 2009 bis Dezember 2012 durchgeführt.
IV
Danksagung
Mein besonderer Dank gilt meinem Doktorvater Prof. Dr. Peter Wasserscheid für
sein in mich gesetztes Vertrauen und die außerordentlich gute Betreuung. Ich
danke ihm für die vielen motivierenden und produktiven Diskussionen.
Frau Prof. Dr. Jana Zaumseil danke ich herzlich für die Übernahme des
Zweitgutachtens.
Weiterhin danke ich dem Bundesministerium für Bildung und Forschung für die
finanzielle Unterstützung meines Forschungsprojektes, als auch allen
Teilnehmern des HEBEL-Projektes für die gute Zusammenarbeit.
Besonderer Dank geht an Dr. Peter Schulz für seine wissenschaftliche Begleitung
in allen Phasen meiner Dissertation.
Ich bedanke mich ganz herzlich bei Frau Prof. Dr. Zaumseil und Dr. Stefan
Thiemann für die Kooperation im Bereich der Ionogele. Dr. Nikolai Ignatiev, Dr.
Marcel Drüschler und Dr. Benedikt Huber danke ich für ihre hilfreichen Hinweise
und die interessanten Gespräche bei Konferenzen und Unterstützung bei
Impedanzmessungen.
Den CRT-Werkstattmitarbeitern gilt mein Dank für die vielfältige Hilfe bei
mechanischen und elektrischen Problemen und Hendryk Partsch mit seinem
Team für die IT-Unterstützung. Ebenfalls bedanke ich mich bei Frau Menuet, Frau
Singer und Frau Bittan.
Ganz besonders bedanken möchte ich mich bei meinem Bachelor-Arbeiter Igor
Landa, meinen Praktikanten Johannes Schwegler und Tobi Fendt und meiner
HiWine Anastasia Lenz, die einen wesentlichen Anteil zum Gelingen dieser Arbeit
beigetragen haben. Ohne Eure Unterstützung wäre ich nicht so weit gekommen.
Allen aktuellen und ehemaligen Mitgliedern des „AK-Wasserscheid“ und
insbesondere meinen Bürokollegen Judith, Markus, Jens, Kerstin, Matthias,
Katharina, Markus, Patrick und Giang danke ich für die hilfreichen Diskussionen
und die schöne Zeit am Lehrstuhl.
Ein ganz besonderes Dankeschön gilt meiner Familie: Meinen Eltern und
Schwiegereltern, die mich stets großartig unterstützt haben. Grischa und Tanja,
ihr seid meine absoluten Vorbilder. Alex, ohne Dich hätte ich es niemals
geschafft.
V
Parts of this thesis have been published already in the following papers:
(1) S. J. Sachnov, P. S. Schulz, P. Wasserscheid, „A convenient method to access
long-chain and functionalised mixed methylphosphonate esters and their
application in the synthesis of ionic liquids”, Chemical Communications, 2011,
47(40), 11234-11236.
(2) S. Thiemann, S. Sachnov, S. Porscha, P. Wasserscheid, J. Zaumseil, „Ionic
Liquids for Electrolyte-Gating of ZnO Field-Effect Transistors”, Journal of Physical
Chemistry C, 2012, 116(25), 13536-13544.
(3) S. Thiemann, S. J. Sachnov, F. Pettersson, R. Bollstroem, R. Oesterbacka, P.
Wasserscheid, J. Zaumseil, „Cellulose-Based Ionogels for Paper Electronics”,
Advanced Functional Materials, 2014, 24(5), 625-634.
(4) S. Thiemann, S. J. Sachnov, M. Gruber, F. Gannott, S. Spallek, M. Schweiger,
J. Krückel, J. Kaschta, E. Spiecker, P. Wasserscheid, J. Zaumseil, „Spray-coatable
ionogels based on silane-ionic liquids for low voltage, flexible, electrolyte-gated
organic transistors”, Journal of Materials Chemistry C: Materials for Optical and
Electronic Devices, 2014, 2(13), 2423-2430.
Furthermore, I have contributed to the following publication:
(5) T. M. Koller, S. R. Schmid, S. J. Sachnov, M. H. Rausch, P. Wasserscheid, A.
P. Fröba, „Measurement and Prediction of the Thermal Conductivity of
Tricyanomethanide- and Tetracyanoborate-Based Imidazolium Ionic Liquids”,
International Journal of Thermophysics, 2014, 35(2), 195-217.
VI
During my PhD research, I have contributed to conferences with the following
posters and oral presentations:
(1) S. Sachnov, P. S. Schulz, P. Wasserscheid, “Ionic Liquids as Additives for Li-
Battery Electrolytes”, Poster, 1st International Conference on Materials for
Energy, 4. - 8. July 2010, Karlsruhe, Germany.
(2) S. Sachnov, P. S. Schulz, P. Wasserscheid, “Synthesis of Ionic Liquids with
alkylphosphonate anions”, Poster, 4th International Congress on Ionic Liquids
(COIL-4), 15 - 18 June 2011, Washington, DC, United States.
(3) S. Sachnov, S. Thiemann, P. S. Schulz, J. Zaumseil, P. Wasserscheid,
“Gelation of functionalised methylphosphonate ionic liquids by addition of
cellulose or chitosan”, oral presentation, 111. Hauptversammlung der Deutschen
Bunsen-Gesellschaft für Physikalische Chemie e.V., 17. - 19. Mai 2012, Leipzig,
Germany.
(4) S. Sachnov, P. S. Schulz, P. Wasserscheid, “Synthesis of Ionic Liquids with
alkylphosphonate anions”, Poster, Green Solvents for Synthesis, 8. - 10.
October 2012, Boppard, Germany.
(5) S. J. Sachnov, S. Thiemann, P. S. Schulz , J. Zaumseil and P. Wasserscheid,
“Gelation of Methylphosphonate Ionic Liquids by Cellulose and Investigation of
the Electrochemical Properties of the Obtained Ionogels”, Poster, 2nd Inter-
national Conference on Materials for Energy, 12. - 16. Mai 2013, Karlsruhe,
Germany.
VII
Abstract
This work reports a new method to synthesize long-chain and functionalized
methylphosphonate esters and the corresponding Ionic Liquids. The synthesis
comprises the formation of dialkyl methylphosphonate esters in a SN2 reaction.
The kinetics of the transesterification reaction were investigated using starting
ILs with anions revealing different nucleophilicities. The esters were subsequently
used as alkylating agents to form the corresponding, new alkyl methyl-
phosphonate ILs. Further, the alkylphosphonate species were compared to other
ester based anions concerning functionalization.
Methylphosphonate ILs carrying polyethylene glycol residues were able to
dissolve cellulose forming biopolymer based ionogels. The investigation of
electrochemical properties of pure ILs as well as ionogels showed conductivities
in order of 0.5 mS cm-1 and relatively high double-layer capacitances in the
range of 9-12 μF cm-2. Thus, these materials showed promising properties for
application in electronic devices such as organic electrolyte-gated field-effect
transistors (FETs).
1
Contents
1. Introduction ....................................................................................................................4
2. State of the art ................................................................................................................7
2.1. Phosphonate Chemistry ..................................................................................................7
2.1.1. Methods for preparation of phosphonates ......................................................................7
2.1.2. H-phosphonates ..............................................................................................................8
2.1.3. Mixed dialkyl alkylphosphonates .....................................................................................9
2.1.4. Alkylating activity of dialkyl H-phosphonates ................................................................. 12
2.1.5. Reaction of dialkyl H-phosphonates with amines ........................................................... 12
2.1.6. Chemical properties of monoalkyl H-phosphonate anions [R(H)PO3]-............................. 14
2.1.6.1. Basicity of [R(H)PO3]- anions .......................................................................................... 14
2.1.6.2. Nucleophilicity of [R(H)PO3]- anions ............................................................................... 15
2.1.7. Rearrangements of [R(H)PO3]- based ILs ........................................................................ 16
2.1.8. Poly(alkylene H-Phosphonate)s ..................................................................................... 18
2.2. Ionic Liquids .................................................................................................................. 20
2.2.1. Synthetic Strategies of TSILs .......................................................................................... 20
2.2.1.1. Functionalized Cations .................................................................................................. 21
2.2.1.2. Functionalized Anions ................................................................................................... 22
2.2.1.2.1. SN2 nucleophilic substitution reaction of selected ionic liquids’ anions .......................... 23
2.2.1.2.2. Kinetics and activation energy of the SN2 nucleophilic substitution reaction .................. 26
2.2.2. Melting point, thermal stability, viscosity and density of ILs .......................................... 28
2.2.3. Water and other impurities ........................................................................................... 31
2.3. Ionogels ........................................................................................................................ 32
2.3.1. Dissolution of carbohydrates in ILs ................................................................................ 35
2.3.2. Biopolymer ionogels for electrochemical devices .......................................................... 37
2.4. Electrochemistry ........................................................................................................... 39
2.4.1. Cyclic voltammetry ........................................................................................................ 40
2.4.1.1. Cyclic voltammetry of RTILs ........................................................................................... 42
2.4.2. Electrochemical double-layer ........................................................................................ 45
2.4.2.1. Alternating voltage ........................................................................................................ 46
2.4.3. Electrochemical impedance spectroscopy (EIS) ............................................................. 47
2.4.3.1. Equivalent circuit representation .................................................................................. 48
2.4.3.2. Graphical representations ........................................................................................... 50
2.4.3.2.1. Constant phase element (CPE) .................................................................................... 53
2.4.3.3. Instrumental limitations ............................................................................................... 55
2.4.3.4. Impedance on ILs.......................................................................................................... 56
2
2.4.3.4.1. Conductivity of ionic liquids........................................................................................... 57
2.4.3.4.2. Fragility ........................................................................................................................ 58
2.4.3.4.3. Walden plot ................................................................................................................. 60
2.4.3.4.4. Electrochemical double-layer in ionic liquids ................................................................. 61
2.4.3.4.4.1. Effect of temperature on the double-layer capacitance of ionic liquids .......................... 62
2.4.3.4.4.2. Effect of ion size on the double-layer capacitance of ionic liquids .................................. 65
3. Results and Discussions ................................................................................................. 69
3.1. Synthesis ....................................................................................................................... 69
3.1.1. Transesterification reaction of [Me(Me)PO3]- ............................................................... 69
3.1.1.1. Kinetics of the transesterification reaction of phosphonate anions with methyl chloroacetate................................................................................................................................... 74
3.1.1.2. Probing other cations, anions and alkylating agents in the transesterification reaction.. 78
3.1.2. Synthesis and physico-chemical characterisation of methylphosphonate ILs .............. 88
3.2. Cellulose based MP ionogels ......................................................................................... 97
3.3. Electrochemistry of MP derived materials ................................................................... 102
3.3.1. Cyclic voltammetry of MP-ILs ...................................................................................... 103
3.3.1.1. Electrochemical windows of MP-ILs .......................................................................... 104
3.3.1.2. Effect of cellulose addition on the electrochemical window .................................... 105
3.3.1.3. Effect of temperature on MP-IL and ionogel electrochemical windows ................... 106
3.3.2. Temperature dependent impedance measurements ................................................... 108
3.3.2.1. Temperature dependent impedance measurements of [EMIM][NTf2] ........................ 108
3.3.2.2. Temperature dependent impedance measurements of [EMIM][Me(EG)1(Me)PO3] .... 110
3.3.2.3. Temperature dependent impedance measurements of MP-based ionogels ................ 112
3.3.3. Influence of anion functionalization on impedance measurements ............................. 114
3.3.3.1. Conductivity of [EMIM]+ based MP-ILs ......................................................................... 114
3.3.3.2. Fragility analysis of MP-ILs........................................................................................... 116
3.3.3.3. Walden plot ................................................................................................................ 117
3.3.3.4. Double-layer capacitance of [EMIM]+ based MP-ILs ..................................................... 118
3.3.4. Influence of cation functionalization on electro-chemical properties of the [Me(EG)1(Me)PO3]- based ILs .......................................................................................................... 120
3.3.5. Influence of cellulose addition on impedance measurements...................................... 122
3.3.5.1. Fragility comparison of MP-ILs and corresponding ionogels ......................................... 124
3.3.5.2. Double-layer capacitance comparison of MP-ILs and corresponding ionogels .............. 125
4. Summary and outlook ................................................................................................. 127
5. Zusammenfassung und Ausblick .................................................................................. 130
6. Experimental ............................................................................................................... 133
6.1.1. Solvents and reagents ................................................................................................. 133
6.1.2. Analytics ..................................................................................................................... 133
6.1.2.1. Nuclear magnetic resonance spectroscopy (NMR) ....................................................... 133
3
6.1.2.2. Gas chromatography Mass Spectrometry (GC-MS) ...................................................... 133
6.1.2.3. Electrospray ionization Mass Spectrometry (ESI-MS) ................................................... 134
6.1.2.4. Differential scanning calorimetry (DSC) ....................................................................... 134
6.1.2.5. Thermogravimetric analysis (TGA) ............................................................................... 134
6.1.2.6. Viscosity measurements .............................................................................................. 134
6.1.2.7. Density measurements................................................................................................ 135
6.1.2.8. Karl-Fischer-titration ................................................................................................... 135
6.1.2.9. Light microscopic investigations .................................................................................. 135
6.1.2.10. Electrochemical measurements................................................................................... 135
6.1.3. Synthesis of asymmetric methyl methylphosphonate esters ....................................... 138
6.1.4. General synthetic procedure of alkylphosphonate ionic liquids from dialkyl alkylphosphonate esters ................................................................................................................. 157
6.1.5. Preparation of cellulose ionogels. ................................................................................ 193
6.2. Determination of the crucible surface ......................................................................... 193
6.3. Derivation of complex capacitance .............................................................................. 193
7. References ................................................................................................................. 194
8. Abbreviations ............................................................................................................. 205
4
1. Introduction
In the last years, the depletion of fossil fuels and global climate change
entailed in Germany the so called “Energiewende”, say promotion of re-
newable energy and sustainable development. Maintaining the current
systems based on fossil fuels becomes more and more difficult and thus
has to be replaced with cleaner and more abundant forms of energy.[1]
The demand is continuousely growing for high performance electronic
devices and improved energy technologies like better solar conversion
methods, better energy storage systems or more efficient ways to use
energy.[1]
The use of low melting salts – ionic liquids – is in focus of interest. This
new class of materials reveals new possibilities in application technology
due to their unique structural, physico- and electrochemical features. The
development of new functionalized molecules with improved or novel
properties is promoted through design, synthesis and characterization.
Thus, originating from the electrochemical research,[2] ILs found
application in various fields including solvent replacement, analytics or
catalysis.[3] Furthermore, a possibility exists to immobilize ILs in solid
devices, while keeping their unique properties, as it is well illustrated by
the current development of supported IL catalysis (SILP). In this case, IL
is confined in porous materials like e.g. silica gel. But also using binding
materials as gelators considerably enlarges the array of applications of
ILs.[89]
Biodegradable composites generated from renewable biomass feedstock
are regarded as promising materials that could replace synthetic polymers
and reduce global dependence on fossil fuel sources.[4] The most abundant
natural polymer in our environment is cellulose with its highly ordered
structure, which is responsible for its desirable mechanical properties but
makes it a challenge to find suitable solvents for its dissolution.[5]
5
The implementation for the Energiewende requires that the research is
carried out in multiple approaches in parallel. Along with optimization of
existing techniques, the development of novel materials and inter-
disciplinary methodologies is indispensable.
The intention of the current work is the synthesis, characterisation and
application of novel ionic liquids based on methylphosphonate (MP)
anions. Leaned on the basics of phosphonate chemistry, the ionic liquid
[EMIM][Me(Me)PO3] represents the crucial precursor of the ionic liquid
synthesis in the current work. A new method to synthesize long-chain and
functionalized methylphosphonate esters through SN2 reaction leads to a
plethora of alkyl methylphosphonates (Scheme 1).
Scheme 1: Synthesis of long-chain and functionalized methylphosphonate esters through SN2
reaction.
The kinetics of this SN2 reaction is investigated in dependence on the
features of the starting phosphonate species. The substituents connected
to phosphorus and / or oxygen influence the nucleophilicity of the phos-
phonate head group and the reaction proceeding. Further, the
functionalization procedures of phosphonate anions are investigated under
consideration of starting materials and compared to the existing
strategies. In particular, the comparison among ester-based anions –
sulfates, phosphates, phosphonates – is carried out. The trans-
esterification under acid catalysis is compared to the alkylation by halo-
alkanes.
The functionalized methylphosphonate esters may be used as alkylating
agents in quaternization reactions to form the corresponding, new alkyl
methylphosphonate ILs (Scheme 2).
6
Scheme 2: Use of functionalized methylphosphonate esters as alkylating agents in quaternization
reactions.
The physico-chemical properties of the MP-IL series are determined and
tuned through functionalization with the goal to optimize electrochemical
and solvation properties. Then they are compared to known sulfate and
phosphate based ILs, to which they differ by high functionalization
possibility and hydrolytic stability. Due to the lack of literature about
functionalized MP-ILs and their implementation potential, they represent a
wide field for further research.
Among the tremendous application possibilities of ILs, the current work
concentrates on the ability of selected MP-ILs to dissolve carbohydrates,
especially cellulose. The product represents corresponding ionogels, which
belong to the family of electrically conductive polymers. The use of the
MP-ILs and carbohydrate based ionogels in flexible electronics requires
electrochemical characterisation of such materials. For this reason,
electrochemical windows, conductivities and double-layer capacitances
were determined by means of impedance spectroscopy for the pure
relevant MP-ILs as well as their ionogels.
7
2. State of the art
2.1. Phosphonate Chemistry
2.1.1. Methods for preparation of phosphonates
The diesters of phosphonic acids can be obtained by several synthetic
procedures. The discovery of phosphonates dates back more than a
century when the very early studies of Michaelis[1] and Arbuzov[7]
appeared. The starting material is the trialkylphosphite (RO)3P which is
produced from phosphorus trichloride and alcohols. In 1905, Arbuzov et
al. showed a reaction of trialkylphosphite (usually trimetyl, triethyl or
triphenyl) and an alkyl halide to form phosphonate esters. This SN2-trans-
formation of esters with trivalent phosphorus to pentavalent phosphorig
esters was named “Michaelis-Arbuzov-reaction” and a two-step
mechanism was suggested (Scheme 3).
Scheme 3: Two-step mechanism of the “Michaelis-Arbuzov-reaction”.
Halogen substituted carbon acid esters react as well with trialkylphos-
phites to carbon acid ester functionalized phosphonates. The electron-
withdrawing properties of the phosphoryl and carbon ester groups make
the protons on -C acidic.[9] Through powerful bases such as NaH, NaNH2
and lithium reagents phosphonate carbanions could be generated with
very high reaction efficiency. The third major breakthrough was reported
8
by Corey and Kwiatkowski et al., in 1966. These authors found that simple
dialkyl alkylphosphonates react with BuLi to give phosphonate carb-
anions.[11] This immediately made phosphonate carbanions competitive
with Wittig reagents for the preparation of olefins. The workup is more
advantageous than in the corresponding Wittig reaction because the phos-
phate by-product can be washed away with water. In the Horner-
Wadsworth-Emmons reaction[10] olefin formation from aldehydes and
phosphonates occurs according to Scheme 4.
Scheme 4: Olefin formation from aldehydes and phosphonates according to the Horner-
Wadsworth-Emmons reaction.[10]
The main two advantages that make the phosphonates such attractive are
their easy synthetic access and the special reactivity of the carbon
adjacent to the phosphoryl group. Most of the recent advances in phos-
phonate chemistry are now based on the carbanionic methodologies. The
conversion to functionalized phosphonates by simple treatment with
electrophiles has great value for the synthesis of complex phosphonates
including aminophosphonates, hydroxyalkyl phosphonates, phosphates,
amidophosphates, nucleoside H-phosphonates, poly(alkylene H-phos-
phonate)s and poly(alkylene phosphate)s, phosphorus-containing poly-
esters, polyurethanes, etc, which have an ever-increasing importance in
organic synthesis as precursors of elaborated organic compounds and bio-
logically important derivatives.[14]
2.1.2. H-phosphonates
Till these days the phosphonate chemistry was mostly concentrated on the
P–C bond in the P-functionalized phosphonates,[13] but for R’ = H in
Scheme 3 H-phosphonates are obtained.[8] The preparation of dialkyl H-
9
phosphonates via direct esterification of H-phosphonic acid with alcohols
was reported in several patents.[18] The process involves among others
heating up to 200 °C. The presence of sulfonic acid[19] or trialkylphos-
phates increases the yield of dialkyl H-phosphonates.[20] At much lower
temperatures, 20 – 50 °C, H-phosphonic acid may be esterified with a
carboxylic acid anhydride and an alcohol.[21]
The diesters of H-phosphonic acid are as well frequent intermediates in
the synthesis of a variety of bioactive products. For this reason they
occupy a major position in organophosphorus chemistry. The versatility of
these compounds is determined by the presence of two types of reaction
centers in the molecule: The phosphorus atom and the -carbon atom of
the alkoxy groups. The strongly polar character of the phosphoryl group of
the H-phosphonates is responsible to a great extent for the reactivity of
this class of compounds.
H-phosphonate diesters exhibit P–H type acidity, although weaker than
the corresponding P–OH type acids, and are therefore tautomeric systems.
The phosphite–phosphonate equilibrium (Scheme 5) is practically entirely
shifted to the four-co-ordinated phosphonate form.[27]
Scheme 5: The phosphite–phosphonate equilibrium.
2.1.3. Mixed dialkyl alkylphosphonates
From the described Michaelis-Arbuzov-reaction (Scheme 3) symmetric
substituted phosphonate esters of the type R2(R’)PO3 are mainly
produced. Much more rarely mixed phosphonate esters with
10
asymmetrically substituted P-atom like RR”(R’)PO3 can be found in the
literature.
Some of the possibilities to obtain mixed phosphonate esters are nicely
summarized by Troev.[14][15] E.g. if the trialkylphosphite synthesis is
carried out with an equimolar mixture of two different alcohols,[16] the
precursors of type (RO)2(R’O)P and (R’O)2(RO)P may be obtained. After
the Arbuzov reaction asymmetric RR”(R’)PO3 species along with symmetric
ones may be found in the reaction mixture.
However, the initial treatment of phosphorus trichloride is usually carried
out with rather short alcohols like methanol, ethanol or butanol. The
synthesis of higher dialkyl phosphonate homologues includes then the
transesterification of the so-formed dimethyl phosphonate with higher
alcohols (Scheme 6).[17] Mixtures of two different alcohols were also used
in this process.
Scheme 6: Transesterification of the dimethyl phosphonate with higher alcohols.
The transesterification is carried out at elevated temperatures, usually in
the range between 95 and 180 °C. The rate of transesterification depends
both on the type of substituents at phosphorus and the nucleophilicity of
the corresponding alcohol.[14] The transesterification rate decreases in the
order CH3O > C2H5O > C3H7O > i-C3H7O depending on the type of the
alkoxy substituents at phosphorus[28] corresponding to the electro-
negativity of these alkoxy groups. Compared to nucleophilic substitution at
a tetrahedral carbon atom, steric factors have significantly lower effects
on the reactivity, since the phosphorus atom has a considerably larger
atomic radius (1.28 Å) than carbon (0.91 Å). Thus, the distances between
phosphorus and its substituents are larger than those at a carbon atom,
providing more space for the incoming nucleophile.[29]
11
The noncatalyzed transesterification of dialkyl H-phosphonates has been
assumed to proceed in a nucleophilic substitution reaction, SN2. Milliken
population analysis of the charge distribution in the dimethyl H-phos-
phonate clearly indicates that the electron density at the phosphorus atom
is the lowest. Thus, the phosphorus atom acts as an electrophilic center.
In the SN2 substitution at the phosphorus atom pentacoordinated inter-
mediates are quite common, in contrast to the corresponding five-co-
ordinated species at a carbon atom, which are only transition structures.
AM1 semiempirical calculations of the model transesterification of dimethyl
H-phosphonate with methanol indicate that in the first stage of this
reaction, dimethyl H-phosphonate and the nucleophile form a pentaco-
ordinated intermediate via a four-centered cyclic transition structure I.
The trigonal-bipyramidal intermediate II undergoes pseudorotation. A new
transition structure III of the same type as I forms the monotrans-
esterificated product (Scheme 7).[31]
PHRO OR
O
R'OHP
RO OR
O
OR'
H
H
PROOR
OH
OR'
H
POR' OR
O
OR
H
H-ROH
PHOR' OR
O
I II III
Scheme 7: Model transesterification of dimethyl H-phosphonate with methanol.
The transesterification reaction of H-phosphonate diesters with nucleo-
philes proceeds both in the absence and in the presence of a catalyst. The
commonly used basic catalysts are alkali metals, alkali alkoxides, and
tertiary amines, whereas H3PO4, CH3CO2H provide acidic catalysis.[14]
Asymmetric, optically active dialkyl H-phosphonates are formed in the dis-
proportionation reaction (Scheme 8) by heating equimolar mixtures of two
symmetric dialkyl H-phosphonates.[22] However, this process has been
shown to be reversible.[23] The equilibrium is established readily at room
temperature.
12
Scheme 8: Disproportionation reaction of symmetric dialkyl H-phosphonates.
2.1.4. Alkylating activity of dialkyl H-phosphonates
The use of phosphonates as alkylating agents for carboxylic acids, phenols
and amines is very attractive and well known. Dialkyl alkylphosphonates
are, in general, less reactive alkylating agents than sulfates or alkyl
halogenides. On the other side, they are more reactive than the
corresponding phosphate esters. The reason is the absence of unpaired
electrons on the carbon in the P–C bond in contrast to the P–O bond,
where p–d contributions are allowed. This rends the phosphorus atom of
phosphonates more electrophilic than the phosphorus atom of the
corresponding phosphate ester. The P–C bond is usually stable to
hydrolytic procedures.[14]
Gray and Smith reported in 1980 demethylation of several phosphates
and phosphonates by t-butylamine and obtained de facto ionic liquids as
products.[12] However, those days, before the IL boom, the
characterization and application possibilities of these salts were not further
investigated.
2.1.5. Reaction of dialkyl H-phosphonates with amines
Although the proton connected to the phosphorus in the dialkyl H-phos-
phonate ester exhibits certain acidity, the only products formed in the
reaction between dialkyl H-phosphonates and amines are alkylammonium
salts of type [NR’3R][R(H)PO3] resulting from alkylation of the
corresponding amine (Scheme 9).[26],[33],[36] The 31P, 1H NMR, and IR
spectroscopic data provide direct evidence for the presence of a P–H bond
in the structure of the final products.[26]
13
Scheme 9: Alkylation of amines by dialkyl H-phosphonates.
The reaction can be carried out with or without a solvent. The ease of de-
alkylation is in the order Me > Et > i-Pr ≈ Bun, according to the trans-
esterification.[28] The alkylation reaction was found to have SN2
substitution transition structure (Figure 1). The electron pair on the
nitrogen atom of the amine attacks the -carbon atom of the alkoxy group
of dialkyl H-phosphonate.[39]
Figure 1: SN2 transition structure for the alkylation of ammonia with dimethyl H-phosphonate
obtained with the HF/6–31+G* basis set.[39]
The formation of the [NR’3R][R(H)PO3] salts is monitored by specific
changes of the 1H, 31P NMR, and IR spectra, which reflect predominantly
the ionic character of the products compared to the starting materials. In
particular, the monoalkyl H-phosphonates exist entirely in their four-co-
ordinate phosphonate form and the delocalization of the negative charge
in the anion is responsible for the upfield shift of the 31P NMR signals
(4.55 ppm) due to the increased electronic shielding compared to the
starting dialkyl H-phosphonates (9-11 ppm).[33] Due to this delocalized
negative charge it is impossible to obtain diammonium salts of phosphonic
acid. The nucleophilic substitution at the second -carbon atom of the
[R(H)PO3] anion is then strongly disfavored.[40]
Even in 2009, when crystal data of [NH3But][R(H)PO3] salts were
determined (Figure 2) by Bryant et al.,[38] the authors did not categorize
them as ionic liquids with subsequent physico-chemical characterization.
14
Figure 2: Ortep diagram of [NH3But][Me(H)PO3]and crystal data and refinements for the anion.[38]
An extensive hydrogen bonding networks could be found. The ions are
hydrogen bonded within layers bounded by the But groups of the cations
and alkyl groups of the anions. In the anion, two oxygen atoms are
covalently bound only to phosphorus. One P–O bond is shorter than the
other and this bond is presumed to have more double bond character. The
double bond oxygen is hydrogen bonded to one cation and the other, de-
protonated oxygen is H-bonded to two cations.[38] The very important
result of the calculations by Georgiev et al. is that the formation of the
hydrogen-bonded contact ion pair is predicted to be an exothermic
process with its total energy being 13.0 kcal mol-1 lower than that of the
reactants.[41]
2.1.6. Chemical properties of monoalkyl H-phosphonate anions [R(H)PO3]-
2.1.6.1. Basicity of [R(H)PO3]- anions
The monoalkyl H-phosphonate anions [R(H)PO3]- exhibit certain basicity.
E.g. in the Atherton–Todd reaction,[33],[34] which is base promoted,[32]
monoalkyl [R(H)PO3]- anions are supposed to deprotonate the dialkyl H-
phosphonate. This hence generated reactive dialkyl phosphite anion reacts
then with the tetrachloromethane to provide highly reactive dialkyl chloro-
phosphates along with the trichloromethanide anion, which are usually not
15
isolated, but rather used in situ under mild conditions.[35] The catalytic
cycle is completed with the reaction of the trichloromethanide anion with
monomethyl H-phosphonate to form chloroform and [R(H)PO3]- (Scheme
10).
Scheme 10: Atherton–Todd reaction for synthesis of dialkyl chlorophosphates.
2.1.6.2. Nucleophilicity of [R(H)PO3]- anions
Further it has been shown that mixed (asymmetrically substituted) dialkyl
H-phosphonates can also be obtained in good yields from phosphonate
monoanions. Zwierzak and Kluba reported multiple step syntheses for
phosphorylation of alkyl halides (Scheme 11).[24],[25] In the first step the
symmetric dialkyl phosphonate is monodealkylated to the anion
[R(H)PO3]- (Scheme 9), where the phosphorus atom acts as a nucleo-
phile.[26],[33] Alternatively, the dealkylation may be carried out by bases
like NaOH or tetrametylammonium hydroxide providing the corresponding
phosphonate salt and alcohol R’-OH as by-product (Scheme 12). For the
Na-phosphonate species, Na is then exchanged by tetrabutylammonium
hydrogensulfate to provide tetrabutylammonium phosphonate. This ion
exchange using CH2Cl2 / H2O mixture is similar to that nowadays
ubiquitously used for the synthesis of e.g. [NTf2]- and [FAP]- based ionic
liquids. In this regard the tetraalkylammonium phosphonate species,
which the authors denote as “a non-crystallisable, hydroscopic syrup”,[24]
represented in fact ionic liquids, widely known today. Finally, the
alkylation through alkyl halides was carried out by means of SN2
substitution reaction and mixed dialkyl phosphonates were obtained.
16
Scheme 11: Synthesis for phosphorylation of alkyl halides, possibility I.
Scheme 12: Synthesis for phosphorylation of alkyl halides, possibility II.
The authors describe further to cleave the But – O bond obtaining the
phosphonate monoacid. However, since such free acid is unstable at
ambient conditions, it was converted into crystalline S-p-chlorobenzyl-
thiuronium salt for reasons of purification and identification.
2.1.7. Rearrangements of [R(H)PO3]- based ILs
Quaternary alkylammonium salts of the monoalkyl H-phosphonic acid
exist as free ions in solution. Having at least one methyl group connected
to the nitrogen atom is a prerequisite for several rearrangements.
Alkylammonium salts with at least one ethyl group in the cation undergo
Hoffman’s elimination (Scheme 13).[26],[33]
17
Scheme 13: Hoffman’s elimination in phosphonate alkylammonium salts.
The phosphonate anion [R(H)PO3]- is supposed to facilitate the -
elimination, which leads to the formation of ethylene. Ionic liquids
obtained by alkylation of primary amines with dimethyl H-phosphonate,
are thermally unstable and yield the corresponding alkylammonium salts
even at room temperature (Scheme 14).[37]
Scheme 14: Dealkylation of phosphonate alkylammonium salts under elevated temperature.
This transformation can be accelerated to quantitative yield by stirring at
50–70 °C under vacuum for several hours. In contrast, quaternary
methylammonium salts obtained by alkylation of tertiary amines (N,N-
dimethyl aniline or triethyl amine) possess quite high thermal stability.[37]
Rearrangements between the two component ions in the alkyl H-phos-
phonate IL are as well possible after the principle denoted in Scheme 15.
PH
O O
O
NR1
R"
PH
O O
O
R'N
R1
R"R'
Scheme 15: Rearrangements between the two component ions in the alkyl H-phosphonate IL.
The 31P NMR spectrum of the reaction mixture obtained by heating diethyl
H-phosphonate and dimethyl aniline shows signals for two types of phos-
phorus atoms: for the [Et(H)PO3]- and the [Me(H)PO3]-.[26],[33]
18
Table 1: 31P NMR data of some H-phosphonic acid diesters.[14]
Compound / ppm Me2(H)PO3 11.61 Et2(H)PO3 9.8 Pr2(H)PO3 7.41 Bu2(H)PO3 7.63 Ph2(H)PO3 0.95 But
2(H)PO3 3.21 MeBenz2(H)PO3 1.3
2.1.8. Poly(alkylene H-Phosphonate)s
Poly(alkylene H-phosphonate)s are an interesting class of phosphorus-
containing polymers because both the polymer backbone and phosphorus
substituents can be modified. The P–H groups are highly reactive and can
be converted into a number of interesting functional groups. The most
important reactions are hydrolysis, oxidation (Atherton-Todd[42],[43]),
addition reactions to the double bonds and to carbonyl group. So, e.g.,
poly-(alkylene phosphate)s can readily be prepared from poly(alkylene H-
phosphonate)s (Scheme 16).
Scheme 16: Synthesis of poly-(alkylene phosphate)s from poly(alkylene H-phosphonate)s.
Poly(alklylene H-phosphonate) containing a nitrogen base in the side chain
was synthesized by polycondensation of dialkyl H-phosphonate with 1-
(2’,3’-dihydroxypropyl)imidazole (Scheme 17).[44]
Scheme 17: Polycondensation of dialkyl H-phosphonate with 1-(2’,3’-dihydroxypropyl)imidazole.
19
However, the transesterification of dimethyl H-phosphonate is
accompanied by side reactions to form phosphonic acid end groups and
ether compounds. The formation of these side products is due to the
nucleophilic attack of the -carbon atom, the second electrophlic center in
the molecule of dimethyl H-phosphonate.
Especially fascinating is the transesterification of the methoxy groups of
dimethyl H-phosphonate Me2(H)PO3 with the hydroxyl groups of poly-
ethyleneglycol (PEG) by linking via one terminus or both termini.[45] After
heating dimethyl H-phosphonate with PEG, there are two signals in the 31P
NMR spectrum. The signal at 11.17 ppm can be assigned to the phos-
phorus atom in the end group of poly(oxyethylene H-phosphonate)s
bonded to OCH3 and OCH2 groups. The signal at 10.46 ppm belongs to the
phosphorus atom in the repeating unit of poly(oxyethylene H-phos-
phonate)s bonded with two OCH2 groups.[14]
PEG is a synthetic polymer, which is extensively studied as a polymer drug
carrier. Its chains can be used as building blocks to construct
functionalized polymers with low toxicity and reduced immunoreactivity.
Tough it is nonbiodegradable, it is well tolerated in the human body. The
incorporation of phosphonate units in the backbone of a PEG-polymer
guarantees solubility and degradation under physiological conditions, as
well as feasible modification at the P-center. Poly(oxyalkylene H-phos-
phonate)s have further advantages: They are water-soluble and the
extent of polymer drug loading is not limited to the two reaction sites at
the termini of the linear PEG molecule. The P–H, hydroxy, carboxy, amino,
oxirane group, and P–O group in the repeating unit of the poly(oxy-
alkylene phosphonate)s determine various chemical functionalities. These
polymers can be prepared either with the hydrophobic main chain and
hydrophilic side chain or vice versa. PEG-containing polymers can be used
to prepare hydrogels, that have the ability to swell in water or aqueous
solvent systems but will not dissolve regardless of the solvent.[46]
20
2.2. Ionic Liquids
The liquid range of the organic salts (m.p. < 100 °C), which is determined
by the combination and structure of the component ions, defines their
classification as “ionic liquids”. Originally investigated as alternatives for
conventional volatile organic solvents (“green” or “designer” solvents),[49]
the fields of applications of these materials expanded explosively in the
last decade including analysis,[53] bioconversion,[54],[68] electrochemical[52]
or engineering applications[55],[78] to name a few. An online ionic liquid
database is available free to users.[50]
The concept of “task-specific” ionic liquids (TSILs)[74] represents the
incorporation of functional groups in the ion structure. TSILs influence the
reaction process of solute materials by their effects on yields, rates,
and / or selectivities. A very prominent example is their application as
water scavenger.[51]
Traditionally, the functionalized ion of a TSIL is the cation. Consequently,
the literature on TSILs mainly contains synthesis routes to functionalized
cations, though the general principles may be transferred to the synthesis
of functionalized anions as well. The choices made in this regard play a
large role in both the chemical and physical properties of the resulting
salts.[74]
Specific functional groups like fluorous tails facilate the emulsification of
perfluorocarbons in the respective IL and free amino or urea groups are
able to capture gases like CO2 or SO2.[77] The solubility of inorganic salts in
ILs may be enhanced through inserting of ether and alcohol
functionalities.[80]
2.2.1. Synthetic Strategies of TSILs
As mentioned in chapter 2.1.4, the existence of salts consisting of
relatively large, structurally complex ions is known much earlier than the
21
concept of ionic liquids was established. Actually, since discovering of the
alkylating and / or protonating activity of some compounds and the
accepting activity of the others, the appearance of the organic salts was
predestined. In 1900, e.g., v. Braun described the reaction of cyanogen
bromide (BrCN) with a series of tertiary amines, among them the
heterocyclic quinoline.[48] The author obtained de facto low melting nitrile
functionalized tetraalkyl ammonium bromide salts (m.p. ≈ 180 °C) and
characterised them as hydroscopic solids. Then the physical properties of
the organic molten salts were described by Walden in 1914 who reported
on [EtNH3][NO3] (m.p. 13-14 °C).[47]
The basic methods to synthesize ILs are direct alkylation or protonation,
acid-base neutralization and ion metathesis. Rather rarely the target ILs
are formed in one step. Usually, the formation of the desired cation comes
first through quaternization of amines or phosphines by haloalkanes and
then the anion is introduced by means of anion exchange reactions.[76]
An overview of nowadays existing cation and anion structures may be
found in several reviews and / or textbooks on ionic liquids.[74]-[76],[196],[199]
2.2.1.1. Functionalized Cations
The core of the cations is usually based on atoms having originally free
electron pairs such as N (ammonium), P (phosphonium) or S (sulfonium)
on which a positive charge may be imposed through the alkylation or
protonation reactions. The heterocycles such as imidazole, pyridine
thiazole or oxazole are as well very frequently used in this context. The
most prominent are, however, 1,3-dialkylimidazolium based cations for
reasons of affordability of the starting materials, easy reaction proceeding
and desirable physico-chemical properties of the resulting ILs. Increasing
size and asymmetry of the cation have decreasing effects on the melting
point, but result in increased viscosity.[74],[76]
22
2.2.1.2. Functionalized Anions
While a plethora of functionalized cations are now available, much less
effort has been devoted to the synthesis of functionalized anions. The
source of the functional group that is to be transferred on the cationic core
is of great importance not only for functionalization reasons but also
regarding the leaving group representing then the anion of the ionic liquid.
The requirements on the leaving group may be threefold: either it is
directly the anion of interest (like e.g. funcionalized carbon acids[73] or
esters) or it is easily modifiable to the respective groups (like e.g. methyl
or ethyl sulfates[57]). If none of that is the case (like e.g. for halogenates),
the anion should be easily exchangeable e.g. by [BF4]-, [PF6]- or [NTf2]-
anions. Salts of alkali metals and silver as well as ion exchange resins
(Dowex®, Amberlite®) are routinely used for this purpose. These readily
available and widely used anions are not involved in any homologues
series. As consequence plenty of research is done on comparing the
effects of systematic cation functionalization on the IL properties and
performance keeping the anion constant.[56] Only for some sulfates,[57],[71]
alkylsulfonates,[58] as well as perfluoroalkyltrifluoroborate derived ILs[81]
similar studies were carried out. For dialkylphosphate ionic liquids the
research is dominated by non-functionalized, short-chain alkyl groups with
dimethylphosphates, diethylphosphates and dibutylphosphates being
described in more detail.[59] The structural anion variability of phos-
phonate ionic liquids has been restricted to date to methyl phosphonate
[Me(H)PO3]-[60] and methyl methyl-phosphonate [Me(Me)PO3]- anions.
[EMIM][Me(H)PO3] and [EMIM][Me(Me)PO3] have been synthesized and
applied in the context of HPLC applications,[62] biomass treatment[61] and
glucose dehydrogenation.[68] This situation is so much the more surprising
as the pool of phosphorus functionalized phosphonate esters and acids is
enormous as shown in chapter 2.1 and described in detail by Savignac et
al.[13] As mentioned in chapter 2.1.5, the ethyl and methyl diesters are
versatile alkylating agents and plenty of ionic liquids may be synthesized
23
by the best known quaternization reaction. Very exotic examples of the
acidic species are shown in Figure 3 representing e.g. fullerene
functionalized phosphonic acids,[67] which may be converted to ILs by
simple neutralization reaction of a hydroxide of the desired cation.
Figure 3: Chemical structure of the SAM forming phosphonic acids 1)C60C18-PA, 2) C10-PA, and
3) C14-PA underneath source and drain electrodes in the Self-Assembled Monolayer Field-Effect
Transistors.[67]
The P–C bond in contrast to the O–C bond in the phosphonate esters is
hydrolytically stable.[14] This property represents one of the prominent
requirements of the TSILs.
2.2.1.2.1. SN2 nucleophilic substitution reaction of selected ionic liquids’ anions
For ionic liquids with alkyl sulfate anions a straightforward synthetic
methodology is known that gives access to a large range of compounds
with different chain lengths and functionalities. The established method
starts from the methylsulfate or ethylsulfate salt and involves an acid
catalysed (usually methane sulfonic acid) transesterification reaction with
any longer chain or functionalized alcohols (Scheme 18).[57] The ionic
24
state sustains for the sulfates species and the group connected to oxygen
is replaced.
Scheme 18: Transesterification reaction of alkyl sulfate anions with any longer chain or
functionalized alcohols.
The oxygen of the alcohol is the nucleophile and the core of the anion the
electron poor center. A possible mechanism for the transesterification of
the sulfate is shown in Scheme 19.
CationSRO
O
O
O
Cation SR"O
O
O
O
HR"OH / -ROH /Cation SRO
O
O
OH
OR"H
SRO
O
O
O H
OR"
H
HCation
Scheme 19: Possible mechanism for the transesterification of the alkyl sulfate anions.
The products are, thus, ionic liquids with modified sulfate anion and a low
weight alcohol that is removed in vacuum.
For the metal cation catalysed transesterification of phosphate anions
(Scheme 20), Morrow et al. described likewise a mechanism of nucleo-
philic displacement of the oxygen tethered group maintaining the anionic
nature in the product.[63]
Scheme 20: Metal cation catalysed transesterification of phosphate anions.
25
Very interesting is now the comparison to the transesterification reaction
of phosphonate anions using alkylating agents as described in chapter
2.1.6.2, Scheme 11 and Scheme 12. The difference in the reaction
mechanism is the alkylation of the electron rich oxygen by the alkyl
halogenide. Thus, formation of the neutral dialkyl ester occurs instead of
the replacement of the oxygen connected group like in the acid catalysed
case (Scheme 21).
Scheme 21: Formation of the neutral dialkyl phosphonate ester.
Routes to synthesize aminopropylsulfonates / aminobutyl-sulfonates[69] or
halogen-propylsulfonate / halogenbutylsulfonate have been described for
alkylsulfonate ionic liquids by reaction of amines or halide salts with the
respective sultone – cyclic sulfonate esters. It is well known that sultones
are alkylating agents and undergo the ring-opening reaction with various
nucleophiles[71] Thus, nucleophile-containing ILs can be applied to the
ring-opening reaction of sultone providing as product an ionic liquid
containing the same cation, but a modified anion. The reaction of 1,4-
butane sultone with imidazolium ILs containing halogenides and / or
hydroxide as nucleophilic species is presented in Scheme 22.[70]
NNR S
O
X
O O
NNR
SX
O
OO
X: F-, Cl-, Br-, OH-
Scheme 22: Reaction of 1,4-butane sultone with imidazolium ILs containing halogenides or
hydroxide.[70]
26
The synthesis of chloro- and bromobutylsulfonate ILs is largely versatile
since plenty of chloride and bromide ILs with a large variety of cations are
commercially available. Furthermore, the preparation of sulfonate ILs with
other residues like e.g. fluorobutyl is as well possible.
The SN2 nucleophilic substitution reaction is furthermore used in the
synthesis of carboxylate esters, which are also obtainable by alkylation of
carboxylate ionic liquids.[64] However, the latter cannot be used as
alkylating agents.
In summary, the IL anions can be functionalized by the SN2 nucleophilic
substitution reaction. However, the nucleophiles are different in the acid
catalysed reaction and alkylation reaction. In the case of using alkylating
agents, the oxygen of the anion is the nucleophile and the halogen
connected carbon is the electron poor group. For sultones, the O-
connected carbon represents the center of the nucleophilic attac.
2.2.1.2.2. Kinetics and activation energy of the SN2 nucleo-philic substitution reaction
The esterification reaction is a nucleophilic substitution reaction of the SN2
mechanism, as the nucleophile displaces the leaving group in the reaction
rate limiting step, which thus depends on both the nucleophile
concentration, and the concentration of substrate, R-X (Equation 1).[65]
Nu(Nu)+RX(R-X) → Nu-R(Nu-R)+X(X) Equation 1
The rate of conversion r respective to the nucleophile is proportional to its
concentration (Equation 2).
푟 =푑[푁푢]푑푡 = −푘[푁푢][푅푋]
Equation 2
In the equimolar case with [Nu] = [R-X] and the quantity [Nu]0 being the
starting concentration of the nucleophile Equation 3 and Equation 4 are
given.
27
푑[푁푢]푑푡 = −푘[푁푢]
1[푁푢] −
1[푁푢] = 푘푡
Equation 3
Equation 4
As can be seen from Equation 5 [ ]
is directly proportional on time, so the
plot [ ]
vs. t is linear with its slope being the rate constant k and the
intersection with the y-axis is at the value of 1.
푘 =1푡
1[푁푢]−
1[푁푢]
Equation 5
The temperature dependence of the reaction rate is referred to as
activation energy since rates of reaction usually go up with increasing
temperature. The common derivation of an activation energy results from
a set of temperature dependent rates. For an elementary reaction, the
temperature dependence of the rate constant is given by the Arrhenius
equation (Equation 6) where k0 is called the preexponential factor
(prefactor) and EA the activation energy.
k(T) = 푘 푒 ( ) Equation 6
The activation energy EA (in kJ mol-1) can be determined from the slope
and the prefactor from the intercept by plotting the logarithm of the rate
constant against the reciprocal temperature (Equation 7).
ln k(T) = ln푘 − Equation 7
In contrast to SN2, in the SN1 mechanism the nucleophile attacks after the
rate-limiting step is over. Under real conditions, SN2 and SN1 are two
extremes of a sliding scale of reactions and many reactions may exhibit
both SN2 and SN1 character in their mechanisms, although SN2 reactions
are more common than SN1 reactions.
SN2 reactions are generally favored in primary alkyl halides or secondary
alkyl halides with an aprotic solvent. They occur at a negligible rate in
tertiary alkyl halides due to steric hindrance.[66]
28
2.2.2. Melting point, thermal stability, viscosity and density of ILs
The key criterion of an IL is its liquid range, which should ideally enclose
the room temperature. Many ILs, however exhibit no melting points, but
rather glass transitions upon cooling. Following features are essential for
low-melting salts: high asymmetry and weak intermolecular interactions.
The latter are achieved through good delocalization of charge and
avoidance of hydrogen bonding activity.[74],[76]
The behavior of ILs upon heating differs from that of conventional solvents
by their negligible vapor pressure even at elevated temperatures. Thus,
no boiling point can be experimentally determined for ILs under ambient
pressure. Only under high vacuum conditions and temperatures of 200-
300 °C, some ILs were reported to be distillable.[77] This extremely low
volatility entails consequently the non-flammability of ILs denoting them
environmentally friendly and highly save.
On the other side, thermal decomposition limits the IL application. This
can be determined by thermogravimetric analysis and lies around 300 °C
for imidazolium based ILs containing anions like [NTf2]-. Are the ILs
exposed to such high temperatures for a longer time, volatile de-
composition products occur and significant mass loss may be detected.[78]
The most important thermophysical properties of ILs are their viscosity
and density. Ionic liquids are mostly Newtonian fluids which are more
viscous than most common molecular solvents. The viscosity of a fluid
manifests itself externally as the resistance of the fluid to flow and is
accessed by means of rotational viscometers. The viscosity of ionic liquids
is strongly dependent on temperature and typically ILs show a concave
curvature instead of a straight line in the Arrhenius plot. This means a
complex interplay of short- and long-range forces is involved in molecule
dissociation and ion motion.[199] Thus, the temperature dependences of
29
the ionic viscosity (and subsequently conductivity, Equation 20, chapter
2.4.3.4.1) show Vogel-Fulcher-Tammann behavior (Equation 8).[198]
휂 = 퐴푒 Equation 8
where A, B and T0 are the fit parameters. T0 is the Vogel temperature, and
for T0 = 0 K Arrhenius behavior is obtained. A seems to represent the
minimum viscosity if temperature were infinite.
The viscosity is determined by intermolecular forces like Coulomb, van der
Waals, and hydrogen bonding.[204] The identity of the organic cation is of
basic importance. For ionic liquids with the same anion the viscosity
increases with larger cation size. Larger alkyl substituents on the
imidazolium cation lead to more viscous fluids, higher degree of branching
has been identified as important prerequisite for low viscosities.[83] (See
also chapter 2.4.3.4.1).
Changing the anion in ILs containing the same cation clearly impacts the
viscosity. Generally, the anion size has less impact on the viscosity than
other anion properties, such as their charge configuration and subsequent
ability to form tighter ion pairs with the cation. More basic anions exhibit
increased intermolecular forces and higher viscosity. On the other side
weakly hydrogen-bonding anions like [NTf2]- and [FAP]- compose ILs with
lowest viscosities.[83],[86],[151]
The phosphonate ionic liquid [EMIM][Me(H)PO3] was investigated among
others by Abe et al.[61] and Hasse et al.[86] These groups derived Vogel-
Fulcher-Tammann coefficients A, B, and T0 for this IL as represented in
Table 2.
Table 2: Fitting parameters of VFT equation for the viscosity of [EMIM][Me(H)PO3] from Ref. [86].
A / Pa s B / K T0 / K 272.73 688.941 182.993
30
Abe et al. investigated further [EMIM]+ based alkyl phosphonate ILs and
the obtained viscosities are shown in Figure 4.
Figure 4: Viscosity of [EMIM]+ based alkyl phosphonate ILs. R: 1) methyl, 5) ethyl, 6) i-propyl, 7)
n-butyl.[61]
Since there is an evident lack of literature data concerning the class of
methyl phosphonate ILs (MP-ILs), these results may be especially
valuable for comparison in this work. However, the viscosity data found in
the literature exhibit a certain variability due to measurements by
different researchers and different content of impurities in the ionic
liquids. It is well known that relatively small amounts of impurities or
cosolvents can have dramatic impact on ionic liquid viscosity (leading
usually to reduced viscosity).[87]
The reported densities of ionic liquids vary between 1.12 and 2.4 g cm-3.
The effects of the particular ions on the density of an IL were described by
Fredlake et al.[82] They found decreasing densities with increasing size of
the cation. The ILs with the smallest [EMIM]+ cation possess the highest
densities. The largest [OMA]+ cation exhibits the lowest density, followed
by the IL containing the [BBIM]+-cation. Tokuda et al. also found a
decreasing density with increasing alkyl chain length in ILs containing 1-
31
alkyl-3-methyl-imidazolium cations.[83] The influence of small anions on
the density is such that an increasing molar weight of the anion leads to
an increase of density. The reason is their ability to occupy the close
positions around the relatively large cation which can lead to the
formation of a gridlike assembly of anions and cations. The hydrogen
bonding, if present, shortens further intermolecular distances. This
behavior was confirmed by Froeba et al. for [EMIM]+ based ILs with
[EtSO4]-, [N(CN)2]- and [NTf2]- ions.[85]
Most of the commonly employed anions are highly symmetric and almost
spherical. Increasing their asymmetry appears to have a marked effect on
the properties like melting point, density and viscosity of the resulting
ionic liquid. Further, the anion has great impact on the miscibility of the IL
with molecular solvents (water, ether, etc.).[74] Hydrophilic anions like
nitrate lead to water miscible and hydroscopic ILs. Only very long aliphatic
chains on the cation may damp this behavior. [BF4]-, [PF6]- or [NTf2]- ions
are known to provide hydrophobicity.
For ILs with larger anions like [OcSO4]-, the density does not match this
behavior. The long C8 side chain prohibits the formation of tight molecular
assemblies leading to a lower density. The impact of impurities on density
appears to be far less dramatic than in the case of viscosity.[208]
2.2.3. Water and other impurities
The impurity content of an ionic liquid is determined in the first place by
its synthetic route since the purification of the final product from residues
of starting materials and by-products is one of the most challenging
issues. The purity of an IL is always mentioned in context with the
application aspects. E.g. halogenides are known to be highly corrosive and
thus halogen-free ILs are of great advantage. On the other side, organic
starting materials may be less crititical. An extensive treatment of this
topic may be found in literature.[74]-[76],[196],[199]
32
Water is one of the most significant impurities in RTILs independent on
the synthetic route and even hydrophobic ones absorb some water from
the atmosphere.[212] The miscibility of aprotic RTILs with water is
determined largely by the anion. The hydrophobicity of anions here follows
the general trend: [FAP]- > [NTf2]- > [PF6]- > [BF4]- > halides, as
previously suggested in the literature.[151],[218] The cation of an ionic liquid
exhibits following effects on the hydrophobicity: Increasing alkyl chain
length increases hydrophobicity of an ionic liquid.[209] None of the physico-
chemical or electrochemical properties of an IL is independent on water
content. Further, water may undergo chemical reactions with IL
components like widely known hydrolysis of [PF6]- and [BF4]-.[214]
Therefore, the water concentration should preferably be measured and
stated when reporting an IL based study.
2.3. Ionogels
ILs are suited for use in electro-chemical devices such as fuel cels and / or
dye sensitized solar cells (DSSCs), double layer capacitors, lithium
secondary batteries and thin film transistors. There is a challenging need
for immobilizing ILs in solid devices, while keeping their specific
properties.[88] Conversely, some effects of confinement can modify some
properties of the guest IL, the most important one being the ion mobility.
One of the most simple and efficient approaches to keep the main features
of ILs, while allowing easy shaping and printability, is based on gelation.
Ionogels form a promising family of solid electrolyte membranes. The
solid-like behaviour of the resulting material is due to the formation of a
three-dimensional network which percolates throughout the IL. The key
point is that ionogels maintain characteristic conductivity of ILs[90] and
have tremendous specific capacitances, in excess of 10 F cm-2, which is
500 times as high as many typical dielectrics and 10 times as high as
some other recently reported dielectric layers.[89] Their high potential in
flexible electronics is thus evident.
33
The various types of ionogels may be separated into physical and chemical
gels. In chemical gels the internal 3D network is cross-linked through
covalent bonding. In physical gels, cross-linkage results from weak and
reversible interactions like hydrogen bonds, hydrophobic interactions,
crystallite junctions etc. The mechanical solidity of physical gels ranges
from free-standing membranes over jellies, slurries to pastes. There are
generally three categories of preparation of ionogels, which depend on the
nature of the solid-like network. The organic route may be carried out by
using an organic gelator like low molecular weight gelators (LMWGs),
gelatin[90] or polymers and biopolymers. The LMWGs are able to self-
assemble in solution through supramolecular bonding such as hydrogen
bonding, – or electrostatic interactions, thus inducing physical
gelation.[91] The use of polymers to immobilize ILs has been widely
developed, particularly as materials for electrochemical devices. A very
important route is the polymerization of monomers in an IL used as a
solvent.[92] The most prominent examples for host polymers revealing
sufficient miscibility between the IL and the polymer are poly(methyl
methacrylate)s (PMMA), poly(ethylene oxide)s (PEO), fluoropolymers and
copolymers, as sulfonated tetrafluoroethylenes (Nafions) and
poly(vinylidene fluoride-cohexafluoropropylene)s (PVdF-HFP).[97] An out-
standing position is due to the tri-block copolymers reported by Lodge et
al.. Two types of polymer blocks (IL soluble and insoluble) build
transparent thermoreversible ionogels by self assembly (Figure 5).[98],[99]
Figure 5: An ABA triblock copolymer with soluble B block (blue) and insoluble A blocks (red) (left)
self-assembles in the presence of an IL (+ and – symbols) to form an ion gel (center). Suitable
34
choice of A block enables thermoreversible gelation when the A blocks become soluble at a higher
temperature (right).[99]
The recent application of such ionogels derived from tri-block copolymers
is as gate dielectric in polymer thin-film transistors. Due to their electrical
properties like high capacitance, high conductivity (Figure 6), and short
polarization response times they can serve as solution processable solid
electrolytes.[100],[101]
Figure 6: Temperature dependence of conductivity (filled symbols, experimental data; and line,
Vogel-Fulcher-Tamman (VFT) fit) for a 13.4 μm thick ion gel film.[101]
Further, gelled systems may be obtained by mixing the polymer and the
IL with or without a co-solvent or simply by swelling a polymer in an IL.[93]
The inorganic synthesis is based on oxide nanoparticles,[94] fumed silica
particles[95] or carbon nanotubes[96] which coagulate the whole assembly.
Finally, hybrid organic–inorganic synthesis is possible using polymers
reinforced with inorganic fillers.
Special mention has to be made of the use of biopolymers, as gelatin and
polysaccharides, which provide sustainable materials, equipped with bio-
molecular functions.[90], [105]-[122]
35
2.3.1. Dissolution of carbohydrates in ILs
Carbohydrates are the most abundant organic compounds on earth. They
are relatively inexpensive and represent a renewable feedstock. Thus,
they find many industrial applications in such diverse areas as chemistry,
fermentation, petroleum production, food, paper, and pharmaceutical
industries. Dependent upon their molecular weight saccharides are
distinguished. Simple, low-molecular-weight carbohydrates represent
mono-saccharides like arabinose, glucose, fructose, mannose, and xylose.
Sucrose, lactose, and maltose are di-saccharides. More complex, high-
molecular-weight poly-saccharides like cellulose, chitin, chitosan, starch,
amylose, amylopectin, agarose, inulin, and xylan are of the highest
importance.[102]
Unfortunately, the main hindrance in the use of carbohydrates is their
poor solubility in almost all solvents. E.g. cellulose is a linear polymer
stabilized by a large number of intra- and intermolecular hydrogen bonds
forming a highly ordered, crystalline structure. In the two last decades,
the biomass processing with ionic liquids gained wide attention. Rogers et
al. demonstrated that some imidazolium-based ILs are capable of
dissolving considerable amounts (up to 25 wt%) of cellulose, forming
highly viscous solutions. They suggested that the IL is capable for
breaking the extensive hydrogen-bonding network in the polysaccharide
and promotes the dissolution being despite this a non-derivatizing
solvent.[103] Remsing et al. demonstrated through 13C and 35/37Cl NMR
relaxation measurements that the interaction between the carbohydrate
and the anion of an IL is predominant compared to the interactions of the
carbohydrate with the cation.[104] If the anion is a strong proton acceptor,
say it exhibits high hydrogen-bond basicity, it plays a key role in the
dissolution process. Anion functionalizations with hydrogen-bond acidity,
on the contrary, will reduce cellulose solubilisation activity by competing
for the hydrogen-bond basic site.[110],[111] The high melting point and
relatively high viscosity of chloride ILs make the processing of carbo-
36
hydrates expensive and inefficient. This demands for newly designed ILs
that exhibit low melting temperature, relatively low viscosity, and
sufficient polarity. ILs containing carboxylate,[105] phosphate[106] and phos-
phonate[61],[62] anions show low viscosity and basicity of hydrogen bonding
stronger than chloride anion. Further, some [BMIM]+ based ILs with O,S-
dimethyl phosphorothioate [dmpt] and O,Se-dimethyl phosphoroselenoate
[dmpSe] anions (examples in Scheme 23) have already been proven as
solvation media for cellulosic materials.[107]
S
P
OO
O
dmpt
Se
P
OO
O
dmpSe
Scheme 23: O,S-dimethyl phosphorothioate (dmpt) and O,Se-dimethyl phosphoroselenoate
(dmpSe) anions.[107]
The named anions are capable of destroying the crystalline structure of
the carbohydrate by separation of the hydroxyl groups of the different
chains.[108] Du et al. carried out quantum mechanical calculations in order
to determine the mechanisms for the superiority of the imidazolium
acetate-based ionic liquids to the corresponding chloride-based ionic
liquids.[109] They showed, that the imidazolium cation can react with the
acetate anion to generate a carbene, which then reacts with cellulose in
addition to the hydrogen bonds formed by the acetate anion. The
drawback of carboxylate IL employment is, however, their relatively low
thermal stability and a multi-step procedure of preparation.
The effect of the anion cannot be considered in isolation. The fact that the
chemical structure of cations affects the carbohydrate dissolution was
already proven. Dissolving cations consist of planar, nitrogen-containing
rings with the ability to delocalize their positive charge within their
aromatic -system. The non-aromatic heteroatoms without the ability to
delocalize the positive charge such as non-cyclic ammonium and phos-
phonium cations belong to poorly dissolving IL cations.[110] Further,
37
Rogers et al. found that the solubility of cellulose decreases with an
increase of the alkyl chain length in the imidazolium cation. Due to the
recognized biocompatibility of PEG, ILs with PEG-containing cations have
been probed as biomaterial processing solvents. The oxygen atom present
in the molecule serves as a hydrogen-bond acceptor and interacts with
carbohydrates to enhance its solubility in the IL. Kimizuka et al. studied
the gelation of ILs with ethylene glycol residues by the addition of L-
glutamic acids or carbohydrates such as -D-glucose and -cyclodextrin.
Strong interactions between carbohydrates and ether groups lead to
formation of fibrous nanostructures.[112],[113]
Water is one of the major impurities in ILs and decreases the solubility of
carbohydrates therein considerably. Carbohydrates interact with the
aqueous environment through numerous hydroxyl groups and build
hydrogen bonds.[103],[122] Water links units of sugar and decreases carbo-
hydrates accessibility causing its aggregation. In addition, water may
hydrolyse IL components and may lead to side reactions. On the other
hand, water can be used for a facile regeneration of carbohydrates already
dissolved in an IL by precipitation.[103]
2.3.2. Biopolymer ionogels for electrochemical devices
The efficient utilization of biodegradable polymers from renewable sources
such as starch,[119] chitosan[120] or agarose[121] has attracted attention in
recent years because of their superior mechanical and electrical properties
as well as diminishing resources of fossil fuels as well as white
pollution.[118]
Rheological investigations were performed on the IL / cellulose and / or
IL / chitosan solutions for [EMIM][Ac] as depicted in Figure 7.[114]-[116]
a) b)
38
c)
Figure 7: a) Viscosity shear rate dependence for microcrystalline (MC) and bacterial cellulose (BC)
dissolved in [EMIM][Ac] and in [BMIM][Cl] at 40 °C.[114]; b) Viscosity as a function of shear rate
(filled symbols) and complex viscosity as a function of angular frequency (hollow symbols) for
cellulose / [EMIM][Ac] solutions at 25 °C and the concentrations indicated.[115]; c) Steady
rheological curves of 8 wt% cellulose / chitosan solutions in [EMIM][Ac] with different ratios at
80 °C.[116]
Although the results differ concerning Newtonian fluidity for mixtures with
higher biopolymer content, the dramatic increase in viscosity at ambient
temperature is evident. Thus, augmenting biopolymer concentration leads
finally to free-standing IL plasticized biopolymer films[117] that have a
potential application as solid biopolymer electrolytes.
Usually, the biopolymer ionogel preparation is carried out by adding finelly
powdered carbohydrate to the ionic liquid under stirring and heating until
the solid material disappears and visually transparent mixtures are
obtained. Finally, viscous clear solutions are allowed to cool for gelling if
needed in the desired form.[121] The electrochemical properties like
39
conductivity (Figure 8) differ only slightly from the neat IL, although the
viscosity increases significantly.
Figure 8: left: Temperature dependence of ionic conductivity in neat RTILs: () [BMIM][MeSO4];
() [BMIM][Cl]; () [OMIM][Cl]. Hollow symbols correspond to agarose–RTIL sol–gels (3 wt%).
The corresponding VFT data are plotted as solid lines;[121] right: The ionic conductivity vs.
composition plot in chitosan / IL polymer electrolyte system.[120]
2.4. Electrochemistry
If a system contains ions and the ions are free to move, the system will be
able to conduct an electric current. The conduction in the solution is ionic
whereas in the electrodes and the connecting wires it is electronic. Across
the interfaces one mode is transferred to the other through the electro-
chemical processes, which are of kinetic nature. Their rates are controlled
by the properties of the surface, solution and the nature of the reacting
species.[123]
If two electrodes are dipped in solution, the interaction of the ions with
the surface can be immediately detected as the open circuit potential (also
called the zero-current potential or the rest potential). The experiments
can be performed in galvanostatic mode, say by controlling the current
externally and measuring the resulting changes in potential at the working
electrode or vice versa in the potentiostatic mode, which is the most
common.[123]
40
2.4.1. Cyclic voltammetry
Is a potential ramp E applied on an electrochemical cell with two blocking
(or polarisable) electrodes, the current response j can be divided roughly
in two parts:[127] At potential values representing the ideally polarisable
region, the nonfaradaic current flow (or double-layer current density) jdl
(Equation 9) results from the mass transport processes like migration of
the ions to the surface and their arrangement there (e.g. double-layer
formation, adsorption). jdl is thus function of the double-layer capacitance
Cdl (see chapter 2.4.2).
푗 = 퐶푑퐸푑푡
Equation 9
Then, as the Red / Ox value of the potential is reached, the faradaic
current jct is measured, which mainly is due to the charge transfer
processes such as oxidation and reduction of the ions at the electrode and
is much higher than the current density jdl (Figure 9).[125]
Figure 9: Current-potential curve of 5.00 mM DmFc in [MIMSBu][PO2(OBu)2] at a glassy carbon
(GC) electrode (Ø = 1 mm) at scan rates of 0.10, 0.20, 0.30, 0.40, 0.50, 0.70, and 1.00 V s-1 with
identification of following regions: 1) polarisable (nonfaradaic), 2) faradaic and 3) uncompensated
resistive drop manifesting in the drawing out of CV waves as the scan rate increases.[128]
1
2 3
41
The faradaic current flow jct can be written as
푗 =푛퐹퐷푐훿
Equation 10
in which nF is the charge transferred per mol (C mol-1), D is the diffusion
coefficient, cS is the bulk concentration and is the Nernst-diffusion layer
thickness, typically in the range 10-3 – 10-1 cm. The most important things
to notice in Equation 10 are that the limiting current density is in-
dependent of potential and that it depends linearly on the bulk
concentration say the solution composition.
For a diffusion controlled process, is proportional to the square root of
time (Equation 11) and hence the limiting current density decreases
gradually with time.
훿 = √휋퐷푡 Equation 11
Taking measurements at short times increases the value of jct allowing the
use of wider range of potentials. Very important is the consequence of this
equation that jct is independent on the kinetics of the reaction, i.e. of the
nature of the surface and its electrochemical activity. These features make
it an ideal tool for probing the concentration of species in solution.[123]
The quantity is the essence of mass transport, a rough estimate of the
distance over which molecules can diffuse in a given time, considered as
“the characteristic length” for diffusion.
Mass transport to the interface can occur through three independent
mechanisms: migration, convection and diffusion. Stirring the solution or
moving the electrode (rotation, vibration) decrease the value of and
hence increase jct.[123]
In contrast to migration and convection that demand an input of external
energy like electric field or mechanical stirring, respectively, the driving
force for mass transport by diffusion is the gradient in concentration. It is
a relatively slow process, with diffusion coefficients for small molecules in
42
dilute aqueous solutions at room temperature, in the range of 10-6 –
10-4 cm2 s-1.[123]
Mass transport – the process to bring the reacting species close enough to
the surface – and charge transfer are two consecutive processes and the
overall rate is determined by the slower of the two. If the charge transfer
is absent, the overall rate is limited by the mass transport.[123] This
situation occurs in the polarisation region. The second case of low charge
transfer is the so-called uncompensated resistive drop (Figure 9). That is
the depletion of charge carriers at the electrode surface due to conditions
resulting in large amplitude currents like large electrode area or fast scan
rates. The current – potential curves tend then to be drawn out over
extended potential ranges.[128]
2.4.1.1. Cyclic voltammetry of RTILs
For ionic liquids the polarisation region is referred to as electrochemical
window calculated by subtracting the reduction potential from the
oxidation potential. In considering RTILs as components in electrochemical
devices, it is important to have information on its electrochemical stability
towards a particular electrode. The basis for the selection of the cut-off
current is suggested to be 1 mA cm-2.[196] If the maximum current density
observed is lower than the cut-off current density, the limiting potential is
taken at the maximum current. On the other side, for current scales much
greater than the cut-off current density, the limiting potential is taken at a
cut-off current density that is as low as possible.[196] However, in RTIL
studies, jct is much smaller than in molecular solvents because of the high
viscosity and low rate of mass transfer, while jdl values are rather similar
to their counterparts in molecular solvents.[203]
Imidazolium-containing ionic liquids are thought to have slightly
constricted reduction limits due to the acidic nature of the proton attached
to the ring carbon between the two heteroatoms.[204],[210] The cathodic
43
limiting reactions of imidazolium cations proceed initially via the reduction
of ring protons to molecular hydrogen.[211] Substituting the proton in
question by a methyl group results in an improvement in the reductive
stability of the imidazolium cation and the potential window is found to be
extended: 5.2 V for [BMMIM][NTf2] compared to 4.8 V for
[BMIM][NTf2].[203] In the imidazolium cation based ILs like e.g.
[RMIM][NTf2] the length of the alkyl chain attached to the cation does not
affect the behavior of the ionic liquid at anodic potentials as the positive
limit is practically identical for all ionic liquids containing the anion [NTf2]-.
In contrast, the negative limit of polarization is clearly dependent on the
cation. Silva et al. observed for imidazolium-based liquids that the limit
increases with increasing length of the alkyl side chain [HMIM]+ >
[BMIM]+ > [EMIM]+. On the other hand, when functionalized groups like
an alkyl-ether chain are introduced, the electrochemical window is
narrowed.[202] Replacing the imidazolium cation by cations with four co-
ordinate species like pyrrolidinium,[205] tetraalkylammonium,[206] or tetra-
alkylphosphonium,[151] leads to wider electrochemical windows. In general,
the apparent overall trend in the electrochemical stability of the ionic
liquid cations follows the order: benzotriazolium < pyridinium <
pyrrolinium < imidazolium ≤ pyrazolium ≤ sulfonium ≤ pyrrolidinium ≤
piperidinium ≈ ammonium ≈ morpholinium.[208] Thus, both the centre of
localization of positive charge (imidazolium or pyrrolidinium) and the size
of the hydrophobic chains were found to affect the reduction potential of
the cation.[150]
The anion influences the electrochemical window in the manner that
slightly wider electrochemical windows are observed for [NTf2]- anions
compared to [BF4]- or [PF6]- ones. Protic ionic liquids containing chloride,
bromide, and iodide show narrower electrochemical windows, since the
oxidation of these halide anions proceeds more easily compared to [BF4]-,
[PF6]-, and [NTf2]-. The anion tris-(pentafluoroethyl)trifluorophosphate
[FAP]- exhibits the highest stability towards oxidation and a very wide
electrochemical window of 7.0 V was reported for [NBu4][FAP].[151]
44
Since CV is very sensitive to impurities, the quality of the RTILs affects
strongly the voltammogram. The effect of water on RTILs has been
studied, since water can be reduced and oxidized within the electro-
chemical potential window of many ionic liquids consequently decreasing
the overall effective electrochemical window.[213] In contrast to glassy
carbon electrodes (GC), the electrochemistry of protonic impurities (e.g.
water) will be strongly observed at Pt electrodes, which exhibit generally
good electrochemical behavior for proton.[208]
Figure 10 displays the full electrochemical window of [EMIM][NTf2] on Pt
under vacuum-dried, atmospheric and wet conditions. The potential
window for vacuum-dried IL shows no other redox features than the limits
of the IL.
Figure 10: Effect of water on the electrochemical window of [EMIM][NTf2] with increasing water
content for vacuum-dried, atmospheric, and wet conditions. Wet conditions were measured at
temperatures 298, 318, and 338 K. CV was carried out on 10 µm Pt electrode vs. Pt wire
(0.3 mm). Each scan was taken at 100 mV s-1.[217]
In atmospheric conditions, the electrochemical windows decrease through
the reduction of atmospheric oxygen at a potential between -1.0
and -1.5 V.[215],[216] The anodic window also decreases significantly. Thus,
the addition of water into the cell and increasing amount of moisture of
the RTILs significantly narrow the electrochemical window in Figure
10.[217]
45
Water can react with the ionic liquid components, especially anions, to
produce products that are electroactive within the electrochemical
potential window of the IL. Ionic liquids containing the [PF6]- and [BF4]-
anions can exhibit the largest electrochemical window, but such anions are
susceptible to hydrolysis when in contact with water and can result in the
undesirable evolution of hydrogen fluoride.[214]
2.4.2. Electrochemical double-layer
The processes at the interface are of greatest interest, however on the
atomic scale they are very difficult to describe. The properties of both
phases at the interface are different from their bulk properties. One
distinct property of the metal / solution interface is, in this regard, a
capacitance, called the double-layer capacitance, Cdl. The double-layer is a
very thin region near the interface, extending about 1 – 10 nm. It results
from the charge separation between the two phases in contact.[123]
The double-layer capacitance Cdl and the faradaic resistance Rct are
intrinsic properties of the interface, whereas the solution resistance (or
bulk resistance) RS is not a property of the interface. It is a term arising
from the fact that the potential in solution is always measured far from
the interface on the molecular scale, typically at a distance of 0.1 –
1 cm.[123] At this point it is important to distinguish the integral and
differential capacitances, since only the latter is accessed
experimentally.[124]
For the basic understanding of the structure of the double-layer one kind
of adsorption - called specific or contact adsorption – should be considered
in more detail. The structure of the double-layer at the surface of
electrodes is quite different depending on the metal used and should be
considered for each solid metal separately. Further it depends on the
degree of solvation of the ions, which is a function of the structure of the
ions and solution composition. E.g. on mercury, small inorganic cations
46
which are usually strongly hydrated in solutions are not specifically
adsorbed. In contrast, small inorganic anions can be in direct contact with
the surface. For larger ions, the situation can change: some highly
symmetrical anions like [ClO4]-, [BF4]-, and [PF6]- are not specifically
adsorbed on mercury. Large organic cations like [NEt4]+ were found to be
specifically adsorbed.[123]
2.4.2.1. Alternating voltage
Is alternating voltage applied on the system, the double-layers are
rearranged in the same frequency. The double-layer capacitance behaves
like a pure capacitor: when charge is brought to one side (plate) of the
capacitor, an equal but opposite charge is induced on the other side. An
excess of electrons on the surface of the metal causes a rearrangement of
the distribution of ions on the solution side of the interface, yielding an
excess of positively charged ions, and vice versa. There is no transfer of
charge across the interface.[123] The current response has then also the
same frequency like the voltage, however phase-shifted, since the re-
generation of the double-layer takes time.[126]
Mathematically this can be expressed as follows: A purely sinusoidal
voltage E = E sin(t) where is the angular frequency, entails a current
j = j sin(t+) shifted by a phase angle, . Usually E is taken as a
reference signal, and is measured with respect to it. For an ideal
capacitor, the phase angle is -90°. For an ideal resistor, the phase angle is
0° (Figure 11).
a) b)
47
Figure 11: a) Relationship between the voltage across a resistor and current through the resistor,
b) Relationship between an alternating voltage across a capacitor and the alternating current
through the capacitor.[125]
In real systems the phase angle will be somewhat in between and will
depend on frequency.[123]
Figure 12 shows the relationship between alternating voltage and current
signals at frequency and subsequent rearrangement of the double-
layer.[125],[126]
Figure 12: The relationship between alternating voltage and current signals at frequency and
subsequent rearrangement of the double-layer upon applying an alternating voltage.[125],[126]
2.4.3. Electrochemical impedance spectroscopy (EIS)
The frequency dependence between alternating voltage and current is
always system specific. This frequency dependence is referred to as
impedance Z. Through variation of the frequency f of the alternating
voltage usually in the range 106 – 10-2 Hz an impedance spectrum can be
obtained with Z = f() and = 2f. The several electrochemical processes
contribute in different ways to the overall impedance and can be detected
through the adjustment of the applied frequency range. Thus, the electro-
chemical impedance spectroscopy is the most appropriate technique to
48
provide accurate measurements on systems consisting of the bulk solution
and the interface over a wide range of experimental conditions.[123],[129]
2.4.3.1. Equivalent circuit representation
The impedance data measured experimentally must be interpreted
theoretically. The representation of impedance spectra of electrochemical
systems in terms of equivalent circuits – the circuit modelling approach –
aided by special computer programs has become a common
practice.[129],[130] The objective is the approximation of the experimental
impedance data Z() by the impedance Zec() of an equivalent circuit
made up of ideal elements say C (capacitance), R (resistance) and L
(inductance). Thus, a cell with two electrodes can be represented by the
equivalent circuit shown in Figure 13. Usually only the part in the blue
framed field is considered, since the experiment is set up in such a way
that only one of the electrodes – the working electrode – is studied at a
time.
Figure 13: ec representation for a two electrode cell.[123]
In such a circuit a resistance represents a conductive path, where a
resistor RS accounts for the bulk conductivity of the material and the
combination of the double-layer capacitance Cdl and the faradaic
resistance Rct represents the interface. The Cdl and Rct must be put in a
parallel rather than in a series combination because a steady direct
current can be observed flowing when the potential is high enough, say
above the minimum prescribed by thermodynamics. Capacitances and
49
inductances will be generally associated with space charge polarization
regions and with specific adsorption and electrocrystallisation processes at
an electrode.[123],[129]
The values of the components and their distribution in the circuit are
chosen to give the best fit to the observed frequency spectral response.
The physical interpretation of the distributed elements in an equivalent
circuit is somewhat more elusive. It should be noted, that a particular
selected circuit model which provides a best mathematical fit, may not
mandatory represent the impedance behavior of the system.[129] On the
other side, the equivalent circuit in Figure 13 represents a large break
down, and interfaces rarely behave that simple.[123]
This interface representation therefore may be supplemented e.g. by at
least two further elements: the Warburg impedance W, associated with
diffusion and a distributed element DE representing the adsorption
processes on the electrode (Figure 14). The adsorption element can be
then splitted in parallel combination of Rad and Cad.[126] In contrast to RS
and Rct, the impedance of the elements Cdl, W and DE is frequency
dependent.[126] Through adjustment of the frequency range they may be
neglected.
Figure 14: Possible ec for the process sequence: mass tansport – adsorption –charge transfer.[126]
50
The most widely used approach involves a study of the variation of the
total impedance Z with frequency for the series combination of resistance
and capacitance (Equation 12)
푍 = 푅 −푖휔퐶 = 푍 − 푖푍"
Equation 12
with 푍 = 푅 (also referred to as resistive impedance) and 푍" = (also
referred to as capacitive impedance) being the real and the imaginary
parts of the impedance, respectively, and 푖 ≡ √−1. For the equivalent
circuit in Figure 13 (framed part), one has the RS and Cdl in series, and Cdl
and Rct in parallel.
푍(휔) = 푅 − 푖푅
휔퐶 푅 − 푖 Equation 13
At low frequencies, the capacitance containing term becomes very high.
So we detect Cdl and Rct in the lower frequency range. As frequency is
increased, the capacitive impedance decreases, while the resistive
impedance is unchanged. At the limit of high frequencies, the faradaic
resistance Rct is effectively predominated by Cdl, and the solution
resistance RS is the only measured quantity.[123]
It follows further that the absolute value of the impedance vector is given
by Equation 14.[123],[125]
|푍(휔)| = (푍 ) + (푍") Equation 14
and it comes out for the phase angle
푡푎푛휑 = 푍"푍′
Equation 15
2.4.3.2. Graphical representations
The results of the impedance measurements can be displayed either in a
complex plane by plotting Z” vs. Z’ (Nyquist plot) for the frequency range
51
or in so-called Bode plots. The latter represent Z and phase-angle, vs,
usually on a logarithmic scale. An advantage of Bode plots is that the
impedance behavior at high frequencies is shown with equal weight, along
the plot, to that at low frequencies whereas, in Nyquist plots, the
frequency data are incorporated implicitly and at high-frequency tend to
become bunched together towards the → ∞ intercept on the Z’ axis. As
can be seen from Equation 15 the phase angle is the angle between the
directions of the real and imaginary components of Z at a given
frequency, corresponding to the shift of the current response applying
alternating voltage -90° for a pure capacitor 0° for a resistor[123],[125],[129]
Figure 15: Simulated impedance plots for the ec of Figure 13 (framed part) with RS = 105 ,
Cdl = 20 nF and Rct = 106 as a) Nyquist and b) Bode representations.
The description of the plots corresponds closely to the specification of the
Equation 13. The imaginary component to the impedance in the circuit of
Figure 13 (framed part) comes solely from Cdl. At high frequencies, its
contribution falls to zero and the only impedance is the solution resistance
RS. As the frequency drops, the finite impedance of Cdl manifests itself as
a significant Z” until at very low frequencies it offers high impedance.
Then the current flow passes mostly through Rct. Thus the imaginary
impedance component falls off again (Figure 15).[125]
a) b)
RS RS +Rct
RS+Rct
RS
RS+Rct RS
→
Z“
/
|Z|
/
φ Z‘ /
52
A high value of Rct is associated with a polarisable interface, whereas a
low value of Rct represents a nonpolarisable interface. In other words, in
the polarisation region, where no charge transfer occurs (Rct→∞), the
equivalent circuit may be shorted to Figure 16. Then at lower frequencies
no inclination of the Nyquist plot to the real axis occurs and the
impedance plot appears as a straight line perpendicular to the real axis
with the intersection being the RS as displayed in Figure 16a. The Bode
representation does not show the second plateau for the Rct anymore and
the phase angle stays constant after having reached -90°.
Figure 16: Simulated impedance plots for the series RC ec with RS = 105 and Cdl = 20 nF as
a) Nyquist and b) Bode representations.
The complex capacitance of this equivalent circuit can be represented in
analogy to the complex impedance by Equation 16 (see chapter 6.3 for
derivation).
RS Cdl
a) b)
Z‘ /
|Z|
/
Z“
/
φ
53
퐶(휔) = 퐶 (휔) − 푖퐶"(휔)
|퐶(휔)| = (퐶 ) + (퐶")
Equation 16
K. S. Cole and R. H. Cole[155],[222] introduced the graphical treatment of the
complex permittivity and their concept may be transferred to the complex
capacitance by plotting C” vs. C’ in a complex plane (Cole – Cole similar
type plot). As result an ideal semicircle is obtained with the diameter
being the Cdl as displayed in Figure 17. For Cs = 0 (bulk capacitance), Cdl
is the intersection of the real axis. In real systems Cs « Cdl, and thus can
be neglected.
Figure 17: Simulated complex capacitance plots for the series RC ec with RS = 105 and
Cdl = 20 nF as a) Cole-Cole and b) quasi-Bode representations.
2.4.3.2.1. Constant phase element (CPE)
However, studies on the double-layer capacitance at solid electrodes
usually show deviations from the ideal behavior. Physical explanations of
this capacitance dispersion consider in this connection microscopic
roughness of the electrode surface due to polycrystallinity, scratches or
pits which lead to current-density inhomogeneities and subsequent
coupling of the solution resistance and the surface capacitance on one
hand and slow interfacial processes like adsorption of ions on the other
hand. A modern view sees capacitance dispersion as interfacial in origin
and caused by adsorption effects, including slow formation of surface
bonds or rearrangement of surface structures. Pajkossy et al.
demonstrated experimentally that roughness and adsorption effects are
a) b)
CS+Cdl
→
Cdl |C|
/ F
C“
/ F
C‘ / F
54
intrinsically coupled: the capacitance dispersion is increased through
increased roughness which broadens the time constant distribution of
adsorption kinetics.[133]
The nonideal behavior usually cannot be represented as a series
connection of the solution resistance and double-layer capacitance, since a
frequency dependent capacitance is observed. The anomalous dispersion
behavior of capacitance manifests itself as a tilted line in the Nyquist plot
and as the absolute value of the phase angle below 90° in the Bode
representation. In the equivalent circuit the capacitance is then replaced
by a so-called “constant phase element” (CPE) which is a complex
impedance having the special property that its phase angle is independent
of frequency.
→
Figure 18: Simulated impedance plots for the series R-CPE ec with RS = 105 Q = 20 nF and
= 0.8 as a) Nyquist with zoom in the insert and b) Bode representations.
The impedance of the CPE ZCPE depends on frequency as displayed in
Equation 17 and contains the double-layer capacitance quantity, Q and
the parameter . This relationship is included in commercial impedance
a) b)
RS CPE
RS Cdl
Z‘ /
|Z|
/
Z“
/
φ
55
fitting softwares used to fit the data to equivalent circuits involving CPE
elements.[134]
푍 =1
푄(푗휔) Equation 17
is related to the phase angle by = (/2) and < 1, usually in the
range 0.9 to 0.99. The smaller , the larger is the deviation from a Z’-
independent vertical line on a Nyquist plot (Figure 18). → 1 obviously
corresponds to true capacitance behavior. Beyond that, for = 0, the CPE
behaves as a resistor, and for = -1, the CPE behaves as an inductor.[135]
The assumption that Q ≈ Cdl can only be justified if = 1. Otherwise it is a
crude numerical approximation.[136] The smoother and cleaner the
electrode, the closer is the parameter to unity. Both and Q are
dependent on the electrode material, its surface preparation (roughness),
electrode potential, temperature, ionic concentrations and whether or not
adsorbable anions are present in the electrolyte.[137] To extract values of
the interfacial capacitance Cdl for a blocking electrode from the parameters
and Q, the Equation 18 was proposed by Brug et al.[132]
퐶 = 푄푅( )Equation 18
2.4.3.3. Instrumental limitations
EIS measurements should be carried out over a wide frequency range in
order to identify all time constants in the circuit. The lowest frequency
typically used is 10-3 Hz. It is relatively easy to get measurements of good
precision for impedances between 1 and 105 at frequencies below
5·104 Hz. However, for lower and higher impedances, distortions may be
observed. The distortion observed at high frequencies leads to large
positive imaginary impedances and corresponds to an inductance in series
with the electrode impedance which arises from that of the leads and the
current measuring resistor. Often, such effects may be minimized by
56
shortening the cables and shielding the set up e.g. by means of a Faraday
cage.[131] Impedance measurements in the high frequency range beyond
the electrolyte resistance RS currently exhibit one or several loops (so-
called “parasitic loops”) irrelevant to the electrode process. In fairly
conductive media, the contribution of the electrolyte resistance is rather
negligible in contrast to low conductivity media. Unfortunately, as the
resistivity of the solution increases these artefacts may appear even at
much lower frequencies, for instance at 10 to 100 Hz. The identification of
the various items contributing to the measured impedance becomes more
and more difficult. In particular, electrolyte resistance no longer appears
as the high frequency limit of the impedance. The parasitic contributions
by capacitive as well as by inductive features intervene in the high
frequency range. A great vigilance and a thorough knowledge of the
phenomena observed in the range of high frequencies when measuring
impedance in low conductivity media is essential for interpreting
experimental diagrams.[138]
2.4.3.4. Impedance on ILs
Room-temperature Ionic Liquids (RTILs) are highly concentrated
electrolytes with large potential windows of electrochemical stability and
high mobilities of the component ions. They have conductivities of
10-2 S cm-1 at room temperature[196] that is lower than all aqueous
solutions and capacitances higher than those in aqueous solutions. Both
properties are a function of temperature.[145],[150] The range of
opportunities offered by RTILs in electrochemistry expands over solar and
fuel cells, batteries and supercapacitors, media for electrodeposition of
semiconductors and metals. All of these applications created interest in
the conductivity of ILs as well as in interface between ionic liquid and
metal or carbon electrodes, respectively, which are determined not only
by the structure and properties of the double layer.[150]
57
2.4.3.4.1. Conductivity of ionic liquids
Ionic conductivity is defined as the reciprocal of proper resistance RS
multiplied by the cell constant k which represents the ratio of the distance
between the electrodes l and the mean area of the working electrode A
(Equation 19).
휎 =푙
푅 퐴 =1푅 푘
Equation 19
The cell constant k has to be determined by measuring the conductivity of
standard solutions like aqueous KCl since the determination of the cell
geometry and the distance between the electrodes in the closed state are
difficult. For the ionic conductivity measurement, the complex impedance
method is used to separate contributions of the sample bulk and electrode
interface.[196]
Ionic liquids possess comparable conductivities (∼10 mS cm-1) to
traditional organic solvents with added inorganic electrolytes but have
advantages over them as their conductivity is intrinsic. However, they are,
in general, significantly less conductive than concentrated aqueous
electrolytes (up to 350 mS cm-1).[197] The conductivity of a pure ionic
liquid depends on the mobility of the available charge carriers, which is
influenced by the ion size and ion association.
The correlation between the ionic liquid conductivity and the size and type
of the cation is such that increasing cation size tends to lead to lower
conductivity, most likely due to the lower mobility of the larger cations.
For the anions, in contrast, higher conductivities may be observed for
ionic liquids with larger anions such as [NTf2]- compared to those with
smaller anions such as [CH3CO2]-. More crucial than ion size seem to be
the interactions and co-ordination ability of the anions.
The conductivity can be, in fact, attributed almost directly to changes in
the viscosity.[208] Large size of constituent ions results in reduced ion
mobility and subsequently high viscosity and average conductivities.
58
Buzzeo et al. reported decreasing conductivity in the order of [EMIM]+ >
[RR’Pyrr]+ > [R4N]+ attributed to the decrease in the planarity of the
cationic core. A higher conductivity was ascribed to the flatness of the
imidazolium ring being an advantage compared to the tetrahedral nature
of the ammonium salts, with the pyrrolidinium cation having an inter-
mediate geometry.[210] At increased temperatures, the conductivity
increases and viscosity decreases due to increased ion
mobility.[166],[207]-[210]
According to the viscosity (chapter 2.2.2), the temperature dependence
of the ionic conductivity is generally depicted by an Arrhenius plot.
Typically ILs show upper convex curvature instead of a straight line in
the Arrhenius plot due to the complex interplay of short- and long-range
forces involved in molecule dissociation and ion motion.[199]
Thus, the temperature dependence of the ionic conductivity shows
Vogel-Fulcher-Tammann behavior (Equation 20).[198]
휎 = 퐴푒 Equation 20
where A, B and T0 are empirically derived constants. According to the
Arrhenius equation, their physical meaning may be deduced. T0 is the
Vogel temperature, at which the conductivity is supposed to drop to zero
(for T0 = 0 K Arrhenius behavior is obtained). The relation between T0 and
Tg is discussed in following section 2.4.3.4.2. A seems to represent the
maximum ionic conductivity if temperature were infinite and the
apparent activation energy for ionic conduction, EA, can be calculated
from B = EA / kB, where kB is the Boltzman constant.
2.4.3.4.2. Fragility
The deviations from Arrhenius behavior may be encompassed in terms
of dimensionsless quantity m, named fragility, that is a qualitative
concept used to study the behavior of glass forming liquids such as
59
ILs.[226] m is determined by Equation 21, taking T0 from the best fitting
of the VFT equation for conductivity measurements. The Tg values have
to be obtained experimentally by differential scanning calorimetry.
푚 =퐵
푙푛10 ∗푇
(푇 − 푇 ) Equation 21
In this connection, glass formers are classified between the extremes of
the “strong” (m ≈ 30) and “fragile” (m ≈ 250) behaviors. Fragile liquids
are those which undergo a rapid breakdown of their configurational
structure when temperature varies near to the glass transition tempe-
rature, Tg. On the other hand, structures of strong liquids do not change
much with temperature.[227] In this context, configurational structure is
associated with the nearest neighbour co-ordination number and the
intermediate range order lying between the first co-ordination shell and
the long range randomness that defines amorphousness. Fragile liquids
have large configurational heat capacities (exception: some hydrogen
bonded liquids), whereas strong liquids have small configurational heat
capacities. The most strong glass forming materials show the smallest
deviations from the Arrhenius law and vice versa. The nonlinearity of
the plot is determined by how close Tg is to T0.
The scaling parameter for temperature in the non-Arrhenius version of
the strong / fragile scheme is thus Tg and fragility is also a function of
. This ratio is related to the non-linear relaxation behavior observed in
the glass transition temperature range and glassy state. Thus, glassy
state behavior can be incorporated into the strong / fragile scheme by
means of the ratio conveniently lying between 0 (strong) and
1 (fragile).[227],[228] and m are equivalent measures of fragility, so that
works on ionic liquids by different authors may be compared easily.
Several pyridinium [BF4] ionic liquids investigated by Bandres et al.
represent ratios close to 1, thus being highly fragile.[229] Also Leys et
60
al. examined imidazolium-based [BF4]- ionic liquids and obtained m values
in the range 60 – 100 indicating relatively high fragilities.[230] The
authors find the trend of decreasing fragility with chain length of the
cation, since the van der Waals forces between the molecules increase
with chain length.
On the other side, the fragility of the ionic liquid [HMIM]Br is found to be
much larger than the fragility of the ionic liquids with fluorinated anions.
The authors explain this anomaly by the fact that the bromide ion is able
to form strong hydrogen bonds with the imidazolium cation. The
hydrogen-bonded network is thus responsible for higher fragility. The
fluorinated anions with negative charge delocalized over a much larger
volume have lower charge density. Their interaction with the imidazolium
cation is much weaker and hence, the IL exhibits lower fragility.[230]
2.4.3.4.3. Walden plot
The conductivity and viscosity of ionic liquids are often combined by a so-
called Walden’s rule (Equation 22)[219]
훬휂 = 푐표푛푠푡 Equation 22
where is the molar conductivity of the ionic liquid, and it is given by
Equation 23
훬 =휎훭휌
Equation 23
where M is the equivalent weight (molecular weight) of the ionic liquid and
is the ionic liquid density. Plotting the molar conductivity instead of
the absolute conductivity , normalizes the effects of molar concentration
and density on the conductivity and, thus, gives a better indication of the
number of mobile charge carriers in an ionic liquid. Ideally, the Walden
product remains constant for a given ionic liquid, regardless of
temperature. The Walden plot log versus log-1 shows better the
61
relationship between conductivity and viscosity. As has been observed
previously, the vast majority of the ionic liquids fall slightly below the ideal
1:1 Walden line (Figure 19),[220] thus being mainly dissociated. [BF4]- and
[PF6]- based ILs lie even on the line or above it due to high charge de-
localization in anions and their non-coordinating nature.
Figure 19: Walden plot of the protonated and methylated imidazolium-based ionic liquids.[220]
The Walden plot proved to be a useful tool in probing ion association in
aprotic ionic liquids. The deviation W from the ideal line indicates the
degree of ion association. For W > 1.0, less than 10 % of the ionic liquid
is dissociated. Say, the majority of ions are “locked” in zero-charged, non-
conductive pairs or clusters. Such neutral species can have the effect of
decreasing the viscosity of the whole medium.[221] The viscosity of a
given system is strongly dependent not only on temperature but also on
impurities, the most important of which is water, even at the low ppm
range (see also chapter 2.2.3).
2.4.3.4.4. Electrochemical double-layer in ionic liquids
Many techniques are nowadays being used for studying the double-layer
at the conductor / ionic liquid interface. Among them are STM (scanning
62
tunnelling microscopy), AFM (atomic force microscopy)[139]-[142] along with
various spectroscopic (Raman)[143],[144] and electrochemical (EIS)[145]-[159]
methods as well as Monte Carlo and molecular dynamics
simulations.[161]-[165] However, the development of a theory of the
molecular structure and behavior of the double-layer suffers from the lack
of systematic experimental data. Recently, an extensive review “Ionic
Liquids at Electrified Interfaces” by Fedorov and Kornyshev summarized
the state of the art.[160] The diversity of ILs with varying levels of purity
complicates further the problem. Results have been derived from different
techniques using single frequency measurements without taking into
account the frequency dispersion observed in the impedance
spectra[166]-[169] and also by fitting complete impedance spectra. In these
latter cases the spectra were fitted with different equivalent
circuits.[137],[145],[153],[171],[172] E.g. Pajkossy et al. proposed more
complicated circuits including double-layer capacitance components in
parallel with the adsorption resistance, Warburg (to describe diffusion-
controlled adsorption) and adsorption capacitance components, to model
the behavior of metallic Au electrodes in the presence of specifically
adsorbed anions.[156] On the other side groups of Locket and Silva used
simple R-CPE sequences for the data fitting.[137],[145],[150],[153] So far the
focus has been placed on measuring the differential double-layer
capacitance in ionic liquids at room temperature in dependence of the
electrode material (Hg,[137],[173] Pt,[137],[169],[171] glassy
carbon,[137],[145],[167],[172] Au,[167]-[169] and Bi[149]) and the potential.[140],[142]
The capacitance / potential curves that resulted had many different
shapes and are hard to interpret. Reliable data from many different
studies will be needed to develop an accurate model.[145]
2.4.3.4.4.1. Effect of temperature on the double-layer capacitance of ionic liquids
The temperature dependence of the double layer structure at an electrode
immersed in an ionic liquid is of great practical and theoretical interest
63
since it provides critical information necessary for the design of electric
double-layer capacitors. However, it has been studied only in few
publications.[137],[150],[170],[174]
For RTILs, the following temperature dependence of double-layer
capacitance has been observed by several authors:[137],[145],[150],[179] Zistler
et al. investigated [EMIM][DCA] and [EMIM][BF4] systems mixed with
[PMIM]I at Pt electrodes and showed an increase in double-layer
capacitance from 10 to 55 F cm-2 in the temperature range of 25 to
60 °C.[179] The capacitance values obtained by Silva et al. for the inter-
faces formed at a solid polycrystalline Pt, liquid Hg and semimetallic
glassy carbon in contact with [BMIM][PF6] within a temperature range
from 20 to 75 °C increased with T in the whole potential region
studied.[137] The capacitance values for the [BMIM][PF6] on Pt lies around
3 - 7 F cm-2. For [EMIM][NTf2], [BMIM][NTf2] and [HMIM][NTf2] on Hg
interfaces the double-layer capacitance increased with temperature with
Cdl of [EMIM][NTf2] on Hg from 14 to 15 F cm-2 at 20 to 60 °C.[150]
Lockett et al. analysed the double-layer properties of [EMIM]+, [BMIM]+
and [HMIM]+ chlorides on glassy carbon electrode in the temperature
range from 80 to 140 °C and later the same group carried out extensive
measurements on Au, Pt and GC electrodes in the ionic liquids [EMIM]+
and [HMIM]+ chlorides, [EMIM][BF4], [BMIM]+ halogenates, [BMIM][PF6],
[BMIM][NTf2], and [BMpyrr][NTf2] at temperatures between 20 and
100 °C.[145],[153] The obtained capacitance values were in the range of 7 to
35 F cm-2. The electrochemical behavior is, however, complicated as
shown by Drüschler et al.. The authors point to the fact that single
frequency measurements may be afflicted by artefacts in the form of
strong temperature dependence since there can be more than one
capacitive process with different temperature dependent relaxation times.
They suggest in particular that Cdl should be determined from broadband
impedance spectra, say 10-1 – 105 Hz, using equivalent circuit based
fitting. In their investigations on [BMpyrr][FAP] on Au(111) at
temperatures between 0 and 90 °C they found a) very weak temperature
64
dependence of the Cdl values and b) two capacitive processes with a
tendency rather to decrease with increasing temperature.[174] The
existence of two capacitive processes for Bi(111) in [EMIM]+, [BMIM]+ and
[BMpyr][BF4] as well as in [EMIM][FAP] and [EMIM][TCB] has been
observed by Siinor et al.[149],[175]-[178] The high-frequency (so-called
‘‘true’’) electrical double-layer differential capacitance, Cdl, and low-
frequency equilibrium adsorption capacitance, Cad, increase both with the
rise of T. It is difficult to avoid the specific dynamic ionic rearrangements
in media where only ions are present. Strong adsorption of both cations
and anions at various interfaces in ionic liquids has been deduced from
spectroscopic and microscopic measurements by means of e.g. high-
resolution in situ STM[139]-[142],[188] or SFG (sum frequency generation
vibrational spectroscopy).[189] Also an increase in differential capacitance
with increasing temperature was observed for high temperature molten
salts.[180] The positive temperature coefficient of Cdl is the opposite of
predictions by the classical double-layer theory which thus reveals in-
appropriate for the description of the double layer of RTILs at electrode
surfaces.[137],[145],[150],[179] In particular, the inadequacy of the Gouy-
Chapman model to describe the structure of the RTIL / electrode interface
has been pointed by Kornyshev on the basis of a local density
approximation dependent mean field theory.[181] The phenomenon of a
positive temperature coefficient of the differential capacitance has not yet
been adequately explained by classical modelling of the electrical double
layer. The role of electrostatic interactions in the highly concentrated ionic
media and the role of ion association are considered very important in this
context.[182] In particular, a multilayer model of the ionic arrangement at
the interface was suggested for molten salts. Cations and anions close to
the electrode are organized in several layers of alternating positive and
negative charge. Investigations in various ionic liquids showed that a
layered several nm thick structure is formed.[185]-[187] The excess charge
extends then for several layers deep into the bulk.[166],[171],[180] With in-
creasing temperature the electrostatic interactions between the ions and
65
ion association are supposed to decrease providing higher availability of
“free” ions which can get closer to the interface as a result of the
breakdown of complexes.[150],[183] This behavior is accelerated by
weakening of the hydrogen bonding between the anion and hydrogen from
the imidazolium ring with increasing temperature.[184]
2.4.3.4.4.2. Effect of ion size on the double-layer capacitance of ionic liquids
It was found that ion size influences the capacitance value of ionic liquids.
To determine the influence of individual ions a series of measurements
were made in different liquids in which one of the ions was fixed and the
other was varied. Locket et al. investigated the difference between
[BMIM]+ halogenate ILs.[153] The double-layer capacitance reveals highest
for the chloride IL followed by bromide and iodide. According to the
authors as the anion size increases the value of the differential
capacitance decreases. This trend was confirmed by calculations for media
without solvent.[193] In contrast to this, in aqueous solutions the solvation
energy contributes significantly to the strength of specific adsorption of
these ions.[194] Comparing the [BF4]-, [PF6]- and [NTf2]- ions in [BMIM]+
ILs suggested that [PF6]- is adsorbed more strongly than [BF4]- on Pt and
more strongly than [NTf2]- on Au.[153] The anions [BF4]-, [TCB]- and [FAP]-
in [EMIM]+ ILs were compared by Siinor et al.[176] Cdl weakly depends on
RTIL studied until there is no specific adsorption of anions (Figure 20).
Figure 20: Dependences of adsorption capacitance Cad (hollow symbols) and double-layer
capacitance Cdl (filled symbols) on electrode potential for Bi(111) in [EMIM][BF4] (triangles),
[EMIM][FAP] (squares) and [EMIM][TCB] (rhombs).[176]
66
The lower capacitance for [EMIM][FAP] can be explained by the larger
molar volume (larger diameter) of [FAP]- anion compared to [BF4]- or
[TCB]-,[176] and thus lower surface activity of [EMIM][FAP] indicating the
dependence of closest approach of anions onto the Bi(111) surface on the
anions chemical structure.[145]
On the cation side the influence of the hydrocarbon chain length on the
imidazolium was investigated by Locket et al. for imidazolium chlorides at
120 °C (Figure 21).[145]
Figure 21: Differential capacitance curves for glassy carbon in [RMIM][Cl] at 100 °C.[145]
It has been shown that, increasing the size of the cation decreases the
overall capacitance. For phosphonium-based ionic liquids with a common
anion on activated carbon electrode similar observations were made.[195]
This decrease in capacitance was suggested to be related to a decrease in
the area of the porous carbon that is accessible to larger cations. Siinor
et al. compared further [BMPyr]+, [EMIM]+ and [BMIM][BF4] on Bi(111) in
negative potential range in order to identify the impact of the cation
hydrocarbon chain.[175],[178]
67
Figure 22: Dependence of calculated double layer capacitance for Bi(111) in [BMIM][BF4] (filled
marks) and in [EMIM][BF4] (open marks) on T at temperatures, noted in figure. (left)[178] and
[BMpyr][BF4] (1)and [EMIM][BF4] (1’) (right). (2, 3 and 4 represent [BMpyr][BF4] solutions in
acetonitrile at different concentrations; irrelevant for the current work.)[175]
Cdl is found to be higher for the [EMIM][BF4] / Bi(111) interface compared
to the [BMIM][BF4] / Bi(111) interface at potentials higher -0.4 V (Figure
22 left)[178] and for the [BMpyr][BF4] / Bi(111) interface in the whole
potential range (Figure 22 right).[175] The authors explain their obser-
vations by the formation of a multilayer structure of electric double-layer
consisting of alternatingly charged layers, caused by the correlation of the
ions. Interestingly, [EMIM][BF4][191] and [BMpyr][BF4][190] ion pairs are
supposed to play a role, namely the [BF4]- does not form H-bonds with
hydrogen atoms of the pyridinium ring, but it does in contrast in the
[EMIM][BF4] ionic complex. From the DFT calculations this feature also
remains for the ion-pair–bismuth cluster complex. The authors draw
parallels to the viscosity behavior of the ILs which is dependent on the
cation structure. Although [BMpyr]+ and [BMIM]+ have almost equal molar
volumes, their RTILs have quite different viscosity due to defects in the
Coulomb lattice leading to more fluid ionic liquids.[192] Obviously, simple
ion pair correlation gives only a clue for the analysis of real structure at
metal / RTIL interface, however, chemical composition influencing the
physico-chemical properties of an IL also influences electric double-layer
formation / rearrangement kinetics.[175]
The measurements comparing [EMIM][BF4] and [BMIM][BF4] (Figure 22
left)[178] show that capacitance of [BMIM][BF4] is higher than that of
68
[EMIM][BF4] at more negative potentials, whereas it is the opposite way
around at potentials from 0 to -0.4 V. Thus, [BMIM]+ cations form a more
stable interfacial layer with co-adsorbed [BF4]- anions than [EMIM][BF4].
The authors ascribe the more constant capacitance values for [BMIM][BF4]
to a compact interfacial adsorption layer caused by the stronger van der
Waals interactions of the [BMIM]+ cations due to longer aliphatic tails
forming stable surface adsorption layers structures.[178]
69
3. Results and Discussions
3.1. Synthesis
3.1.1. Transesterification reaction of [Me(Me)PO3]-
The ionic liquid [EMIM][Me(Me)PO3] was used as starting material for the
synthesis of asymmetric metyl methylphosphonate esters. This
compound itself can readily be produced following literature procedures
by solvent-free methylation of ethyl imidazole using dimethyl methyl-
phosphonate at 100 °C (Scheme 24).[62]
Scheme 24: Synthesis of [EMIM][Me(Me)PO3] by solvent-free methylation of ethyl imidazole by
dimethyl methylphosphonate.
Taking advantage of the high nucleophilicity of [Me(Me)PO3]-, a SN2-type
substitution reaction was carried out with a variety of different,
functionalized alkylating agents R’–X (Table 3) as shown in Scheme 25.
Scheme 25: Alkylation of [Me(Me)PO3]- by a variety of different, functionalized alkylating agents
R’–X.
The synthesis of mixed and functionalized methylphosphonate esters of
the general type R’Me(Me)PO3 (R’ = functionalized alkyl group) was
carried out in most cases without solvent and the amount of
[EMIM][Me(Me)PO3] was slightly higher than equimolar in order to have
70
full conversion concerning the alkylating agent. The formation of the
alkyl methyl methylphosphonate esters was monitored by 31P and 1H
NMR. [Me(Me)PO3]- of the starting IL gives rise to a peak at 18–20 ppm
in the 31P NMR spectrum, whereas the ester product R’Me(Me)PO3
shows its peak in the 31P spectrum at 34–36 ppm. During the reaction
proceeding only the two species, starting anion and the asymmetric
ester are observed. No signs of disproportion, consecutive reactions or
rearrangements could be detected. Furthermore, the obtained
asymmetric esters are stable at ambient conditions and show no re-
arrangements like described for H-phosphonates in section 2.1.3.
Among the applied alkylation agents R’–X, the alkyl halides with longer
alkyl chains were less reactive than those with shorter ones. As
expected, the use of alkyl iodides and alkyl bromides resulted in faster
transformation compared to the reaction with alkyl chlorides. The
product was then isolated by extraction with diethyl ether or toluene.
The alkylating agents and product methylphosphonate esters are
depicted in Table 3. All esters (except entry 14) R’Me(Me)PO3 were
obtained after extraction solvent removal in a completely halide-free
quality (confirmed by Ag[NO3] test).[236]
The stoichiometric by-product represented a valuable IL material
consisting of [EMIM]+ and the leaving group of the alkylating agent as
anion. It could be isolated from the reaction mixture by crystallisation
after cooling in cases of Br- and I-. Thus, the reaction may be seen as
completely atom efficient.
71
Table 3: Alkylation of [EMIM][Me(Me)PO3] with different alkylating agents R’–X to form the
corresponding esters.
Entry R’–X R’Me(Me)PO3 Reaction Cond.:
T, t, Yield 1 1-bromobutane
BuMe(Me)PO3
65 °C 20 h 85 %
2 1-bromohexane
HexMe(Me)PO3
65 °C 25 h 83 %
3 1-bromooctane
OcMe(Me)PO3
70 °C 30 h 89 %
4 1-iodododecane
DodMe(Me)PO3
80 °C 40 h 92 %
5 2-methyl chloro-acetate
MeAcMe(Me)PO3
25 °C 12 h 74 %
6 2-bromoacetonitrile
AcNMe(Me)PO3
25 °C 5 h
42 %
7 2-chloro-acetophenone
(2-oxo-2-phenylethyl)Me(Me)PO3
25 °C 12 h 62 %
8 Cinnamylchloride
CinnamylMe(Me)PO3
25 °C 12 h 48 %
9 1-(chloromethyl)-4-vinylbenzene
4-vinylbenzylMe(Me)PO3
25 °C 12 h 43 %
10 Me(EG)1-benzenesulfonate
Me(EG)1Me(Me)PO3
25 °C 12 h 90 %
11 Me(EG)2-benzenesulfonate
Me(EG)2Me(Me)PO3
25 °C 12 h 91 %
72
12 Me(EG)3-benzenesulfonate
Me(EG)3Me(Me)PO3
25 °C 12 h 87 %
13 1,4-butane sultone
[EMIM][Me(Me)PO3BuSO3]
25 °C 24 h 95 %
14 2-bromoethylamine HBr
Pyridine
NH2EtMe(Me)PO3HBr/[Pyr]Br
P
OO
O
H2N
HN
Br
25 °C 24 h 89 %
Two very interesting examples are represented in entries 13 and 14. 1,4-
butane sultone was probed as well as alkylating agent according to the
reaction described in Scheme 22 (chapter 2.2.1.2.1). As result [EMIM]+
sulfonate IL was obtained carrying a methyl methylphosphonate group as
functionalization (Scheme 26).
Scheme 26: Alkylation of [Me(Me)PO3]- by 1,4-butane sultone.
The attempt to insert an amino group as functionalization lead to the
presumable product in entry 14. The reaction required a base since HBr of
2-bromoethylamine may protonate the methyl methylphosphonate anion
so that the nucleophilic activity got lost (Scheme 27).
73
Scheme 27: Protonation of the methyl methylphosphonate anion by HBr and subsequent loss of
the nucleophilic activity.
However, no neutral species were extractable by solvents like toluene. The
transesterification reaction proceeded only upon addition of pyridine
(Scheme 28).
Scheme 28: Transesterification reaction of methyl methylphosphonate anion after addition of
pyridine.
However, this solvent could not be removed completely after the end of
conversion. Rather, the amount of residual pyridine was equimolar to the
anion as indicated by 1H NMR spectra. Also, no neutral species could be
extracted. The product mixture consisted thus of the cations 1-ethyl-3-
methylimidazolium, pyridinium and the product [NH2EtMe(Me)PO3], which
was not extractable by toluene or diethylether. As anion served bromide.
The uptake of the complete reaction mixture in dichloromethane resulted
in precipitation of white solid. The 1H NMR analysis of the precipitate
showed no signals for [EMIM]+, only those for pyridinium and
functionalized methyl methylphosphonate in equimolar amounts. The
conclusion is, thus, that [EMIM][Br] was washed out by CH2Cl2. The ESI-
MS analysis of the final product showed pyridinium and 2-aminoethyl
methyl methylphosphonate ester as well as bromide, whose amount
74
could not be specified. The co-ordination situation of these three species
could not be identified in this work.
3.1.1.1. Kinetics of the transesterification reaction of phos-phonate anions with methyl chloroacetate
P-functionalized phosphonates may be used for alkylation reactions hence
providing a variety of functionalized anions. The transesterification
reaction was reported so far by Troev et al. only for the H-phosphonate
anions[14] and by our group for [Me(Me)PO3]-.[236] It is obvious that the P-
attached as well as O-attached residues influence the nucleophilicity of the
reactive oxygen in the phosphonate head group through inductive and
mesomeric effects.[66] The question arising now is: Which effects do these
residues have on the kinetics of the transesterification reaction of phos-
phonates?
To clarify this, kinetic experiments were conducted with a series of P- and
O-functionalized anions. Methyl chloroacetate (Table 3, entry 5) was
always used as alkylating agent for reasons of availability and appropriate
activity. To exclude influences of the ionic liquid’s cation on the reaction
kinetics, [EMIM]+ was always the cation of choice. To investigate the
effect of the P-connected residue, starting materials with anions depicted
in Figure 23 (II-IV) were used in the transesterification reaction. These ILs
were synthesized by ethylation of methylimidazole by corresponding
diethyl alkylphosphonates. Thus, the O-connected ethyl chain could be
kept constant. The effect of the O-connected residue was probed
comparing starting materials with [Me(Me)PO3]- and [Et(Me)PO3]- anions
(Figure 23; I-II). The P-connected methyl was kept constant in this case.
75
I [Me(Me)PO3]-
II [Et(Me)PO3]-
III [Et(Et)PO3]-
IV [Et(EtPh)PO3]-
Figure 23: Anions of the [EMIM]+ based ILs used to investigate the influence of P- and O-attached
residues on the kinetics of the transesterification reaction.
The reaction was assumed to be of second reaction order as it is a SN2
substitution reaction. The reactants were mixed in equimolar con-
centrations and the rate constant as well as activation energies were
calculated according to section 2.2.1.2.2. The reaction progress was
monitored by 31P NMR.
Figure 24 displays the conversion vs. time plot of the ester formation for
the ethyl ethylphosphonate anion (III) in the temperature range of 25 to
60 °C. The initial rate was determined from the data up to 40 %
conversion, since above this conversion the reaction conditions alter
due to lack of reactants.
76
Figure 24: Conversion vs time plot of the ester formation from ethyl ethylphosphonate anion and
methyl chloroacetate at different temperatures.
According to the assumption of second reaction order and [Et(Et)PO3]-
being the nucleophile,[ ]
was plotted vs. time (Figure 25).
Figure 25: Determination of the reaction order in respect to [Et(Et)PO3]-.
The plots are linear obeying Equation 4: y=kt+1, so that the second
reaction order can be seen as verified. Table 4 summarizes the rate
constants k, the intersection with the y-axis and the coefficient of
determination of the linearity of the plot at different temperatures.
77
Table 4: Rate constant k, intersection with the y-axis for the reaction of [Et(Et)PO3]- with methyl
chloroacetate at different temperatures.
T / °C k / min-1 y(t=0) R2 25 0.0011 1.01 0.994 40 0.0048 1.02 0.991 45 0.0083 1.01 0.994 50 0.0133 0.99 0.992 55 0.0195 1.00 0.997 60 0.0235 0.99 0.998
The same procedure was carried out for the anions depicted in Figure 23
and Arrhenius plots were obtained as shown in Figure 26. The activation
energies of the reactions and the prefactors were calculated according to
Equation 7 and the results are depicted in Table 5 in comparison to those
with [Et(Et)PO3]-.
Figure 26: Arrhenius plots for the transesterification of [Me(Me)PO3]-, [Et(Me)PO3]-, [Et(Et)PO3]-
and [Et(EtPh)PO3]- anions.
78
Table 5: Kinetic parameters for the transesterification of [Me(Me)PO3]-, [Et(Me)PO3]-, [Et(Et)PO3]-
and [Et(EtPh)PO3]- anions.
Anion Slope k0 / min-1 EA / kJ mol-1 [Me(Me)PO3]- -9.462 1.21E+11 78.67 [Et(Me)PO3]- -7.870 8.61E+08 65.43 [Et(Et)PO3]- -8.933 1.22E+10 74.27 [Et(EtPh)PO3]- -14.117 5.12E+16 117.37
The activation energy increases with increasing size of the residue
connected to phosphorous in the ethyl alkylphosphonate anion. This
observation is contra-intuitive at the first glance since larger alkyl
residues shoud provide larger inductive effects, thus increasing the
electron density of the reacting oxygen, and thus increasing the nucleo-
philicity. On the other side, steric effects may play an important role
and inhibit the electrophilic attack of the alkylating agent for anions
with larger residues like [Et(EtPh)PO3]-. Especially the presence of an
aromatic ring may compete to some extent with the oxygen for the
electrophile.
The influence of the O-connected chain of the alkyl alkylphosphonate
anion on the transestrerification rate may be encompassed by
comparing [Me(Me)PO3]- and [Et(Me)PO3]-. The slightly higher activation
energy of 78.67 kJ mol-1 found for IL with [Me(Me)PO3]- may be
explained by the lower electron density on the phosphonate group and
thus lower nucleophilicity. The ethyl group has higher inductive effect than
methyl so that the alkylation of [Et(Me)PO3]- requires less activation.
3.1.1.2. Probing other cations, anions and alkylating agents in the transesterification reaction
As outlined in chapter 2.2.1.2.1, several ester-based anions undergo SN2
substitution reactions. However, the reaction conditions, alkylating agents
and anions themselves determine the reaction pathway and products. In
this chapter, the alkylation reaction as described for methyl methylphosp-
79
phonate anions is compared to the transesterification under acid catalysis
as described for alkyl sulfate anions.
Leitner et al. described the participation of the [EMIM]+ cation in the
transesterification reaction of carboxylates through its acidic proton on C2
(Scheme 29), which is supposed to be promoted by CH2Cl2.[64]
Scheme 29: Participation of [EMIM]+ in the transesterification reaction of carboxylates through its
acidic proton on C2. The reaction is supposed to be promoted by CH2Cl2.[64]
To exclude the formation of byproducts influenced by the acidic C2-H in
the transesterification of methyl methylphosphonates, the reaction was
also probed with [BMPyrr][Me(Me)PO3]. As can be seen from the 31P NMR
spectrum in Figure 27, the reaction is exactly the same as for the [EMIM]+
based IL: Only two phosphonate species are present during the reaction,
the starting anion and the product ester without any intermediates or side
products. At the end of the conversion, only one product is present
without side- or consecutive reactions.
80
Figure 27: Proceeding of the transesterification of [BMPyrr][Me(Me)PO3] with methyl
chloroacetate.
A very interesting precursor represents the methyl phosphonate anion in
the ionic liquid [EMIM][Me(H)PO3]. The phosphonate chemistry described
in chapter 2.1 considers profoundly this species. The reaction of
[EMIM][Me(H)PO3] with methyl chloroacetate according to Scheme 30 was
probed.
N N PH
OO
O
Cl
O
O65 °C series of consecutive reactions,
mixture of productsCl- X
Scheme 30: Reaction of [EMIM][Me(H)PO3] with methyl chloroacetate.
Figure 28 shows the 31P NMR spectra of the [EMIM][Me(H)PO3] IL before
the reaction with methyl chloroacetate at 65 °C and of the reaction
mixture after 30 min and 2 d. Since the starting material was used as
received and without purification the collateral peaks at 5.1 and 2.8 ppm
were probably impurities and were not further considered. The high
reactivity of the starting materials revealed a mixture of products as well
81
as possible consecutive reactions, so that a simple SN2 reaction
mechanism cannot be assumed.
According to Table 1 (chapter 2.1.7), neutral H-phosphonate dimethyl and
diethyl esters, respectively, are expected in the range 9 – 13 ppm in 31P
NMR. They are clearly present after 30 min of reaction and disappear after
two days. The peaks at 2.2 and 2.9 ppm belonging to a kind of final
product were not further specified in this work.
Figure 28: 31P NMR spectra of the [EMIM][Me(H)PO3] IL before the reaction and of the reaction
mixture after 30 min and 2 d.
The transesterification under acid catalysis was probed for
[EMIM][Me(H)PO3] with methane sulfonic acid as catalyst and 1-butanol
(Scheme 31).
N N PH
OO
O
70 °C / series of consecutive reactions,intermediates
HHO
Scheme 31: Reaction of [EMIM][Me(H)PO3] with 1-butanol under acid catalysis.
82
The reaction proceeding at 70 °C is displayed in Figure 29. Pure
[EMIM][Me(H)PO3] exhibits in the 31P NMR spectrum a singlet at 3.9 ppm.
The peak shifts to 5.2 ppm upon mixing of the reactants due to solvent
effects. Similarly to the reaction with methyl chloroacetate, a mixture of
products can be detected after 6 h. However, all signals appear between
1 – 6 ppm and no signals can be found in the 9 – 13 ppm range (chapter
2.1.7). From this observation, one could assume that a replacement of the
methyl group at the anion takes place and no neutral species occur in
contrast to the alkylation reaction. Further, the alkylation seems to be far
more reactive than the acid catalyzed transesterification.
The final products of the transesterification of [EMIM][Me(H)PO3] with 1-
butanol under the acid catalysis were not further specified in the scope of
the current work.
Figure 29: 31P NMR spectra of the [EMIM][Me(H)PO3] IL with 1-butanol and acid catalyst at the
reaction start, 4 h and 6 h at 70 °C.
83
Further, the transesterification under acid catalysis as described for sulfate
anions was tried for [EMIM][Me(Me)PO3] and 1-butanol at 60 °C (Scheme
32).
N N P
OO
O
60 °C / no conversionHHO
Scheme 32: Reaction of [EMIM][Me(Me)PO3] with 1-butanol under acid catalysis.
Pure [EMIM][Me(Me)PO3] exhibits in the 31P NMR spectrum a singlet at
19.7 ppm. Figure 30 shows 31P NMR spectra for 0 and 40 min after
reaction start. As can be seen, the peak shifts to 23.5 ppm upon mixing of
the reactants, however no further conversion occurs.
Figure 30: 31P NMR spectra for the reaction of [EMIM][Me(Me)PO3] and 1-butanol at 60 °C under
acid catalysis at 0 and 40 min after reaction start.
Under the conditions of acid catalysed transesterification, dimethylphos-
phate was investigated (Scheme 33).
84
O
P
OO
O
N N 60 °C / HHO no conversion
Scheme 33: Reaction of [MMIM][Me2PO4] with 1-butanol under acid catalysis.
The results are depicted in Figure 31. Pure [MMIM][Me2PO4] exhibits in the 31P NMR spectrum a singlet at 2.2 ppm. Upon acid addition the peak shifts
to 0.9 ppm and after 1-butanol addition to 1.1 ppm. However no further
conversion occurs during 20 min at 60 °C.
Figure 31: 31P NMR spectra for the reaction of [MMIM][Me2PO4] and 1-butanol at 60 °C under acid
catalysis at 0 and 20 min after reaction start.
As outlined in chapter 3.1.1, [Me(Me)PO3]- reacts with alkylating agents
under formation of neutral phosphonate esters without catalyst. The
reaction of [MMIM][Me2PO4] with methyl chloroacetate was probed at
65 °C as depicted in Scheme 34.
O
P
OO
O
N N Cl
O
O65 °C
series of consecutive reactionsCl- X
Scheme 34: Reaction of [MMIM][Me2PO4] with methyl chloroacetate.
85
The results are shown in the 31P spectrum in Figure 32. Obviously, the di-
methyl phosphate anion reacts with 2-chloroacetate in several consecutive
reactions until the final product is formed. The signal of the starting
material at 2.2 ppm diminishes with time and new peaks arise and
disappear again in the upfield region until only one peak at -0.7 ppm is
present. This product as well as the intermediates, however, were not
isolated and further specified in the scope of this work. The item was only
to investigate if the reaction takes place. Unfortunately, the trialkyl phos-
phate esters as well as dialkyl phosphate anions exhibit chemical shifts in
the range -20 to 0 ppm in the 31P NMR spectrum, so that one cannot
distinguish if neutral species occur or if the anionic form is maintained.
However, the Ag[NO3] test of the reaction mixture shows free chloride
anions. If an other chloroalkyl was generated, the Ag[NO3] test would be
negative.
Figure 32: 31P NMR spectra for the reaction of [MMIM][Me2PO4] and methyl chloroacetate at 60 °C
during ca. 24 h after reaction start.
86
Possibly, a neutral dimethyl methylacetatophosphate is formed first and
then undergoes disproportionation reactions similarly to Scheme 8
(chapter 2.1.3).
Finally, the reaction of [EMIM][EtSO4] with methyl chloroacetate was
probed (Scheme 35).
Scheme 35: Reaction of [EMIM][EtSO4] with methyl chloroacetate.
Also in this case, free chloride could be detected by the Ag[NO3] test
indicating methyl chloroacetate conversion. The reaction was monitored
by 1H NMR in this case and the spectra are depicted in Figure 33 (signals
of imidazolium core not shown). The signals for methyl chloroacetate at
3.7 and 4.3 ppm diminish during 12 h at 60 °C, but no changes of the
ethyl sulfate ion (signals at 1.1 and 3.8 ppm) can be seen. Also, no new
species are observable. These findings are very difficult to interpret. One
speculation e.g. may be some kind of decomposition of methyl chloro-
acetate in presence of [EMIM][EtSO4] and evaporation of its fragments.
87
Figure 33: 1H NMR spectra for the reaction of [EMIM][EtSO4] and methyl chloroacetate (framed)
at 60 °C during ca. 12 h after reaction start.
Table 6 summarizes the observations of the reaction of ester-based ionic
liquid anions with 1-butanol under transesterification conditions and with
methyl chloroacetate under alkylating conditions. Very surprising is the
absence of reaction of [Me(Me)PO3]- and [Me2PO4]- anions with 1-butanol
under acid catalysis. On the other side, only [Me(Me)PO3]- reacts with
methyl chloroacetate under formation of clean easily isolatable product.
88
Table 6: Reactions of ester-based anions under acid catalytic conditions with alcohols and with
alkylating agents.
Anion Alkylating agent
alcohol methyl chloroacetate
with acid catalysis displacement of the O-connected group on the anion
formation of ROH[57]
free chloride occurs; positive Ag[NO3] test
reaction proceeding unclear
with metal catalysis replace-ment of the O-connected group on the anion
formation of ROH[63]
with acid catalysis no reaction
free chloride occurs; positive Ag[NO3] test
series of consecutive reactions
PH
OO
O
with acid catalysis series of
consecutive reactions, inter-mediates
free chloride occurs; positive Ag[NO3] test
series of consecutive reactions
no reaction
free chloride occurs; positive Ag[NO3] test
formation of clean and easily
isolatable neutral asymmetric dialkyl methylphosphonate ester
3.1.2. Synthesis and physico-chemical characterisation of methylphosphonate ILs
Using the alkylating activity of some commercially available symmetric
dialkyl alkylphosphonate esters as well as of some selected asymmetric
dialkyl methylphosphonate esters from Table 3, ionic liquids with the
functionalized group R’ attached to the anion were synthesized
according to Scheme 36 and are summarized in Table 7, entry 1-5.
Scheme 36: Synthesis of phosphonate-based ionic liquids.
89
As acceptors were used 1-methyl, 1-ethyl and 1-butyl imidazole as well
as N-butyl pyrrolidine. Any other amines or phosphines represent as
well possible acceptors. In this work, we focused on imidazolium and
pyrrolidinium derivatives (Table 7, entry 6) to ensure easier comparison
to the existing literature in the field of ionic liquids.
Table 7: Anions and cations of the MP-ILs of the current study.
Entry Chemical structures of the anions 1 [Et(Me)PO3]-
[Et(Et)PO3]-
[Et(EtPh)PO3]-
2 [Me(EG)1(Me)PO3]-
[Me(EG)2(Me)PO3]-
[Me(EG)3(Me)PO3]-
3 [Me(H)PO3]-
[Me(Me)PO3]-
[AcN(Me)PO3]-
[MeAc(Me)PO3]-
4 [HAc(Me)PO3]-
5 [Me(Me)PO3BuSO3]-
[(Me)PO3BuSO3]-
6 Chemical structures of the cations
[EMIM]+
[BMIM]+
[BMPyrr]+
90
It was found that the alkylating power of the phosphonate esters (Table 3,
chapter 3.1.1) decreases with increasing length of the alkyl residue for
symmetric as well as as for the asymmetric phosphonate esters. The
reaction conditions were adjusted accordingly to realise full conversion.
Whereas the methylation reaction by the AcNMe(Me)PO3 ester (Table 3,
entry 6) required acetonitrile as a solvent and 90 °C for 20 h, the
alkylation using the dodecyl methyl methylphosphonate ester required a
reaction temperature of 160 °C for three days under solventless
conditions.[236]
The phosphonate group of the anion [Me(Me)PO3BuSO3]- (Table 3, entry
13) exhibited as well methylating activity, so that upon reacting
[EMIM][Me(Me)PO3BuSO3] with 1-ethyl imidazole the ionic liquid
[EMIM]2[(Me)PO3BuSO3] containing a dianion was formed.
Unfortunately, the methyl methylphosphonate esters of entries 7 – 9 and
14 in Table 3 revealed unstable under alkylation conditions, so that no ILs
with the corresponding anions could be obtained. The reason for this
observation could not be further specified. Common to this series of
methyl methylphosphonates is the presence of an aromatic ring and a
double bond.
In contrast to functionalization carrying species, asymmetric methyl
methylphosphonate esters with aliphatic O-connected residues in Table 3
entry 2-4 reacted in a different way. The aliphatic alkyl groups were
transferred along with the methyl group since they exhibited similar
reactivity. As product, binary mixtures of two different ILs were obtained
according to Scheme 37. Since the methyl group exhibited considerably
higher reactivity, the distribution was typically around 90 : 10.[236]
91
Scheme 37: Binary mixtures of two different ILs as result of using aliphatic asymmetric methyl
methylphosphonate esters as alkylating agents.
The obtained binary IL mixtures are well defined and may find applications
as mixtures e.g. in tribology applications. From a synthetic perspective,
however, pure R’ methylphosphonate ionic liquids are interesting. This
requires a selective transfer of the alkyl residue which can be realised if
the two alkyl ester groups are very different in their reactivity or if the two
residues are the same.
Following the latter approach, the binary IL mixtures obtained in the first
alkylating step were subjected to another SN2 substitution reaction
(Scheme 38).
Scheme 38: SN2 substitution reaction of the binary IL mixtures obtained in the first alkylating
step.
The so obtained neutral phosphonate esters (in relation 90 : 10) were
separated by distillation (T = 160 °C, p = 1 bar) to yield the pure
alkylating agent R’2(Me)PO3. Obviously, by using R’2(Me)PO3 in the
quaternization of different amines, pure ionic liquids were obtained.
92
However, the latter carry the alkyl group functionality R’ in both the cation
and the anion (Scheme 38).[236] The so obtained ILs are shown in Table 8
and their physicochemical properties are discussed in the following. The
obtained data are interpreted in connection with the intermolecular inter-
actions present in ILs and the respective thermophysical properties are
compared to the available literature.
Table 8: Phosphonate ionic liquids of type [R‘MIM][R’(R)PO3].
Structure of the component ions Reaction Cond.:
T, t, Yield
H2O content /
ppm
Tg / °C
Tdec / °C
[BMIM][Bu(Me)PO3]
130 °C 2 d
94 %
2345 -73 253
[OMIM][Oc(Me)PO3]
150 °C 3 d
92 %
2771 -60 258
[OMIM][Oc(Ph)PO3]
150 °C 3 d
99 %
1972 -61 281
[DodMIM][Dod(Me)PO3]
160 °C 3 d
98 %
n.a. solid
Tm: 60 °C
261
[Me(EG)3MIM][Me(EG)3(Me)PO3]
140 °C 3 d
92 %
8280 -70 243
93
The ionic liquids presented in this study are liquids at room temperature
and exhibit glass transitions in the range -50 to -70 °C upon cooling.
Exceptions are the solids NH2EtMe(Me)PO3HBr/[Pyr][Br] in Table 3 entry
14 and [DodMIM][Dod(Me)PO3] in Table 8, which possess melting points
of 119 and 60 °C, respectively.[236]
The ILs prepared in this study are characterised by reasonable thermal
stabilities (TGA, chapter 6.1.4) with values between 216 and 281 °C. The
same temperature range was recently reported also for the short-chain,
non-functionalized phosphonate ILs.[61],[62]
Very remarkably, the hydrolytic stability of the investigated phosphonate
ionic liquids was found to be very high. This property was probed by
heating for 5 h in excess water at 100 °C (reflux) for ILs in Table 8 as well
as for ILs in Table 7 entry 3 and should be seen as transferrable to the
other species. No changes in the NMR spectra were observed for all ILs
except for [EMIM][MeAc(Me)PO3]. In this latter case, there was also no
hydrolytic reaction at the phosphorous observed but cleavage of the
carboxylic ester to form the corresponding acid functionality (Scheme
39).[236] In the product anion [HAc(Me)PO3] the proton may be
delocalized between the phosphonate and carbonate groups.
Scheme 39: Delocalization of the proton between the phosphonate and carbonate groups.
The signal for the methyl group at 3.56 ppm in the 1H NMR spectrum
disappeared upon refluxing in water and the signal for phosphorous
shifted downfield from 19 to 25 ppm. However, this step may be
regarded as further functionalization possibility (Table 7, entry 4). The
free carboxylic acid group is now disposable for coupling reactions like
esterification or amidation or may be deprotonated to provide a dianion.
94
The experimental density data obtained for the temperatures 293.15 and
298.15 K at atmospheric pressure are summarized in Table 9. For the di-
anion containing ionic liquid [EMIM]2[(Me)PO3BuSO3] and the hydrolysis
product [EMIM][HAc(Me)PO3] no density was determined for the reasons
of very high viscosity and subsequent filling problems of the densitymeter.
Table 9: Density values of phosphonate-based ILs at 293.15 and 298.15 K.
Entry Ionic liquid / g cm-3 293.15 K 298.15 K
1 [EMIM][Me(H)PO3] 1.200 1.196 2 [EMIM][Me(Me)PO3] 1.170 1.167 3 [EMIM][Et(Me)PO3] 1.138 1.134 4 [EMIM][Et(Et)PO3] 1.118 1.121 5 [EMIM][Et(EtPh)PO3] 1.141 1.139 6 [EMIM][MeAc(Me)PO3] 1.230 1.226 7 [EMIM][AcN(Me)PO3] 1.210 1.207 8 [EMIM][Me(EG)1(Me)PO3] 1.160 1.156 9 [BMIM][Me(EG)1(Me)PO3] 1.116 1.112 10 [BMPyrr][Me(EG)1(Me)PO3] 1.092 1.088 11 [EMIM][Me(EG)2(Me)PO3] 1.162 1.158 12 [EMIM][Me(EG)3(Me)PO3] 1.150 1.146 13 [EMIM][Me(Me)PO3BuSO3] 1.286 1.283 14 [BMIM][Bu(Me)PO3] 1.075 1.072 15 [OMIM][Oc(Me)PO3] 0.997 0.993 16 [OMIM][Oc(Ph)PO3] 1.023 1.019 17 [Me(EG)3MIM][Me(EG)3(Me)PO3] 1.169 1.165
In accordance to the trends described in chapter 2.2.2, the density
decreases with longer alkyl substituents on the cation as can be seen in
Table 9, entry 8-10. Very low densities show ILs with large asymmetric
ions in entries 14-16. PEG funcionalized species, although large molecules,
exhibit relatively high densities of around 1.16 g cm-3. Similar findings
were made for functionalized alkylsulfate and phosphate ionic
liquids.[57],[59]
The results for the dynamic viscosity at temperatures between 293.15 and
333.15 K at atmospheric pressure are shown in Figure 34 a-e. The lowest
viscosity exhibits the ionic iquid [EMIM][Me(H)PO3]. The experimentally
obtained data in this work can be compared to the measurements carried
out by Hasse et al. (Figure 35). The data deviation lies around 16 % for
95
the value at 20 °C. The reason for this deviation may lie in different
qualities of used ILs regarding the impurities and water content.
a
b
c
d
e
Figure 34: Dynamic viscosity of phosphonate-based ionic liquids at temperatures between 293.15
and 333.15 K at atmospheric pressure.
96
Figure 35: Comparison of the viscosity values of [EMIM][Me(H)PO3] obtained in this work with
that reported by Hasse et al. in Ref. [86].
The viscosity data demonstrate a strong influence of the functionalization.
In Figure 34 a [EMIM]+ based MP-ILs with polar functionalities are
compared. The presence of cyano or ester functionalities leads to viscosity
increase. The anion [Me(Me)PO3BuSO3]- provides the IL with highest
viscosity in this group. Figure 34 b shows viscosities of ILs from Table 8
carrying the same functionality in the cation and anion. The viscosity
increases with alkyl chain length as expected. In contrast to aliphatic
chains, the PEG residues lead to significant viscosity decrease, so that the
ionic liquid [Me(EG)3MIM][Me(EG)3(Me)PO3] exhibits even lower viscosity
than [EMIM][Me(Me)PO3]. Further PEG functionalized ILs are displayed in
Figure 34 c. The longer the ethylene glycol residue, the lower the viscosity
for ILs keeping the cation constant. The cation variation leads as expected
to a viscosity increase according to [EMIM]+ < [BMPyrr]+ < [BMIM]+. The
reason for the higher viscosity of [BMIM]+ in comparison to [BMPyrr]+
based MP-ILs with the same anion can be explained by the evident ability
of the methylphosphonate anions to act as hydrogen bonds acceptor.
Imidazolium cations are better H-bond donors than pyrrolidinium.
Finally, Figure 34 d and e show the dramatic viscosity difference between
the MP-anions and MP-dianions for sulfonate-based ILs
[EMIM][Me(Me)PO3BuSO3] and [EMIM]2[(Me)PO3BuSO3] as well as for ILs
97
[EMIM][MeAc(Me)PO3] and [EMIM][HAc(Me)PO3]. Due to the double
negative charge on one species, it interacts with two cations and makes
larger and stronger clusters and agglomerates. The ionic motion in
solution becomes thus more hindered. A limited number of studies
exists for ILs with doubly charged ions, predominantly dications.[234] In
comparison, dicationic ether functionalized [NTf2]- ILs exhibit viscosities
of around 7 Pa s at room temperature.[235]
3.2. Cellulose based MP ionogels
As described in chapter 2.3.1, phosphonate-based ILs are able to dissolve
carbohydrates, such as cellulose or chitosan. Furthermore, the special
impact on the biopolymer dissolution of the oxygen carrying groups like
polyethylene glycol (PEG) functionalities is also mentioned. For these
reasons, the [EMIM]+ based ILs with anions consisting of methyl methyl-
phosphonate headgroups and a PEG residue (Table 9, entry 8 in chapter
3.1.2) as well as with a methyl acetate residue (Table 9, entry 6 in
chapter 3.1.2) were used for ionogel synthesis. As carbohydrate material
microcrystalline cellulose was applied (Figure 36).
Figure 36: Light microscope picture of microcrystalline cellulose.
In comparison to the MP-based ILs, the dissolution of cellulose in
[EMIM][Ac] was also probed. A 4 wt% mixture of cellulose in the IL
exhibited distinct turbidity, thus indicating fine dispersion in contrast to
real dissolution. As can be seen from Figure 37 a, crystalline structure of
100 m
98
the cellulose is maintained. On the other hand, dissolving 5 wt% of
cellulose in [EMIM][MeAc(Me)PO3] (Figure 37 b) results in nearly complete
disappearance of crystals, so that the mixture is not only visually
transparent, but also on the microscopic level. Mixing
[EMIM][MeAc(Me)PO3] with 20 wt% of cellulose provides as well visually
clear material, the microscopic investigation reveals, however, residual
cellulose crystals (Figure 37 c). The saturation situation depends in this
case, however, not only on the solute / solvent ratio, but also on the
strong viscosity increase of the solution, which inhibits dissolving of
further carbohydrate amounts. Whereas 10 wt% ionogel still shows some
fluidity (not measurable with rheometer), the 15 wt% ionogels are at
hand as free-standing membranes. The initial viscosity of the solvent IL
plays here a significant role. The much lower viscous
[EMIM][Me(EG)1(Me)PO3] shows an uptake of 30 wt% of cellulose (Figure
37 d) showing less residual particles than in Figure 37 c.
99
a
b
c
d
Figure 37: Light microscope pictures of a) 4 wt% cellulose in [EMIM][Ac], b) 5 wt% cellulose in
[EMIM][MeAc(Me)PO3], c) 20 wt% cellulose in [EMIM][MeAc(Me)PO3], d) 30 wt% cellulose in
[EMIM][Me(EG)1(Me)PO3].
The thermal stability of the cellulose ionogels is slightly lower than that of
the pure IL as was observed for [EMIM][Me(EG)3(Me)PO3] and the
corresponding 5 wt% cellulose ionogel in Figure 38 above. The values are
266 °C for the pure IL and 251 °C for the ionogel.
100 m 100 m
100 m 100 m
100
Figure 38: TGA (above) and DSC (below) analyses of pure [EMIM][Me(EG)3(Me)PO3] IL and the
5 wt% cellulose ionogel.
A glass transition was detected by DSC for the MP-IL / cellulose mixtures
of 5 wt% allowing classifying them as glass formers. For this amount of
carbohydrate, the Tg value of the ionogel (-68 °C) was nearly the same as
for the pure IL (-71 °C) (Figure 38 below). The fragility analysis for the
ionogels will follow in chapter 3.3.5.1.
As outlined in chapter 2.3.1, humidity inhibits the IL / cellulose inter-
actions. Storing the MP ionogels under ambient conditions leads to IL
leaching. Thus, originally dry ionogel segregates IL. Finally, the cellulose
pure [EMIM][Me(EG)3(Me)PO3]
5 wt% cellulose ionogel
pure [EMIM][Me(EG)3(Me)PO3]
5 wt% cellulose ionogel
101
can be regenerated from the ionogels by water. Its microcrystalline nature
is then, however, lost and the carbohydrate is obtained as a blend (Figure
39).
Figure 39: Light microscope picture of cellulose regenerated from the ionogel with
[EMIM][Me(EG)1(Me)PO3].
Besides cellulose, MP-based ILs are able to dissolve a series of other
carbohydrates, in particular chitosan, agarose, -cyclodextrin and
2-hydroxypropyl--cyclodextrin. A new class of ionogels based on micro-
cellulose and several methylphosphonate ionic liquids from Table 7 (in
chapter 3.1.2) were synthesized and investigated for their electrochemical
properties (chapter 3.3) in course of this work. As they show promising
properties for application in electronic devices like low voltage inorganic
and organic electrolyte-gated field-effect transistors (FETs), these
substances were transferred to Nanomaterials for Optoelectronics (NMOE)
research group, University Erlangen-Nuremberg, for further investi-
gations.[237]
100 m
102
Figure 40: Optical image of a free-standing ionogel based on microcellulose and
[EMIM][Me(EG)2(Me)PO3]; Optical image of ZnO nanorod FET on multilayer-coated paper with
laminated ionogel during inward bending with a bending radius of 1.1 mm.[237]
Figure 40 displays the free-standing ionogel based on microcellulose and
[EMIM][Me(EG)2(Me)PO3] as well as a ZnO nanorod FET on multilayer-
coated paper. So obtained materials match the performance of more
commonly used ionogels based on synthetic copolymers in terms of
specific capacitance and response time, while being biofriendly and
derived from cheap and renewable resources. Ionogels based on anion
mixtures of [NTf2]- and methylphosphonates are well-suited for side-gated
FETs due to their higher ionic mobility while ionogels with lower specific
capacitances result in higher electron mobilities in spray-coated ZnO FETs.
Due to their features like transparency, flexibility, transferability, high
capacitance and easy processing, they may also find applications in paper-
based supercapacitors or actuators.[237]
3.3. Electrochemistry of MP derived materials
The aim of this work is the investigation of the electrochemical properties
like electrochemical window (EW), conductivity and double-layer
capacitance of phosphonate ionic liquids represented in Table 10. The ILs,
that were used for the ionogel synthesis are marked with the asterisk. The
structures of the component ions are displayed in section 3.1.2.
103
Table 10: Ionic liquids used for electrochemical investigations.
Entry Ionic Liquid 1 [EMIM][Me(H)PO3] 2 [EMIM][Me(Me)PO3] 3 [EMIM][AcN(Me)PO3] 4 [EMIM][MeAc(Me)PO3]* 5 [EMIM][Me(EG)3(Me)PO3]* 6 [EMIM][Me(EG)2(Me)PO3] 7 [EMIM][Me(EG)1(Me)PO3]* 8 [BMIM][Me(EG)1(Me)PO3] 9 [BMPyrr][Me(EG)1(Me)PO3]
The features considered in particular are:
- Influence of the anion functionalization on electrochemical
properties keeping the cation [EMIM]+ constant for ILs in Table 10,
entries 1 – 7.
- Influence of the cation keeping the anion [Me(EG)1(Me)PO3]- con-
stant for ILs in Table 10, entries 7 – 9.
- Comparison of pure ILs with 5 wt% cellulose ionogels for the ILs
marked with the asterisk.
Influence of temperature on the electrochemical properties of phos-
phonate ILs and ionogels.
3.3.1. Cyclic voltammetry of MP-ILs
To study correctly the formation / rearrangement of the electric double-
layer, experimental data must be collected within a narrow potential
range with no intensive perturbations, since they are greatly affected by
faradaic or other oxidation / reduction pseudo-processes. Therefore
cyclic voltammograms (CVs) were recorded for ionic liquids presented in
Table 10 in order to determine the electrochemical windows. In addition,
CV measurements provide an ionic liquid purity test. Within the
potential range, where the current density is small, the electrode is
considered as nearly ideally polarisable and any current is non-faradaic,
say entirely due to electric double-layer charging / adsorption.
Practically, there are always ‘‘trace redox’’ processes in real systems
104
and such electrodes as Au, Pt and GC can be only regarded as close to
‘‘ideally polarisable’’.[125]
The ‘‘cut-off’’ current density for the determination of electrochemical
windows for ionic liquids was suggested as 1 mA cm-2 [196] and thus used
in this work. Oxidation of anions establishes the positive limit and
reduction of cations the negative limit of the electrochemical windows.
3.3.1.1. Electrochemical windows of MP-ILs
For pure MP-ILs in Table 10 the CVs were compared as displayed in
Figure 41. In general, the ‘‘redox’’ processes due to trace
contaminations and water (up to 0.7 wt%) are especially seen in the
anodic part. They are characteristic for the phosphonate family and
could not be removed even after prolonged drying in vacuum. At the
open circuit potential they are below the ‘‘cut-off’’ current density and
thus neglected.
Figure 41 a represents selected [EMIM]+ MP-ILs in comparison. Very
important is, that in the range between -0.5 to 0.5 V only polarization
currents occur. This is especially crucial for the impedance
measurements, which are conducted at open circuit potential (ocp), say
without imposing voltage externally.
At higher voltages the ions undergo reduction / oxidation at the
electrodes. The reductive behavior of [EMIM]+ is independent on the MP
anion within the MP-ILs. The methylphosphonate anions show relatively
high oxidative stability. However, exact oxidative limits may be difficult
to determine since the transport rates of the oxidation products away
from the electrode are correlated to the diffusive effects within the IL.
The accumulation of the decomposition products near the electrode and
the passivation effects are then responsible for supposed higher anion
stability.
105
Figure 41 b displays CVs of [Me(EG)1(Me)PO3]--ILs with different
cations. According to literature,[205]-[208] the stability of the cations
increases in the manner [EMIM]+ < [BMIM]+ < [BMPyrr]+. The oxidative
behavior of [Me(EG)1(Me)PO3]- anion is independent on the cation within
the [Me(EG)1(Me)PO3]--ILs.
Figure 41: Effect of the a) anion and b) cation functionalization on the electrochemical window of
MP-ILs measured at 293 K. Each scan was taken at 10 mV s-1
3.3.1.2. Effect of cellulose addition on the electrochemical window
To study the effect of cellulose addition on the EW, pure MP-ILs
(marked with asterisk in Table 10) and the corresponding 5 wt%
cellulose ionogels were compared. As can be seen in Figure 42, cellulose
addition does not alter the EW dramatically, although the viscosity
increases considerably upon gelation.
a)
b)
106
Figure 42: Effect of gelation on the electrochemical windows of a) pure [EMIM][Me(EG)1(Me)PO3]
and 5 wt% cellulose ionogel, b) pure [EMIM][Me(EG)3(Me)PO3] and 5 wt% cellulose ionogel,
c) pure [EMIM][MeAc(Me)PO3] and 5 wt% cellulose ionogel.
3.3.1.3. Effect of temperature on MP-IL and ionogel electrochemical windows
The effect of temperature on the EW has been studied for pure
[EMIM][Me(EG)1(Me)PO3] and the corresponding 5 wt% cellulose
ionogel. The results are shown in Figure 43. It is evident that with
a)
b)
c)
107
increasing temperature a contraction of the EW occurs. For the pure
ionic liquid as well as for the ionogel a contraction of up to 1 V is
observed upon temperature increase from 10 to 60 °C. The origin of
these contractions lies on one side in viscosity decrease and subsequent
mobility of ions and decomposition products in the vicinity of the
electrode and on the other side in the effect on the kinetics of the redox
processes and subsequent chemical reactions, which effectively define
the potential limits in a voltammetric scan. Such shrinking of the
electrochemical window has been reported by several authors. In
particular by Locket et al.[145] for imidazolium based chlorides and by
Best et al.[201] for the bis(fluorosulfonyl)imide [FSI]- based ILs. At
highest temperature the EW of the ionogel extends from -0.5 V to
0.5 V, which still encloses the open circuit potential at which the
impedance studies were carried out.
Figure 43: Effect of increasing temperature on the electrochemical window of
a) [EMIM][Me(EG)1(Me)PO3], b) [EMIM][Me(EG)1(Me)PO3] 5 wt% cellulose ionogel.
a)
b)
108
3.3.2. Temperature dependent impedance measurements
3.3.2.1. Temperature dependent impedance measurements of [EMIM][NTf2]
The electrochemical properties like conductivity and capacitance of MP
ionic liquids were determined from one impedance measurement over a
frequency range from 1 to 106 Hz. The temperature dependence investi-
gations were carried out in the temperature range from 6 to 75 °C.
Figure 44 displays temperature dependent impedance data of
[EMIM][NTf2], which was used as reference system. First of all, the
capacitive behavior can be seen, which is characteristic for ionic liquids.
The Nyquist plot in Figure 44 a represents straight, tilted lines. Due to
this non-ideal behavior the constant phase element (CPE) instead of an
ideal capacitor was used for the fitting procedure according to section
2.4.3.2.1. In the Bode representations (Figure 44 b) only one plateau –
for RS – is present and the phase angle stays constant after having
reached a value < 90°. All these observations indicate the absence of
charge transfer under measurement conditions. The points below the
zero line in the Nyquist plot are identified as high frequency artefacts
derived from cable inductances and other perturbances. They were not
involved in the fitting procedure.
Rising temperature shifts the intercept with the Z’ axis in the Nyquist
plot to the left. RS gets lower and the conductivity increases. The
increase of the inclination of the plots is reflected in the decreasing
values of the CPE and indicates further processes taking place at the
interface. In the Cole-Cole plot (Figure 44 c), these slow processes like re-
orientations or adsorption of ions at the electrode,[132],[133] can be seen at
lower frequences, where a new increase of C” and C’ occurs. Apart from
that, the Cole-Cole plot shows nearly ideal semicircles, which are hardly
109
dependent on temperature. Also in the capacitance vs. frequency plot
(Figure 44 d), the graphs of all temperatures converge to nearly one
value, indicating very low temperature dependence of the double-layer
capacitance.
Figure 44: Temperature dependent impedance measurements on [EMIM][NTf2]. Arrows indicate
temperature increase. a) Nyquist plot, b) Bode plot, c) Cole-Cole plot, d) |C| vs. plot.
110
3.3.2.2. Temperature dependent impedance measurements of [EMIM][Me(EG)1(Me)PO3]
Figure 45 displays temperature dependent broadband impedance data of
[EMIM][Me(EG)1(Me)PO3]. The patterns in Nyquist (Figure 45 a) and
Bode (Figure 45 b) plots are represantative for all functionalized MP-ILs
studied in this work. The fitting of the data points was carried out in the
same manner as for [EMIM][NTf2]. However, not only the high
frequency points below the zero line were excluded, but also those of the
parasitic loops. In contrast to [EMIM][NTf2] and non-functionalized phos-
phonates, the MP-ILs in Table 10, entry 3-9, exhibit such inductive loops
at low temperatures. Upon temperature increase, the loops regress and
disappear finally above ambient temperature. As described in detail in
section 2.4.3.3, low conductivity media are rather prone to this
phenomenon. As the resistivity of the solution increases, these artefacts
may appear even at much lower frequencies. For this reason, single
frequency measurements, that may be afflicted by artefacts in the form of
strong temperature dependences, were in any case avoided.
The semicircles in the Cole-Cole plot (Figure 45 c) and the capacitance vs.
frequency plot (Figure 45 d) show similarly to [EMIM][NTf2] very weak
dependence of the double-layer capacitance on temperature for the MP-
ILs (deviation within 0.5*10-9 F). In contrast to [EMIM][NTf2], however,
one observes in these raw data a kind of decrease of double-layer
capacitance with increasing temperature. This behaviour will be
reflected and discussed in Figure 50 in chapter 3.3.3.4. Furthermore, it
can be stated, that only one process, namely the double-layer formation,
is observed in the chosen frequency range. This fast capacitive process
includes all interfacial charge redistributions and is determined by the
activation energy required for the charge transport in the bulk of the IL.
Slower processes at low frequences become noticeable only at higher
temperatures.[158]
111
Figure 45: Temperature dependent impedance measurements on [EMIM][Me(EG)1(Me)PO3].
Arrows indicate temperature increase. a) Nyquist plot, b) Bode plot, c) Cole-Cole plot, d) |C| vs.
plot.
112
3.3.2.3. Temperature dependent impedance measurements of MP-based ionogels
Figure 46 displays temperature dependent impedance data of the
[EMIM][Me(EG)1(Me)PO3] 5 wt% cellulose ionogel. The parasitic loops in
the Nyquist plot (Figure 46 a) are more pronounced than in the pure IL
and shifted to lower frequences. They comprise more data points, so that
less points are available for fitting. Also in ionogels, the loops regress and
disappear upon temperature increase. It can be estimated already at
this stage, that ionogels represent highly resistive media. The same
behavior is found also for [EMIM][Me(EG)3(Me)PO3] and
[EMIM][MeAc(Me)PO3] ionogels in comparison to the respective pure
ionic liquids.
The semicircles in the Cole-Cole plot (Figure 46 c) and the capacitance vs.
frequency plot (Figure 46 d) show similar to [EMIM][NTf2] and MP-ILs
very weak dependence of the double-layer capacitance on temperature
(deviation within 0.5*10-9 F).
113
Figure 46: Temperature dependent impedance measurements on [EMIM][Me(EG)1(Me)PO3]
5 wt% cellulose ionogel. Arrows indicate temperature increase. a) Nyquist plot, b) Bode plot,
c) Cole-Cole plot, d) |C| vs. plot.
114
3.3.3. Influence of anion functionalization on impedance measurements
Figure 47 a and b shows the comparison of [EMIM][NTf2] and [EMIM]+
based MP-ILs (Table 10, entry 2 – 9) at 24 °C in the Nyquist and Bode
plots, respectively. Yet from this diagram the conductivity behavior can be
estimated: [EMIM][NTf2] is by far higher conductive than the [EMIM]+
based MP-ILs.
The semicircles in the Cole-Cole plot (Figure 47 c) let estimate lower
double-layer capacitance of [EMIM][NTf2] in comparison to the [EMIM]+
based MP-ILs.
Figure 47: Comparison of impedance measurements on [EMIM][NTf2] (arrow) and [EMIM]MP-ILs
at 24 °C. a) Nyquist plot, b) Bode plot, c) Cole-Cole plot.
3.3.3.1. Conductivity of [EMIM]+ based MP-ILs
In Figure 48, an overview of the conductivity values obtained for [EMIM]+
based MP-ILs as a function of inverse temperature is shown. The values of
are significantly lower than for [EMIM][NTf2] and lie below 1 mS cm-1 at
ambient temperature (Table 11). The conductivity decreases further with
115
respective chain functionalization. Being related to the mobility of the
charge carriers, this can be explained by the higher viscosity induced by
the stronger intermolecular interactions. Hydrogen bonds can be easily
formed between the imidazolium cations and phosphonate anions
providing charge-neutral ion pairs or quadrupols and / or charged clusters
of very high molecular weight. Furthermore, functionalization results in
larger anion size and consequently in larger volume fraction of the anion.
Figure 48: Conductivity values and VFT fit of [EMIM]+ based MP-ILs as a function of the inverse
temperature in comparison to [EMIM][NTf2].
From the significant deviation from linearity in Figure 48 the VFT behavior,
which is characteristic for glass-forming liquids is evident. Therefore the
conductivity data were fitted to the Equation 20. The results of the fitting
are summarized in Table 11. Unfortunately, the fitting parameters cannot
be linked to the chemical structure of the ions.
116
Table 11: Conductivity (at 24 °C), glass transition temperature and fitting parameters of the VFT
equation for the studied ILs.
Entry Ionic Liquid / mS cm-1 @24 °C
A B T0 / K
1 [EMIM][Me(H)PO3] 2.63 3820 1221 129 2 [EMIM][Me(Me)PO3] 0.86 2974 1149 155 3 [EMIM][AcN(Me)PO3] 0.47 5929 1350 154 4 [EMIM][MeAc(Me)PO3] 0.30 1686 1079 172 5 [EMIM][Me(EG)3(Me)PO3] 0.35 2361 1526 124 6 [EMIM][Me(EG)2(Me)PO3] 0.41 1598 1252 146 7 [EMIM][Me(EG)1(Me)PO3] 0.51 10618 1863 110 8 [BMIM][Me(EG)1(Me)PO3] 0.16 4782 1790 123 9 [BMPyrr][Me(EG)1(Me)PO3] 0.23 9616 2048 105 10 [EMIM][NTf2] 8.16 362 546 153
3.3.3.2. Fragility analysis of MP-ILs
The glass transition onset points and corresponding ratios and fragility
parameters m are collected in Table 12. At first glance, the difference
between the T0 and Tg temperatures indicates that phosphonate ILs
represent relatively strong liquids. Values of the ratios are in the range
of 0.5 – 0.8 and values of m are < 50. The correlation of fragility and
ionic liquid structure is difficult to establish. Although able to build
hydrogen bonds, the fragility of the MP-ILs is not only considerably lower
than that of [HMIM]Br, but also even lower than of [BF4]- based
systems.[229],[230] For the IL [EMIM][NTf2] glass transition point of
175 K[231] as well as melting point of 258 K[232] are reported in literature.
It is not unusual for ILs to have both a glass transition temperature and a
melting temperature.[233] Melting happens when the molecules or ions fall
out of their crystal structures, and become disordered liquid. The glass
transition is a transition which happens from solid state to amorphous
solid. In case of ILs, crystalline solids may also have some amorphous
portion. Due to this ambivalency, the fragility of [EMIM][NTf2] cannot be
directly compared to those of MP-ILs.
117
Table 12: Comparison of the Tg, T0, 푻ퟎ푻품 and m values for MP-ILs in Table 10.
Entry Ionic Liquid Tg(onset) / K T0 / K 푻ퟎ푻품
m
1 [EMIM][Me(H)PO3] 191 129 0.68 27 2 [EMIM][Me(Me)PO3] 203 155 0.77 45 3 [EMIM][AcN(Me)PO3] 210 154 0.73 39 4 [EMIM][MeAc(Me)PO3] 219 172 0.78 46 5 [EMIM][Me(EG)3(Me)PO3] 202 124 0.61 22 6 [EMIM][Me(EG)2(Me)PO3] 206 146 0.71 31 7 [EMIM][Me(EG)1(Me)PO3] 205 110 0.53 18 8 [BMIM][Me(EG)1(Me)PO3] 205 123 0.60 24 9 [BMPyrr][Me(EG)1(Me)PO3] 197 105 0.53 21
3.3.3.3. Walden plot
Figure 49 shows a Walden plot for the family of phosphonate ILs. The
“ideal” Walden line representative for fully dissociated systems like
aqueous KCl solution runs from corner to corner of a square diagram. All
investigated ionic liquids lie significantly below this line and are thus
assumed to be weakly dissociated. For comparison, the position of
[EMIM][NTf2] may be seen from Figure 19[220] (chapter 2.4.3.4.3). The IL
[EMIM][MeAc(Me)PO3] lies nearly on the line representing high
dissociation. This finding must be explained by the molecular structure of
the anion. On the one site, the presence of the ester group is supposed to
rather increase its overall co-ordinative nature. On the other site, the
electron density of the phosphonate group decreases due to the electron
withdrawing nature of the ester group.
118
Figure 49: Walden plot of functionalized phosphonate ILs at 20 and 25 °C. The viscosity is
represented in Poise units for comparison with literature like Ref. [128].
3.3.3.4. Double-layer capacitance of [EMIM]+ based MP-ILs
The double-layer capacitance of the investigated ILs was determined in
dependence on temperature. Whereas Cdl of [EMIM][NTf2] seems to
increase (although slightly) with temperature, the Cdl values of the
[EMIM]+ based MP-ILs stay rather constant in the investigated
temperature range.
Already Drüschler et al. reported very weak temperature dependence of
double-layer capacitance for [BMpyrr][FAP] on Au(111).[174] Also,
decreasing as well as increasing Cdl behavior with increasing temperature
were reported by several authors.[149],[175]-[178]
119
Figure 50: Double-layer capacitance values of [EMIM]+ based MP-ILs as a function of temperature
in comparison to [EMIM][NTf2].
In contrast to the physico-chemical properties, the interpretation of the
double-layer capacitance results taking into account the ionic structure is
very elusive. At a first glance, [EMIM]+ based MP-ILs reveal higher Cdl
values than [EMIM][NTf2] under identical conditions. As stated before, the
comparison to literature is somewhat difficult, since measurement metho-
dologies and setups vary significantly. E.g. Drüschler et al. report Cdl
values for [EMIM][FAP] on Au(111) electrode being 6 – 8 F cm-2 [155] and
for [EMIM][NTf2] on Au(111) electrode being around 10 F cm-2.[225] The
Cdl values of [EMIM]+ based MP-ILs lie in the range 9 – 12 F cm-2, but the
influence of the particular anion functionalization is hardly identified as for
properties like electrochemical window or viscosity. Even
[EMIM][Me(H)PO3] does not leave the set of curves of phosphonate ILs, so
that only the charge carrying group and not the attached tails seem to be
the determining factor. The reason for this may be the fact, that the
double-layer capacitance represents not a “bulk” property like other
physico-chemical or electrochemical properties, but rather an interface
property. According to Drüschler et al. not the ion size is decisive, but
rather the ratio of the anion and cation sizes. It determines how dense the
ions are bunched at the interface and how close the charge can be
120
brought to the elctrode.[225] The formation of multilayers of alternating
ionic charge in accordance to molten salt models should also be
considered.
3.3.4. Influence of cation functionalization on electro-chemical properties of the [Me(EG)1(Me)PO3]- based ILs
Variations of the cationic structures cause differences in physico-chemical
properties. The conductivity, which is tightly linked to the viscosity is not
an exception. The influence of the cation on the impedance of the
[Me(EG)1(Me)PO3]- based ILs is displayed in Figure 51. Not surprising is
the low resistivity (Figure 51 a and b) and subsequently higher
conductivity (Figure 52) of the [EMIM]+ based IL.
Figure 51: Influence of the cations [EMIM]+, [BMIM]+, and [BMPyrr]+ on impedance of
[Me(EG)1(Me)PO3]- based ILs at 24 °C. a) Nyquist plot, b) Bode plot, c) Cole-Cole plot.
The comparison of [BMIM]+ and [BMPyrr]+ cations reveals, however,
slightly higher conductivity for [BMPyrr]+ than for [BMIM]+. This result is
in accordance with viscosity data, where [BMPyrr][Me(EG)1(Me)PO3]
121
shows lower viscosity than [BMIM][Me(EG)1(Me)PO3], both having nearly
identical moisture contents (0.88 and 0.79 %, respectively). However, the
general statement of Buzzeo et al.[210] predicting higher conductivity for
imidazolium-based ILs than for pyrrolidinium-based ones is in some
contrast to the determined data. The reason for this may be the evident
ability of the methylphosphonate anions to form hydrogen bonds. Since,
imidazolium is rather able to participate in such H-bonds compared to
pyrrolidinium, the higher viscosity and lower conductivity of imidazolium-
based MP-ILs can be explained. The VFT parameters for [BMIM]+ and
[BMPyrr][Me(EG)1(Me)PO3] may be found in Table 11, the fragility values
in Table 12 and the Walden behavior in Figure 49.
Figure 52: Conductivity values and VFT fit of [EMIM]+, [BMIM]+, and [BMPyrr][Me(EG)1(Me)PO3]-
ILs as function of inverse temperature.
Cdl of [BMIM]+ and [BMPyrr][Me(EG)1(Me)PO3]-ILs stays constant with
temperature increase, as can be seen in Figure 53. This is consistent with
the findings made with other MP-ILs. However, Cdl of [BMIM]+ and
[BMPyrr]+-ILs was found to be lower than that of
[EMIM][Me(EG)1(Me)PO3], as could be already estimated from the Cole-
Cole plot in Figure 51 c.
122
Beyond that, differences concerning cation size within MP-ILs may be
identified. ILs with the larger [BMIM]+ and [BMPyrr]+ cations reveal lower
Cdl values than that with smaller [EMIM]+ cation. As expected from
Ref. [225]: Increasing the size of the cation decreases the overall
capacitance of an activated carbon electrode for phosphonium-based ionic
liquids with a common anion.[195] Further, for [EMIM][BF4] and
[BMIM][BF4] the capacitance was also found to decrease as the cation size
increased,[223] as well as for high-temperature alkali halide melts at a gold
electrode.[224]
The double-layer capacitance values for [BMIM]+ and
[BMPyrr][Me(EG)1(Me)PO3]-ILs are such similar that their aromatic and
steric natures may be hardly addressed for interpretation.
Figure 53: Influence of temperature and cation structure on the double-layer capacitance of
[EMIM]+, [BMIM]+, and [BMPyrr][Me(EG)1(Me)PO3]-ILs.
3.3.5. Influence of cellulose addition on impedance measurements
The influence of the cellulose addition on the impedance of some [EMIM]+
based MP-ILs was investigated in oder to compare the properties of the
123
ionogels to the pure ILs. In Figure 54 impedance spectra of
[EMIM][Me(EG)1(Me)PO3] and the corresponding ionogel are displayed.
Figure 54: Influence of the cellulose addition on impedance of [EMIM][Me(EG)1(Me)PO3] at 24 °C.
a) Nyquist plot, b) Bode plot, c) Cole-Cole plot.
Not surprising is the lower resistivity (Figure 54 a and b) and
subsequently higher conductivity (Figure 55) of the IL in comparison to
the ionogel. Very interesting is, however, the extent of the conductivity
decrease. Bearing in mind that the viscosity of the ionogel increases
drastically compared to the pure IL, its conductivity comes into the set of
curves for pure MP-ILs.
124
Figure 55: Influence of the cellulose addition on the conductivity of a) [EMIM][Me(EG)1(Me)PO3],
b) [EMIM][Me(EG)3(Me)PO3] and c) [EMIM][MeAc(Me)PO3].
3.3.5.1. Fragility comparison of MP-ILs and corresponding ionogels
Table 13 represents glass transition onset points, corresponding ratios
and fragility parameters m for the pure MP-ILs in comparison to the
corresponding 5 wt% cellulose ionogels. The difference between the Tg
temperatures of the pure IL and the ionogel is, at the first glance, nearly
negligible. Much larger, however, is the difference for the T0 temperatures
obtained from the VFT fits. The ratios and fragility parameters m
increase slightly upon cellulose addition for ILs with [MeAc(Me)PO3]- and
[Me(EG)1(Me)PO3]- anions. Especially for the [EMIM][MeAc(Me)PO3] IL
Tg nearly equals T0 which leads to the ratio of 1 and m of 3*106. On the
other side, the cellulose addition has no great effect for the
[EMIM][Me(EG)3(Me)PO3] IL.
125
Table 13: Comparison of the Tg, T0, 푻ퟎ푻품 and m values for selected MP-ILs and corresponding
ionogels.
Entry Ionic Liquid pure gel [EMIM] Tg / K T0 / K 푻ퟎ
푻품 m Tg / K T0 / K 푻ퟎ
푻품 m
1 [MeAc(Me)PO3] 219 172 0.78 46 206 206 1.00 3*106 2 [Me(EG)3(Me)PO3] 202 124 0.61 22 205 131 0,64 25 3 [Me(EG)1(Me)PO3] 205 110 0.53 18 202 124 0.61 22
The correlation of fragility with the cellulose MP-anion interactions is very
elusive. The formation of hydrogen bonded networks plays in any case a
very important role.
3.3.5.2. Double-layer capacitance comparison of MP-ILs and corresponding ionogels
Figure 56 shows besides the independence on temperature of the ionogel
double-layer capacitance Cdl also its constancy upon cellulose addition,
which can be estimated from the Cole-Cole plot in Figure 54 c for
[EMIM][Me(EG)1(Me)PO3]. The slightly higher Cdl of the
[EMIM][MeAc(Me)PO3] ionogel in comparison to the pure IL should be
considered rather as deviation within the measurement error limits.
126
Figure 56: Influence of the cellulose addition on the double-layer capacitance of
a) [EMIM][Me(EG)1(Me)PO3] b) [EMIM][Me(EG)3(Me)PO3] and c) [EMIM][MeAc(Me)PO3].
127
4. Summary and outlook
In this thesis a new and very general synthetic route to long-chain and
functionalized phosphonate esters and the corresponding ionic liquids is
presented. The key-step in this method is the formation of neutral esters
with subsequent quaternization of amines or phosphines (Scheme 40).
Scheme 40: Synthesis of long-chain and functionalized methylphosphonate esters and ILs.
First, the anion of a phosphonate ionic liquid is reacted in SN2 mechanism
with a suitable alkylating agent R’–X to form the neutral esters
R’Me(Me)PO3 or R’2(Me)PO3. The basis for this reaction is the high
nucleophilicity of the anion. In most cases, the stoichiometric by-product
represented itself a valuable IL.
In the kinetics experiments all used ILs were [EMIM]+ based and methyl
chloroacetate was used as alkylating agent. The reaction progress was
monitored by 31P NMR. The reactivity and kinetics of the transesterification
step are dependent on the O-attached and P-attached residues on the
phosphonate anion. Inductive effects and sterical hindrance have opposing
influences: longer alkyl chains are supposed to increase the nucleophilicity
of the anion, but on the other side they sterically inhibit reaction
proceeding. Phosphonate species carrying hydrogen connected to phos-
phorus are the most reactive ones and several byproducts are observed
during the reaction of the phosphonate anion with alkylating agents.
Comparison of the methyl methyl phosphonate anion with other ester-
based anions commonly used in ionic liquids reveals that alkyl phos-
phonate species are the most convenient for transesterification. Using
halogenated alkylation agents is superior to acid catalysis for reasons of
proper separation of the product from the reaction mixture. Phosphates
128
and H-phosphonates are rather inconvenient due to the byproduct
occurrence during the reaction.
The asymmetric dialkyl methylphosphonate esters can be isolated by
extraction or distillation and can serve as alkylating agents to quaternize
amines or phosphines. By this reaction dianionic IL
[EMIM]2[(Me)PO3BuSO3] was as well accessible. The alkylating activity is
dependent on the length of the alkyl residue. In case of different aliphatic
residues both may be transferred to the acceptor providing binary
mixtures of ILs. In this work, we concentrated on pure ILs. The obtained,
functionalized phosphonate salts are free of halogen impurities and display
interesting physico-chemical properties including a wide liquid range,
moderate viscosities (taking into account their size and molecular weight)
as well as reasonable thermal and very good hydrolytic stability. Several
interesting applications may be anticipated for these new ionic liquid
structures in separation technologies (e.g. liquid–liquid extraction),
lubrication and carbohydrate chemistry.
The PEG and ester functionalized MP-ILs were used to dissolve several
carbohydrates. With micro-cellulose biopolymer based ionogels were
formed. The characterisation was carried out by light microscopy. The
ability to dissolve cellulose is for these MP-based ILs much higher than for
[EMIM][Ac]. An uptake of up to 30 wt% was possible for
[EMIM][Me(EG)1(Me)PO3]. The thermal stability of the ionogels was
slightly lower than that of the corresponding IL. The ionogels were stable
under inert conditions and humidity lead to separation of IL and cellulose.
Since these ionogels showed promising properties for application in
electronic devices such as low voltage inorganic and organic electrolyte-
gated field-effect transistors (FETs) their electrochemical properties were
investigated. The comparison to the pure MP-ILs was performed. Wide
potential windows could be observed for MP-ILs and the cellulose addition
had nearly no negative effects on this feature. Both for the pure ILs and
for ionogels the EW shrinked with increasing temperature.
129
The conductivity and capacitance were determined from impedance
measurements. The frequency range from 1 to 106 Hz and temperature
range from 6 to 75 °C were used. The constant phase element (CPE)
instead of an ideal capacitor was used for the fitting procedure.
Conductivities in order of 0.5 mS cm-1 were typical for MP-ILs as well as
VFT viscosity behaviour. The conductivities of the corresponding ionogels
were only slightly lower. Relatively high double-layer capacitances could
be found for MP-ILs in the range of 9-12 F cm-2. The double-layer
capacitance showed very weak dependence on temperature. Also it was
unaffected by the cellulose addition. On the other side, the addition of
cellulose provided moldable, free-standing materials. Finally, the ionogels
obtained by combination of MP-ILs and paper were successfully probed in
thin layer transistors.[237]
All these and other aspects provide a lot of room for future research that
leads to further developments and applications. Especially in the field of
functional electronic components, including solar cells, sensors, biomedical
devices, conductive circuits, and FETs cost-effective and environmentally
friendly materials may be used as substrates. The substrate occupies a
large part of a device and renewable and biodegradable ionogels based on
biopolymers represent an excellent alternative high capacitance
dielectrics.
130
5. Zusammenfassung und Ausblick
In dieser Arbeit wird eine neue allgemeine Syntheseroute zur Herstellung
von langkettigen und funktionalisierten Phosphonatestern und ent-
sprechenden ionischen Flüssigkeiten beschrieben. Der Schlüsselschritt
dieser Methode ist die Veresterung mit nachfolgender Quaternisierung der
Amine und Phosphine (Reaktionsgleichung 1).
Reaktionsgleichung 1: Synthese langkettiger und funktionalisierter Methylphosphonatester und
ILs.
Als erstes reagiert das Anion einer Phosphonat-basierten ionischen
Flüssigkeit mit einem Alkylierungsmittel R’–X in einer SN2-Reaktion unter
Bildung von neutralen Estern R’Me(Me)PO3 or R’2(Me)PO3. Der Grund für
die Reaktion ist die hohe Nukleophilie des Anions. In den meisten Fällen
stellt das stöchiometrische Nebenprodukt der Reaktion selbst eine
wertvolle IL dar.
Die kinetischen Untersuchungen der Veresterung wurden mit [EMIM]+-ba-
sierten ILs durchgeführt. Methylchloroacetat wurde als Alkylierungsmittel
eingesetzt. Der Reaktionsfortschritt wurde mittels 31P NMR verfolgt. Die
Reaktivität und Kinetik des Veresterungsschritts hängen von den O- und
P-gebundenen Resten des Phosphonatanions ab. Induktive Effekte und
sterische Hinderung haben gegensätzliche Einflüsse: Lange Alkylketten
erhöhen die Nukleophilie des Anions, behindern allerdings sterisch den
Angriff. Phosphonate mit P-H-Bindung besitzen die höchste Reaktivität
und zahlreiche Nebenprodukte entstehen während der Reaktion mit
Alkylierungsmitteln.
Der Vergleich von Methyl Methylphosphonatanion mit anderen Ester-
basierten Anionen in ILs zeigt, dass Alkylphosphonatspezies sich am
besten zur Veresterung eignen. Die Verwendung von halogenierten
131
Alkylierungsmitteln hat Vorteile gegenüber der Säurekatalyse wegen der
besseren Abtrennung der Produkte aus der Reaktionsmischung. Phosphate
and H-Phosphonate sind eher unbrauchbar wegen der Bildung von
Nebenprodukten während der Reaktion.
Die asymmetrischen Dialkylmethylphosphonatester können durch
Extraktion oder Destillation isoliert werden und dienen selbst als
Alkylierungsmittel zu Quaternisierung der Amine oder Phosphine. Durch
diese Reaktion konnte die dianionische IL [EMIM]2[(Me)PO3BuSO3]
synthetisiert werden. Die Alkylierungsstärke hängt von der Länge des
Alkylrestes ab. Im Fall von unterschiedlich langen aliphatischen Resten
können beide auf den Akzeptor übertragen werden, so dass binäre IL-
Mischungen entstehen. In dieser Arbeit konzentrierten wir uns auf reine
ILs. Die funktionalisierten Phosphonat-ILs sind alle halogenfrei und zeigen
interessante physikochemische Eigenschaften wie breiten Flüssigkeits-
bereich, niedrige Viskositäten (in Anbetracht ihrer Molekulargröße und –
gewicht), thermische und hydrolytische Stabilität. Viele interessante
Anwendungen wären denkbar für diese neuartigen IL-Strukturen: In
Trenntechnologien (z.B. Flüssig-flüssig-Extraktion), als Schmiermittel und
in der Chemie der Kohlenhydrate.
Die PEG- und Ester-funktionalisierten MP-ILs wurden als Lösungsmittel für
verschiedene Kohlenhydrate eingesetzt. Mit Mikrocellulose entstanden
biopolymer-basierte Ionogele. Sie wurden mittels Lichtmikroskopie
charakterisiert. Diese ILs waren in der Lage viel mehr Cellulose zu lösen
als [EMIM][Ac]. Eine Aufnahme von bis zu 30 wt% war möglich bei
[EMIM][Me(EG)1(Me)PO3]. Die thermische Stabilität der Ionogele war
etwas niedriger als bei entsprechenden ILs. Die Ionogele waren stabil
unter Schutzgas, während Feuchtigkeit zu Trennung von IL und Cellulose
führte.
Da diese Ionogele vielversprechende Eigenschaften zur Anwendung in
elektronischen Instrumenten wie Elektrolyt-gesteuerten Dünnschichttran-
sistoren zeigten, wurden sie elektrochemisch untersucht. Der Vergleich zu
132
reinen ILs wurde durchgeführt. Große elektrochemische Fenster wurden
beobachtet bei MP-ILs und die Zugabe von Cellulose hatte kaum negativen
Einfluss darauf. Sowohl für die reinen ILs, als auch für Ionogele
verkleinerte sich das elektrochemische Fenster mit steigender
Temperatur.
Die Leitfähigkeit und die Doppelschichtkapazität wurden bestimmt durch
Impedanzmessungen. Der Frequenzbereich war 1 - 106 Hz und
Temperaturbereich 6 - 75 °C. Das sog. constant phase element (CPE)
wurde in der Datenauswertung anstatt des idealen Kondensators
eingesetzt.
Leitfähigkeiten in der Größenordnung von 0.5 mS cm-1 und das VFT
Verhalten der Viskosität waren typisch für die MP-ILs. Die Leitfähigkeiten
der entsprechenden Ionogele waren nur etwas niedrieger. Relativ hohe
Doppelschichtkapazitäten im Bereich von 9 - 12 F cm-2 wurden für MP-ILs
gemessen. Die Doppelschichtkapazität zeigte nur schwache Abhängigkeit
von der Temperatur und war kaum beeinflusst von der Cellulosezugabe.
Andererseits führte die Cellulosezugabe zu formbaren, freistehenden
Materialien. Die Ionogele, die aus MP-ILs und Papier hergestellt wurden,
fanden Einsatz in Elektrolyt-gesteuerten Transistoren.[237]
133
6. Experimental
6.1.1. Solvents and reagents
Solvents for syntheses were of spectroscopic grade.
The ionic liquid [EMIM][Me(Me)PO3] was received from BASF AG.
Methylimidazole, butylpyrrolidine (Aldrich, > 99 %), butylimidazole
(solvent innovation, > 99 %) were distilled prior to use. All further
reagents were purchased from Aldrich, Fluka, Merck KGaA with synthesis
grades and used as received.
Cellulose: Sigmacell Cellulose Type 101.
6.1.2. Analytics
6.1.2.1. Nuclear magnetic resonance spectroscopy (NMR)
NMR measurements were carried out with Jeol ECX 400 MHz
spectrometer(1H: 400 MHz, 13C: 100 MHz, 31P: 162 MHz). The spectra
were taken at 20 °C (except for temperature dependent measurements)
und referenced to deuterated solvents CDCl3, d6-DMSO and D2O,
respectively. Chemical shifts are reported in ppm relative to tetramethyl-
silan (TMS) while spin-spin coupling constants are given in Hz. 13C-NMR
spectra were taken decoupled from 1H.
6.1.2.2. Gas chromatography Mass Spectrometry (GC-MS)
All the structures of the synthesized methyl phosphonate diesters in this
work were confirmed by gas chromatographic mass spectroscopy (GC-
MS). GC-MS experiments were carried out on Varian 450 GC Gas
134
Chromatograph with Varian 220 MS IT Mass Spectrometer. All fragments
are denoted in m / z. The chemical structures of the fragments are
depicted without charge for simplicity.
Phosphonate esters are reported to form [2M+H]+ species during mass
spectrometric measurement.[238]
6.1.2.3. Electrospray ionization Mass Spectrometry (ESI-MS)
All the structures of the ILs synthesized in this work were confirmed by
the electrospray ionization mass spectroscopy (ESI-MS). ESI-MS
experiments were carried out on Bruker Daltonics Esquire 6000 and
Applied Biosystems MDS SCIEX Q TRAP LC/MS/MS Systems, respectively.
6.1.2.4. Differential scanning calorimetry (DSC)
DSC measurements were carried out with Netzsch DSC 204 under
nitrogen atmosphere with a heating rate of 4 K min-1 in the temperature
range of -130°C to 120°C. Alumina pans were used. Glass transition
points or melting points were determined from the onsets of the alteration
of the heat capacity.
6.1.2.5. Thermogravimetric analysis (TGA)
TGA measurements were accomplished on a Setsys 1750 CS Evolution
from Setaram instrumentation (software: calisto processing) by heating
samples in a quartz container from 30 to 400°C with heating rate of
10 K min-1 in a constant nitrogen flow.
6.1.2.6. Viscosity measurements
The viscosities were measured with the Anton Paar Physica MCR 100
Rheometer at temperatures from 20°C, 25°C, 40°C, 60°C to 80°C with
135
shear rate varying from 1 s-1 to 1000 s-1 at each temperature under argon
atmosphere.
6.1.2.7. Density measurements
The density was measured by Anton Paar DMA 4500 density and sound
velocity meter at 20 and 25 °C.
6.1.2.8. Karl-Fischer-titration
For the determination of the water content Metrohm 756 KF was used.
6.1.2.9. Light microscopic investigations
The light microscope pictures were made with Nikon eclipse 50i equipped
with DS-Fi1 camera and with linkam scientific Instruments LinkPad T95-PE
heating table.
6.1.2.10. Electrochemical measurements
For the electrochemical measurements, a temperature-controlled Microcell
HC (rhd instruments) as depicted in Figure 57 was used which was filled
with IL under Ar atmosphere inside the glovebox.
136
Figure 57: Setup for electrochemical measurements[200]
A three-electrode configuration was used for CV measurements with a
polycrystalline Pt wire incorporated in the cell top cover dipped in the IL
acting as the working electrode (WE; AWE = 0,00049 cm2) and the Pt-
coated crucible acting as the container for the RTIL and as the counter
electrode (CE; ACE ≈ 5.2 cm2). Another Pt wire from the cell top cover was
used as pseudo reference. The electrodes were polished prior to use. The
electrochemical measurement cell was connected to Metrohm Autolab
potentiostat PGSTAT 30 with a frequency response analyzer module
(FRA). Before carrying out an EIS measurement, a cyclic voltammogram
with a scan rate of 10 mV s-1 was recorded in order to determine the
electrochemical window. The potential scan was started at 0.0 V versus
the REp. The potential was first swept into the anodic direction up to 5 V,
then into the cathodic direction up to -2.5 V, and finally back to 0.0 V. The
scan Nr. 4 was taken as representative.
The ac impedance measurements were carried out in two-electrode
configuration at open circuit potential using the ac voltage peak-to-peak
perturbation of 0.05 V at frequencies logarithmically distributed in the
range from 1 to 106 Hz (69 points) and at temperatures ranging from 6 to
137
75 °C. Heating and cooling was done by means of a peltier element with
an accuracy of 0.01 °C.
The cell constant, k, was determined by a calibration measurement with a
standard conductivity solution (specific conductivity: 1.413 mS cm-1 at
25 °C from VWR BDH Prolabo) and was used to calculate the specific ionic
conductivity σ of the ionic liquid from the measured impedances.
The double-layer capacitance Cdl was normalized to the area of the wor-
king electrode (AWE = 0,00049 cm2). The overall capacitance is defined as
1퐶 =
1퐶 +
1퐶
퐶 =퐶 ∗ 퐶퐶 + 퐶
Equation 24
The area of the counter electrode is approximately 5.2 cm2. The ratio
between the areas of the working electrode and the counter electrode is
nearly 1 : 104. Since Cdl ~ A and AWE « ACE, CWE « CCE. According to
Equation 25, the total measured impedance is approximately equal to CWE.
퐶 =퐶 ∗ 퐶퐶 + 퐶 =
퐶 ∗ 10 퐶퐶 + 10 퐶 ≈ 퐶
Equation 25
138
6.1.3. Synthesis of asymmetric methyl methylphosphonate esters
The general synthesis procedure of methyl alkyl methylphosphonate
esters in Table 3, entry 1 – 4 was carried out as follows:
1.0 eq of alkylating agent R’-X (Table 3, entry 1 – 4) was weighted into
a dry Schlenk flask and 1.2 eq of the ionic liquid [EMIM][Me(Me)PO3]
was added. The reaction mixture was stirred for 20 - 40 h at 50 - 80 °C
under Ar atmosphere. The reaction mixture was subsequently extracted
four times with toluene. The combined toluene phases were
concentrated to a small volume and dried under reduced pressure to
yield the product ester.
139
BuMe(Me)PO3
Yield 85 %.
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.88 (t, 3H, J = 7 Hz, 6-H),
1.35 (m, 2H, 5-H), 1.40 (d, 3H, J = 17 Hz, 2-H), 1.56 (m, 2H, 4-H),
3.59 (d, 3H, J = 11 Hz, 1-H), 3.91 (m, 2H, 3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.00 (d, J = 141 Hz, C-2),
13.72 C-6, 18.45 C-5, 32.55 C-4, 51.94 C-1, 64.90 C-3.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 31.92.
M=166.16
[2M+H] = 333
Spectrum Plot - 3/15/2013 9:35 AM1 A Scan 968 from x:\data\sas\u14_3-14-2013_6-33-21 pm_swetlana.sms
50 100 150 200 250 300 350 m/z
0%
25%
50%
75%
100%
330.9 944618
331.8 1.306e+6
332.8 4.922e+6
333.7 446541
Spectrum 1A6.136 min, Scans: 967-969, 50:650, Ion: 32 us, RIC: 1.282e+7BP: 332.8 (4.922e+6=100%), u14_3-14-2013_6-33-21 pm_swetlana.sms
140
HexMe(Me)PO3
Yield 83 %.
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.86 (t, 3H, J = 7 Hz, 8-H),
1.27 (m, 6H, 5-H, 6-H and 7-H), 1.39 (d, 3H, J = 18 Hz, 2-H), 1.57 (m,
2H, 4-H), 3.58 (d, 3H, J = 11 Hz, 1-H), 3.90 (m, 2H, 3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.10 (d, J = 142 Hz, C-2),
14.14 C-8, 22.55 C-7, 25.23 C-5, 30.43 C-4, 31.37 C-6, 51.93 C-1,
65.27 C-3.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 31.98.
M=194.21
[2M+H] = 389
Spectrum Plot - 3/15/2013 9:39 AM1 A Scan 1326 from x:\data\sas\u15_3-14-2013_4-28-11 pm_swetlana.sms
100 200 300 400 500 600 m/z
0%
25%
50%
75%
100%
387.1 900428
388.1 1.952e+6
389.0 5.641e+6
389.8 662596
Spectrum 1A7.637 min, Scans: 1325-1327, 50:650, Ion: 37 us, RIC: 1.234e+7BP: 389.0 (5.641e+6=100%), u15_3-14-2013_4-28-11 pm_swetlana.sms
141
OcMe(Me)PO3
Yield 89 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.85 (t, 3H, J = 7 Hz, 10-H),
1.24 (m, 10H, 5-H to 9-H), 1.39 (d, 3H, J = 18 Hz, 2-H), 1.57 (m, 2H,
4-H), 3.58 (d, 3H, J = 11 Hz, 1-H), 3.89 (m, 2H, 3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.00 (d, J = 141 Hz, C-2),
14.21 C-10, 15.46 C-9, 22.64 C-7, 25.59 C-6, 29.20 C-5, 30.53 C-4,
31.82 C-8, 51.93 C-1, 65.18 C-3.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 31.86.
M=222.26
[2M+H] = 445
Spectrum Plot - 3/15/2013 9:47 AM1 A Scan 1592 from x:\data\sas\u7_3-14-2013_4-49-02 pm_swetlana.sms
100 200 300 400 500 600 m/z
0%
25%
50%
75%
100%
441.3 712788
443.1 1.443e+6
444.1 3.936e+6
445.1 8.487e+6
445.9 1.042e+6
Spectrum 1A8.978 min, Scans: 1591-1593, 50:650, Ion: 30 us, RIC: 2.687e+7BP: 445.1 (8.487e+6=100%), u7_3-14-2013_4-49-02 pm_swetlana.sms
142
DodMe(Me)PO3
Yield 92 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.82 (t, 3H, J = 7 Hz, 10-H),
1.22 (m, 18H, 5-H to 13-H), 1.38 (d, 3H, J = 18 Hz, 2-H), 1.55 (m, 2H,
4-H), 3.57 (d, 3H, J = 11 Hz, 1-H), 3.86 (m, 2H, 3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.15 (d, J = 141 Hz, C-2),
14.26 C-14, 22.68 C-13, 25.31 C-6, 29.30 C-7 to C-11, 29.67 C-5,
30.55 C-4, 31.93 C-12, 51.99 C-1, 65.20 C-3.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 31.92.
M=278.37
[2M+H] = 557
Spectrum Plot - 3/15/2013 9:43 AM1 A Scan 2009 from x:\data\sas\u11_3-14-2013_6-54-13 pm_swetlana.sms
100 200 300 400 500 600 m/z
0%
25%
50%
75%
100%
536.2 959688
552.7 1.499e+6
555.6 5.041e+6
556.5 8.424e+6
557.3 1.427e+7
558.3 2.186e+6
Spectrum 1A11.271 min, Scans: 2008-2010, 50:650, Ion: 18 us, RIC: 7.468e+7BP: 557.3 (1.427e+7=100%), u11_3-14-2013_6-54-13 pm_swetlana.sms
143
The general synthesis procedure of methyl alkyl methylphosphonate
esters in Table 3, entry 5 and 7 – 9 was carried out as follows:
1.2 eq of the ionic liquid [EMIM][Me(Me)PO3] was weighted into a dry
Schlenk flask and 1.0 eq of alkylating agent R’-X (Table 3, entry 5-9)
was added dropwise (solids were dissolved in acetone) under stirring.
The reaction mixture was stirred for 12 h at room temperature under Ar
atmosphere. The solventless reaction mixture was subsequently
extracted four times with toluene. The combined toluene phases were
concentrated to a small volume and dried under reduced pressure to
yield the product ester.
144
MeAcMe(Me)PO3
Yield 74 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.50 (d, 3H, J = 17 Hz, 2-H),
3.62 (d, 3H, J = 12 Hz, 1-H), 3.69 (s, 3H, 5-H), 4.59 (d, 2H, J = 12 Hz,
3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.75 (d, J = 155 Hz, C-2),
52.12 C-1, 52.42 C-5, 62.04 C-3, 169.53 C-4.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 34.11.
M=182.11
[2M]-89=275
100 200 300 400 500 600m/z
0%
25%
50%
75%
100%
275.2 2.415e+6
276.0 158972
332.9 134985
349.0 499536
Spectrum 1A6.813 min, Scans: 1119-1121, 50:650, Ion: 49 us, RIC: 4.702e+6BP: 275.2 (2.415e+6=100%), u6_3-22-2013_10-54-29 am_swetlana.sms
145
AcNMe(Me)PO3
Yield 42 %
1.2 eq of the ionic liquid [EMIM][Me(Me)PO3]was weighted into a dry
Schlenk flask and 1.0 eq of bromoacetonitrile was added dropwise
under stirring. The reaction mixture was stirred for 5 h at room
temperature under an Ar atmosphere. The reaction mixture was
subsequently extracted four times with diethylether. The combined
diethylether phases were concentrated to a small volume and dried
under reduced pressure to yield the product ester.
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.55 (d, 3H, J = 18 Hz, 2-H),
3.66 (d, 3H, J = 11 Hz, 1-H), 4.90 (d, 3H, J = 12 Hz, 3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.00 (d, J = 140 Hz, C-2),
50.62 C-1, 52.75 C-3, 116.85 C-4.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 35.33.
M=149.08
[2M]-56=242
100 200 300 400 500 600m/z
0%
25%
50%
75%
100%
63.0 4449
79.0 16511
118.9 7151
122.9 13382
150.0 67639
195.9 9398
203.0 3948
242.1 34591
266.8 3974
Spectrum 1A5.830 min, Scans: 916-918, 50:650, Ion: 538 us, RIC: 262630BP: 150.0 (67639=100%), u12_3-25-2013_9-24-51 am_swetlana.sms
NO
P
O
O
Chemical Formula: C2H2N•
Exact Mass: 40,02
Chemical Formula: C2H6O3P•
Exact Mass: 109,01
146
2-oxo-2-phenylethyl Me(Me)PO3
PO O
O
O
3
6
4
2
1 5
7
8
910
Yield 62 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.57 (d, 3H, J = 17 Hz, 2-H),
3.66 (d, 3H, J = 11 Hz, 1-H), 5.44 (m, 2H, 3-H), 7.55 (m, 2H, 7-H and
9-H), 7.68 (m, 1H, 8-H), 7.95 (m, 2H, 6-H and 10-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 11.05 (d, J = 141 Hz, C-2),
52.17 C-1, 68.17 C-3, 129.00 C-6 to C-10, 134.40 C-5, 194.30 C-4.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 33.87.
M=228.18
Fragments 93 105 119 123
Spectrum Plot - 3/15/2013 9:48 AM1 A Scan 1938 from x:\data\sas\u23_3-14-2013_5-09-56 pm_swetlana.sms
100 200 300 400 500 600 m/z
0%
25%
50%
75%
100%
51.2 15470
63.2 8524
77.1 36307
91.1 6411
93.0 11498
104.9 122121
118.1 50619
119.0 8380
229.0 9299
Spectrum 1A10.411 min, Scans: 1937-1939, 50:650, Ion: 731 us, RIC: 332604BP: 104.9 (122121=100%), u23_3-14-2013_5-09-56 pm_swetlana.sms
147
Cinnamyl Me(Me)PO3
Yield 48 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.49 (d, 3H, J = 17 Hz, 2-H),
3.63 (d, 3H, J = 11 Hz, 1-H), 4.63 (m, 2H, 3-H), 6.39 (m, 1H, 4-H),
6.70 (m, 1H, 5-H), 7.26 (m, 1H, 9-H), 7.34 (m, 2H, 7-H and 11-H),
7.46 (m, 2H, 8-H and 10-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.45 (d, J = 140 Hz, C-2),
52.25 C-1, 65.78 C-3, 125.25 C-4, 127.06 C-9, 128.85 C-7, C-8, C-10,
C-11, 133.06 C-5, 136.46 C-6.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 32.71.
M=226.21
Fragments 117 [2M]-109=343
Spectrum Plot - 3/15/2013 9:48 AM1 A Scan 1855 from x:\data\sas\u24_3-14-2013_5-30-43 pm_swetlana.sms
100 200 300 400 500 600 m/z
0%
25%
50%
75%
100% 117.3
1.557e+6
118.2 150513
233.3 98377
342.9 342014
Spectrum 1A10.528 min, Scans: 1854-1856, 50:650, Ion: 36 us, RIC: 3.265e+6BP: 117.3 (1.557e+6=100%), u24_3-14-2013_5-30-43 pm_swetlana.sms
148
4-vinylbenzyl Me(Me)PO3
Yield 43 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.47 (d, 3H, J = 17 Hz, 2-H),
3.58 (d, 3H, J = 11 Hz, 1-H), 4.99 (m, 2H, 3-H), 5.26 (d, 1H, J =11 Hz,
11a-H), 5.84 (d, 1H, J =18 Hz, 11b-H), 6.73 (m, 1H, 10-H), 7.38 (m,
2H, 5-H and 9-H), 7.48 (m, 2H, 6-H and 8-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 12.80 (d, J = 134 Hz, C-2),
52.47 C-1, 66.63 C-3, 115.22 C-11, 122.43 C-6 and C-8, 124.03 C-5
and C-9, 127.65 C-10, 137.07 C-7.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 32.89.
M=226.21
Fragments 117 133
Spectrum Plot - 3/15/2013 9:46 AM1 A Scan 1840 from x:\data\sas\u32_3-14-2013_5-51-34 pm_swetlana.sms
100 200 300 400 500 600 m/z
0%
25%
50%
75%
100%
50.1 2877
51.1 4516
63.2 8950
79.0 28635
91.1 6555
93.1 8365
94.1 29998
103.0 4886
115.1 35216
116.2 7230
118.1 3345
130.1 12580
133.0 27980
226.0 40873
226.8 20065
227.8 2916
232.0 8372
233.0 5487
Spectrum 1A10.117 min, Scans: 1839-1841, 50:650, Ion: 660 us, RIC: 374206BP: 226.0 (40873=100%), u32_3-14-2013_5-51-34 pm_swetlana.sms
149
The general synthesis procedure of methyl PEG methylphosphonate
esters in Table 3, entry 10 – 12 was carried out as follows:
1.0 eq of PEG-benzenesulfonate[59] was weighted into a dry Schlenk
flask and 1.2 eq of the ionic liquid [EMIM][Me(Me)PO3] was added. The
reaction mixture was stirred for 12 h at RT under Ar atmosphere. The
reaction mixture was subsequently extracted four times with toluene.
The combined toluene phases were concentrated to a small volume and
dried under reduced pressure to yield the product ester.
150
Me(EG)1Me(Me)PO3
Yield 90 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.42 (d, 3H, J = 17 Hz, 2-H),
3.26 (s, 3H, 5-H), 3.48 (t, 2H, J = 5 Hz, 4-H), 3.58 (d, 3H, J = 11 Hz,
1-H), 4.01 (m, 2H, 3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.20 (d, J = 141 Hz, C-2),
52.10 C-1, 58.45 C-5, 64.46 C-3, 71.58 C-4.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 32.71.
M=168.13
[2M]=336 [2M]-75=261 137
100 200 300 400 500 600m/z
0%
25%
50%
75%
100%
137.3 139563
138.3 40896
181.9 44938
247.2 78993
260.9 424260
281.0 44352
333.7 45000
336.3 154894
Spectrum 1A6.289 min, Scans: 1012-1014, 50:650, Ion: 43 us, RIC: 3.228e+6BP: 260.9 (424260=100%), peg1mep_3-22-2013_10-12-43
151
Me(EG)2Me(Me)PO3
P
OO
O
O
O
3 6
42
1
5
7
Yield 91 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.43 (d, 3H, J = 17 Hz, 2-H),
3.23 (s, 3H, 7-H), 3.43 (m, 2H, 4-H), 3.55 (m, 4H, 5-H and 6-H), 3.59
(d, 3H, J = 11 Hz, 1-H), 4.01 (m, 2H, 3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.15 (d, J = 141 Hz, C-2),
51.95 C-1, 58.49 C-7, 64.74 C-3, 70.05 C-4, 70.14 C-5, 71.77 C-6.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 32.65.
M=212.18
Fragments 109 137
Spectrum Plot - 3/15/2013 9:41 AM1 A Scan 1445 from x:\data\sas\peg2mep_3-14-2013_6-12-25 pm_swetlana.sms
100 200 300 400 500 600 m/z
0%
25%
50%
75%
100%
103.1 584730
137.1 867299
213.1 3.392e+6
226.9 303674
Spectrum 1A8.324 min, Scans: 1444-1446, 50:650, Ion: 54 us, RIC: 6.715e+6BP: 213.1 (3.392e+6=100%), peg2mep_3-14-2013_6-12-25 pm_swetlana.sms
152
Me(EG)3Me(Me)PO3
Yield 87 %
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.43 (d, 3H, J = 17 Hz, 2-H),
3.24 (s, 3H, 9-H), 3.42 (m, 2H,4-H), 3.51 (m, 8H, 5-H to 8-H), 3.59 (d,
3H, J = 11 Hz, 1-H), 4.03 (m, 2H, 3-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.20 (d, J = 141 Hz, C-2),
51.98 C-1, 58.51 C-9, 64.75 C-3, 70.19 C-4 to C-7, 71.81 C-8.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 32.65.
M=256.23
Fragments 109 137
100 200 300 400 500 600m/z
0%
25%
50%
75%
100%
57.9 20468
59.0 48363
110.9 34689
137.0 293566
167.0 30438
181.1 80228
256.9 166153
Spectrum 1A10.042 min, Scans: 1784-1786, 50:650, Ion: 231 us, RIC: 916742BP: 137.0 (293566=100%), peg3mep_3-22-2013_10-33-34
153
[EMIM][Me(Me)PO3BuSO3]
Yield 95 %
H2O content / ppm Tg / °C Tdec / °C 1009 -53 (1) 213
(2) 328
1.0 eq of 1,4-butane sultone was weighted into a dry Schlenk flask and
1.0 eq of the ionic liquid [EMIM][Me(Me)PO3] was added. The reaction
mixture was stirred for 12 h at RT under Ar atmosphere.
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.36 (m, 6H, 6-H and 7-H),
1.85 (m, 2H, 10-H), 2.48 (t, 2H, J = 8 Hz, 11-H), 3.53 (d, 3H, J = 12
Hz, 8-H), 3.83 (s, 3H, 4-H), 3.95 (m, 2H, 9-H), 4.17 (q, 2H, J = 7 Hz,
5-H), 7.72 (s, 1H, 2-H), 7.81 (s, 1H, 3-H), 9.23 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 10.00 (d, J = 140 Hz, C-7),
15.63 C-6, 27.16 C-10, 36.09 C-4, 44.56 C-5, 48.06 C-8, 52.14 C-11,
64.73 C-9, 122.50 C-3, 124.06 C-2, 137.02 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 32.41.
Entire Cation Anion Mass calculated 342.35 111.16 231.18
154
155
NH2EtMe(Me)PO3HBr/[Pyr]Br
Yield 89 %
Tm / °C Tdec / °C 119 (1) 189
(2) 264
1.0 eq of 2-bromoethylamine HBr was weighted into a dry Schlenk flask
and 1.0 eq of the ionic liquid [EMIM][Me(Me)PO3] was added. As solvent
pyridine was used. The reaction mixture was stirred for 12 h at RT
under Ar atmosphere and was subsequently washed four times with
toluene to remove pyridine. The product phase was dried under reduced
pressure to remove residual solvent and was then uptaken in CH2Cl2.
The product precipitated as white solid which was filtered and washed
with CH2Cl2 three times.
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.01 (d, 3H, J = 17 Hz, 2-H),
3.34 (d, 3H, J = 10 Hz, 1-H), 3.39 (m, 2H, 4-H), 4.92 (t, 2H, J = 5 Hz,
3-H), 8.20 (m, 2H, 6-H and 8-H), 8.63 (m, 1H, 7-H), 9.24 (m, 2H, 5-H
and 9-H).
13C-NMR (D2O, 100.4 MHz, ppm): δ = 10.35 (d, J = 139 Hz, C-2), 39.16
C-4, 51.13 C-1, 57.90 C-3, 128.90 C-6 and C-8, 144.91 C-5 and C-9,
147.10 C-7. Measured in D2O since in dmso C-4 coinsides with dmso-
peak.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 22.03.
Entire 2-aminoethyl methyl (Me)PO3
Pyridinium Bromide
Mass calculated 394.04 154.12 80.11 79.90
156
157
6.1.4. General synthetic procedure of alkylphosphonate ionic liquids from dialkyl alkylphosphonate esters
1.0 eq of the respective alkylphosphonate ester was weighted into a dry
Schlenk flask and 1.0 eq of the corresponding amine was added. Then
the reaction mixture was stirred at elevated temperature. The required
conditions are given for each compound.
[EMIM][Me(Me)PO3]
N N
36
54
2
1
8
7
P
OO
O
Phosphonate
ester Amine T / °C t / d Yield / % H2O content /
ppm Tg / °C Tdec / °C
Me2(Me)PO3 EtIm 100 1 82 2744 -70 270 1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.88 (d, 3H, J = 15.24 Hz, 7-
H), 1.35 (t, 3H, J = 7.42 Hz, 6-H), 3,27 (d, 3H, J = 1.03 Hz, 8-H), 3.92
(s, 3H, 4-H), 4.25 (q, 2H, J = 0.74 Hz, 5-H), 8.08 (s, 1H, 2-H), 8.22 (s,
1H, 3-H), 10.26 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 12.80 (d, J = 131 Hz, C-7),
15.30 C-6, 35.35 C-4, 43.83 C-5, 50.20 C-8, 122.23 C-3, 123.71 C-2,
137.71 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 18.98.
Entire Cation Anion
Mass calculated 220.21 111.16 109.04
158
159
[EMIM][Et(Et)PO3]
P
OO
O
N N
1 7
6
54
32
8
9
10
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Et2(Et)PO3 MIm 100 1.5 96 338 -75 250
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.88 (m, 3H, 8-H), 1.04 (t, 3H,
J = 7 Hz, 6-H), 1.21 (m, 2H, 7-H), 1.38 (t, 3H, J = 7 Hz, 10-H), 3.65
(m, 2H, 9-H), 3.89 (s, 3H, 4-H), 4.23 (q, 2H, J = 7 Hz, 5-H), 7.88 (s,
1H, 2-H), 7.98 (s, 1H, 3-H), 10.03 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 8.94 C-8, 15.79 C-6, 17.46
C-10, 20.82 (d, J = 134 Hz, C-7), 35.95 C-4, 44.37 C-5, 58.65 C-9,
122.57 C-3, 124.09 C-2, 138.00 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 22.70.
Entire Cation Anion Mass calculated 248.26 111.16 137.09
160
161
[EMIM][Et(Me)PO3]
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Et2(Me)PO3 MIm 100 1.5 90 1965 -68 258
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.90 (d, 3H, J = 16 Hz, 7-H),
1.03 (t, 3H, J = 7 Hz, 6-H), 1.38 (t, 3H, J = 7 Hz, 9-H), 3.63 (m, 2H, 8-
H), 3.89 (s, 3H, 4-H), 4.23 (q, 2H, J = 7 Hz, 5-H), 8.05 (s, 1H, 2-H),
7.94 (s, 1H, 3-H), 10.14 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 14.00 (d, J = 131 Hz, C-7),
15.78 C-6, 17.34 C-9, 35.90 C-4, 44.31 C-5, 58.56 C-8, 122.60 C-3,
124.09 C-2, 138.11 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 17.88.
Entire Cation Anion Mass calculated 234.23 111.16 123.07
162
163
[EMIM][Et(EtPh)PO3]
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Et2(EtPh)PO3 MIm 120 2 93 794 -66 255
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.08 (t, 3H, J = 7 Hz, 6-H),
1.39 (t, 3H, J = 7 Hz, 16-H), 1.57 (m, 2H, 7-H), 2.70 (m, 2H, 8-H),
3.71 (m, 2H, 15-H), 3.88 (s, 3H, 4-H), 4.22 (q, 2H, J = 7 Hz, 5-H),
7.16 (m, 5H, 10-H to 14-H), 7.82 (s, 1H, 2-H), 7.92 (s, 1H, 3-H), 9.86
(s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 15.76 C-6, 17.54 C-16,
29.89 C-7, 31.09 C-8, 36.05 C-4, 44.46 C-5, 58.84 C-15, 122.52 C-3,
124.08 C-2, 125.89 C-12, 128.50 C-10 and C-14, 137.77 C-1, 144.24
C-9.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 19.96.
Entire Cation Anion Mass calculated 324.36 111.16 213.19
164
165
[EMIM][AcN(Me)PO3]
N N
36
54
2
1
8
7
P
OO
O
N
9
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
AcNMe(Me)PO3 EtIm 90 1 82 4494 -63 236 in acetonitrile
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.96 (d, 3H, J = 16 Hz, 7-H),
1.40 (t, 3H, J = 7.41 Hz, 6-H), 3.87 (s, 3H, 4-H), 4.21 (q, 2H, J = 7 Hz,
5-H), 4.50 (d, 2H, J = 10 Hz, 8-H), 7.78 (s, 1H, 2-H), 7.87 (s, 1H, 3-
H), 9.60 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 14.00 (d, J = 132 Hz, C-7),
15.20 C-6, 35.52 C-4, 43.98 C-5, 49.02 C-8, 118.87 C-9, 122.09 C-3,
123.61 C-2, 137.15 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 23.55.
Entire Cation Anion
Mass calculated 245.22 111.16 134.05
166
167
[EMIM][MeAc(Me)PO3]
N N
36
54
2
1
8
7
P
OO
O
O
O
9
10
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
MeAcMe(Me)PO3 EtIm 75 1 86 3304 -56 216
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.91 (d, 3H, J = 15 Hz, 7-H),
1.40 (t, 3H, J = 7 Hz, 6-H), 3.60 (s, 3H, 9-H), 3.87 (s, 3H, 4-H), 4.21
(q, 2H, J = 0.72 Hz, 5-H), 4.24 (d, 2H, J = 0.86 Hz, 8-H), 7.78 (s, 1H,
2-H), 7.87 (s, 1H, 3-H), 9.66 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.80 (d, J = 133 Hz, C-7),
15.27 C-6, 35.49 C-4, 43.96 C-5, 51.31 C-9, 61.02 C-8, 122.15 C-3,
123.67 C-2, 137.40 C-1, 171.26 C-10.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 18.37.
Entire Cation Anion
Mass calculated 278.24 111.16 167.08
168
169
[BMIM][Bu(Me)PO3]
3
6
54
2
1
8
7
10
911
N N P
OO
O
13
12
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Bu2(Me)PO3 MIm 130 2 94 2345 -73 253
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.78 (t, 3H, J = 7.42 Hz, 8-H),
0.84 (t, 3H, J = 7.41 Hz, 13-H), 1.19 (d, 3H, J = 16.07 Hz, 9-H), 1.27
(m, 4H, 7-H and 12-H), 1.47 (m, 2H, 6-H), 1.75 (m, 2H, 11-H), 3.74
(q, 2H, J = 0.66 Hz, 5-H), 3.96 (s, 3H, 4-H), 4.19 (t, 2H, J = 0.74 Hz,
10-H), 7.19 (s, 1H, 2-H), 7.31 (s, 1H, 3-H), 10.75 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.00 (d, J = 133 Hz, C-9),
13.14 C-8, 13.54 C-13, 18.60 C-12, 18.75 C-7, 31.56 C-11, 32.87 C-6,
35.37 C-4, 48.17 C-5, 62.33 C-10, 122.38 C-3, 123.58 C-2, 137.86 C-
1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 22.70.
Entire Cation Anion Mass calculated 290.34 139.22 151.12
170
171
[OMIM][Oc(Me)PO3]
N N P
OO
O3
6
54
2
1
8
7
10
9 11
13
12
17
16
15
14
21
20
19
18
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Oc2(Me)PO3 MIm 150 3 92 2771 -60 258
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.74 (m, 6H, 12-H and 21-H),
1.19 (d, 3H, J = 15.65 Hz, 13-H), 1.13 (m, 20H, 7-H – 11-H and 16-H
to 20-H), 1.48 (m, 2H, 6-H), 1.75 (m, 2H, 15-H), 3.71 (q, 2H, J = 0.70,
Hz 5-H), 3.96 (s, 3H, 4-H), 4.16 (t, 2H, J = 0.74 Hz, 14-H), 7.15 (s,
1H, 2-H), 7.29 (s, 1H, 3-H), 10.92 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.00 (d, J = 132 Hz, C-13),
13.74 C-12, 22.11 C-21, 22.59 (2C, C-11 and C-20), 28.54 – 31.34 (m,
10C, C-6 – C-10 and C-15 – C-19), 35.39 C-4, 48.46 C-5, 62.66 C-14,
122.36 C-3, 123.53 C-2, 137.82 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 22.40.
Entire Cation Anion Mass calculated 402.55 195.32 207.23
172
173
[OMIM][Oc(Ph)PO3]
N N P
OO
O3
6
54
2
1
8
7
10
9 11 13
12
22
21
20
19
26
25
24
18
17
16
15
14
23
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Oc2(Ph)PO3 MIm 150 3 99 1972 -61 281
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.84 (m, 6H, 12-H and 26-H),
1.14 – 1.21 (m, 20H, 7-H to 11H and 21-H to 25-H), 1.35 (m, 2H, 6-H),
1.74 (m, 2H, 20-H), 3.50 (q, 2H, J = 0.66 Hz, 5-H), 3.85 (s, 3H, 4-H),
4.16 (t, 2H, J = 0.74 Hz, 19-H), 7.25 (m, 3H, 15-H –17-H), 7.60 (m,
2H, 14-H and 18-H), 7.79 (s, 1H, 2-H), 7.89 (s, 1H, 3-H), 10.78 (s, 1H,
1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.82 C-12, 22.08 C-26,
25.40 to 31.25 (m, 12C, C-6 to C-11 and C-20 to C-25), 35.46 C-4,
48.55 C-5, 62.89 C-19, 122.30 C-3, 123.49 C-2, 127.05 (m, 2C, C-15
and C-17), 128.3 (m, 2C, C-13 and C-16), 131.08 (m, 2C, C-14 and C-
18), 137.48 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 9.33.
Entire Cation Anion Mass calculated 464.62 195.32 269.30
174
175
[Me(EG)3MIM][Me(EG)3(Me)PO3]
P
OO
O
O
O
O
N N
OO
O
3
54
2
1
6
8
7
9
15 18
13
1214
16
10
17
11
19
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
(Me(EG)3)2(Me)PO3 MIm 140 3 92 8280 -70 243
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.98 (d, 3H, J = 16.10 Hz, 12-
H), 3.22 (m, 6H, 11-H and 19-H), 3,41 (m, 4H, 8-H and 18-H), 3.44 –
3.51 (m, 12H, 7-H – 9-H and 15-H – 17-H), 3.54 (m, 2H, 6-H), 3.70 –
3.80 (m, 4H, 5-H and 14-H), 3.88 (s, 3H, 4-H), 4.38 (t, 2H, J = 0.5 Hz,
13-H), 7.77 (s, 1H, 2-H), 7.80 (s, 1H, 3-H), 9.45 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.00 (d, J = 134 Hz, C-12),
35.51 C-4, 48.50 C-5, 58.01 (2C, C-11 and C-19), 60.0 – 70.70 (m,
10C, C-6 – C-10 and C-14 – C-18), 71.30 C-13, 122.70 C-3, 123.39 C-
2, 137.62 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 20.32.
Entire Cation Anion Mass calculated 470.49 229.30 241.20
176
177
[DodMIM][Dod(Me)PO3]
3
54
2
1
6 8
7 9 15
18
13
12 14 1610
17
11
19
P
OO
O
N N
20
21
24
23
22
29
28
27
26
25
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tm / °C Tdec / °C
Dod2(Me)PO3 MIm 160 3 98 n.a. 60 261
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.83 – 0.89 (m, 9H, 16-H, 17-
H and 29-H), 1.23 (m, 36H, 7-15-H and 20-28-H), 1.43 (m, 2H, 6-H),
1.77 (m, 2H, 19-H), 3.56 (q, 2H, J = 0.66 Hz, 5-H), 3.86 (s, 3H, 4-H),
4.15 (t, 2H, J = 0.70 Hz, 18-H), 7.71 (s, 1H, 2-H), 7.78 (s, 1H, 3-H),
9.41 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.00 (d, J = 132 Hz, C-17),
13.69 (2C, C-16 and C-29), 22.00 (2C, C-15 and C-28), 25.42 – 31.22
(m, 18C, C-6 – C-14 and C-19 – C-27), 35.43 C-4, 48.72 C-5, 62.80 C-
18, 121.96 C-3, 123.27 C-2, 137.57 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 16.97.
Entire Cation Anion Mass calculated 514.76 251.43 263.33
178
179
[EMIM][Me(EG)1(Me)PO3]
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Me(EG)1Me(Me)PO3 EtIm 100 1.5 97 3586 -68 271
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.72 (d, 3H, J = 16 Hz, 7-H),
1.38 (t, 3H, J = 7 Hz, 6-H), 3.18 (s, 3H, 10-H), 3.35 (t, 2H, J = 6 Hz,
9-H), 3.67 (q, 2H, J = 5 Hz, 8-H), 3.88 (s, 3H, 4-H), 4.22 (q, 2H, J = 7
Hz, 5-H), 7.87 (s, 1H, 2-H), 7.99 (s, 1H, 3-H), 10.03 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 14.10 (d, J = 132 Hz, C-7),
15.77 C-6, 35.92 C-4, 44.35 C-5, 58.43 C-10, 62.22 C-9, 72.95 C-8,
122.50 C-3, 124.08 C-2, 138.01 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 17.64.
Entire Cation Anion Mass calculated 264.26 111.16 153.09
180
181
[BMIM][Me(EG)1(Me)PO3]
P
OO
O
ON N
1 7
6
54
32
8
11
10
9
12
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Me(EG)1Me(Me)PO3 BuIm 100 1.5 95 7865 -68 261
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.87 (m, 6H, 8-H, 9-H), 1.23
(m, 2H, 7-H), 1.75 (m, 2H, 6-H), 3.20 (s, 3H, 12-H), 3.36 (t, 2H, J = 5
Hz, 11-H), 3.66 (q, 2H, J = 5 Hz, 5-H), 3.87 (s, 3H, 4-H), 4.18 (t, 2H, J
= 7 Hz, 10-H), 7.81 (s, 1H, 2-H), 7.88 (s, 1H, 3-H), 9.85 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.67 C-8, 14.89 C-9, 19.3
C-7, 31.97 C-6, 36.05 C-4, 48.79 C-5, 58.46 C-12, 62.16 C-11, 72.96
C-10, 122.79 C-3, 124.10 C-2, 138.07 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 17.45.
Entire Cation Anion Mass calculated 292.31 139.22 153.09
182
183
[BMPyrr][Me(EG)1(Me)PO3]
P
OO
O
O
1
7
6
5
43
2
8
11
10
9
12N
13
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Me(EG)1Me(Me)PO3 BMPyrr 100 2 97 8782 -76 283
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.84 (d, 3H, J = 15.25 Hz, 10-
H), 0.91 (t, J = 7.4 Hz, 3H, 9-H), 1.28 (m, 2H, 8-H), 1.65 (m, 2H, 7-H),
2.05 (m, 4H, 2-H and 3-H), 3.00 (s, 3H, 5-H), 3.21 (s, 3H, 13-H), 3.35
(m, 4H, 6-H and 12-H), 3.50 (m, 4H, 1-H and 4-H), 3.36 (m, 2H, 11-
H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.99 C-9, 14.78 C-10,
19.86 C-8, 21.48 C-2 and C-3, 25.51 C-7, 47.63 C-5, 58.48 C-13,
62.14 C-6, 63.61 C-1, C-4 and C-12, 73.02 C-11.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 17.15.
Entire Cation Anion Mass calculated 295.36 142.26 153.09
184
185
[EMIM][Me(EG)2(Me)PO3]
P
OO
O
O
ON N
1
6
54
32
7
8
9 10
1211
Phosphonate
ester Amine T / °C t / d Yield / % H2O content /
ppm Tg / °C Tdec / °C
Me(EG)2Me(Me)PO3 EtIm 100 2 93 3734 -67 265
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.92 (d, 3H, J = 16 Hz, 7-H),
1.38 (t, 3H, J = 7 Hz, 6-H), 3.18 (s, 3H, 12-H), 3.36 (m, 2H, 9-H), 3.44
(m, 4H, 10-H and 11-H), 3.70 (q, 2H, J = 6 Hz, 8-H), 3.88 (s, 3H, 4-H),
4.23 (q, 2H, J = 7 Hz, 5-H), 7.90 (s, 1H, 2-H), 8.01 (s, 1H, 3-H), 10.03
(s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 14.00 (d, J = 131 Hz, C-7),
15.73 C-6, 35.92 C-4, 44.36 C-5, 58.51 C-12, 62.62 C-11, 70.00 C-10
71.43 C-9, 71.80 C-8, 122.58 C-3, 124.08 C-2, 137.97 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 18.43.
Entire Cation Anion Mass calculated 308.31 111.16 197.15
186
187
[EMIM][Me(EG)3(Me)PO3]
P
OO
O
O
OON N
1
6
54
32
7
8
9 10
1211
13 14
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
Me(EG)3Me(Me)PO3 EtIm 130 2 92 3447 -71 266
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.92 (d, 3H, J = 15 Hz, 7-H),
1.37 (t, 3H, J = 7 Hz, 6-H), 3.18 (s, 3H, 14-H), 3.41 (m, 10H, 9-H to
13-H), 3.71 (q, 2H, J = 6 Hz, 8-H), 3.90 (s, 3H, 4-H), 4.24 (q, 2H, J =
7 Hz, 5-H), 7.94 (s, 1H, 2-H), 8.06 (s, 1H, 3-H), 10.12 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 14.00 (d, J = 131 Hz, C-7),
15.72 C-6, 35.86 C-4, 44.33 C-5, 58.48 C-14, 62.62 C-11, 70.23 C-10
and C-12, 71.44 C-13, 71.78 C-8, 122.61 C-3, 124.09 C-2, 138.06 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 18.37.
Entire Cation Anion Mass calculated 352.36 111.16 241.20
188
189
[EMIM]2[(Me)PO3BuSO3]
N N
1
6
54
32
7
8
9
10P
O O
O
S
O
O
O
2
Phosphonate ester
Amine T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
[Me(Me)PO3BuSO3]- EtIm 130 2 98 1208 -44 285
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 0.88 (d, 3H, J = 15 Hz, H-7)
1.38 (t, 6H, J = 7 Hz, 6-H), 1.73 (m, 2H, 9-H), 2.41 (m, 2H, 10-H),
3.61 (q, 2H, J = 6 Hz, 8-H), 3.86 (s, 6H, 4-H), 4.20 (q, 4H, J = 7 Hz, 5-
H), 7.79 (s, 2H, 2-H), 7.88 (s, 2H, 3-H), 9.64 (s, 2H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 14.13 (d, J = 131 Hz, C-7),
15.72 C-6, 27.85 C-9, 36.07 C-4, 44.48 C-5, 49.17 C-10, 62.63 C-8,
122.53 C-3, 124.08 C-2, 137.50 C-1.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 17.64.
Entire Cation Anion Mass calculated 438.47 111.16 216.15
190
191
[EMIM][HAc(Me)PO3]
N N
36
54
2
1
8
7
P
OO
O
OH
O
9
Phosphonate ester
T / °C t / d Yield / % H2O content / ppm
Tg / °C Tdec / °C
[MeAc(Me)PO3]- H2O 100 0.5 87 7219 -43 249
1H-NMR (DMSO-d6, 400 MHz, ppm): δ = 1.10 (d, 3H, J = 15 Hz, 7-H),
1.40 (t, 3H, J = 7 Hz, 6-H), 3.84 (s, 3H, 4-H), 4.18 (m, 4H, 5-H and 8-
H), 7.71 (s, 1H, 2-H), 7.79 (s, 1H, 3-H), 9.23 (s, 1H, 1-H).
13C-NMR (DMSO-d6, 100.4 MHz, ppm): δ = 13.00 (d, J = 136 Hz, C-7),
15.65 C-6, 36.14 C-4, 44.57 C-5, 63.06 C-8, 122.48 C-3, 124.25 C-2,
137.06 C-1, 172.62 C-9.
31P-NMR (DMSO-d6, 162 MHz, ppm): δ = 26.06.
Entire Cation Anion
Mass calculated 264.22 111.16 153.05
192
193
6.1.5. Preparation of cellulose ionogels.
The preparation of cellulose ionogels was carried out by adding finelly
powdered microcrystalline cellulose under stirring and heating to the
respective ionic liquid until the material disappeared and visually
transparent mixtures were obtained. Finally, viscous clear solutions
were allowed to cool for gelling. The ionogels were stored under Ar at
room temperature.
6.2. Determination of the crucible surface
Figure 58: Cross-section of the crucible.
퐴 = 13.9mm ∗휋 ∗ (12.4푚푚 + 8.8푚푚)
2 + 휋 ∗ (4.4푚푚) = 5.23푐푚
6.3. Derivation of complex capacitance
푍 = 푅 +1푖휔퐶 = 푍 − 푖푍"
with 푖 ≡ √−1
for consideration of the impedance of the capacitive part, R=0 and
푍 =1푖휔퐶 = 푍 − 푖푍"
퐶 =1푖휔푍 =
1푖휔(푍 − 푖푍")
Using the absolute value of the impedance vector |푍(휔)| =
(푍 ) + (푍") (see also Equation 14)
194
퐶 =푍 + 푖푍"푖휔|Z| =
푍푖휔|Z| +
푖푍"푖휔|Z| = −
푖푍휔|Z| +
푍"휔|Z|
with 퐶′ = "| |
and 퐶" =| |
the Equation 16: 퐶(휔) = 퐶 (휔) − 푖퐶"(휔) is
obtained.
7. References
[1] U. Bardi, Front. Energy Res, 29 August 2013, doi: 10.3389/fenrg.2013.00002.
[2] A. D. Graves, J. Electroanal. Chem., 25, 1970, 349-356; A. D. Graves, D. Inman, J. Electroanal. Chem., 25, 1970, 357-372; F. H. Hurley, T. P. Wier, J. Electrochem. Soc., 98, 1951, 203-206.
[3] Ionic Liquids in Synthesis, Eds. P. Wasserscheid and T. Welton, 2007, Wiley, Weinheim.
[4] H. Xie, A. King, I. Kilpelainen, M. Granstrom, D. S. Argyropoulos, Biomacro-molecules, 8, 2007, 3740.
[5] D. S. Argyropoulos, H. Xie, World Patent, WO/2008/098037, 2008; T. Arioli, L. Peng, A. S. Betzner, J. Burn, W. Wittke, W.Herth, C. Camilleri, H. Hofte, J. Plazinski, R. Birch, A. Cork, J. Glover, J. Redmond, R. E. Williamson, Science, 279, 1998, 717.
[6] A.Michaelis, R. Kaehne, Berichte der Deutschen Chemischen Gesellschaft, 31, 1898, 1048-55.
[7] A. E. Arbuzov, Diss., 1905, St. Petersburg [8] W. Gerrard, J. Chem. Soc., 1944, 85; W. Gerrard, M. J. D. Isaacs, G.
Machell, K. B. Smith, P. L. Wyvill, J. Chem. Soc., 1953, 1920. [9] A. E. Arbuzov, A. A.Dunin, Zhurnal Russkago Fiziko-Khimicheskogo
Obshchestva, 46, 1914, 295-302. [10] L. Horner, H. Hoffmann, H. G. Wippel, G. Klahre, Chem. Ber., 92, 1959,
2499; W. S. Wadsworth Jr., W. D. Emmons, J. Am. Chem. Soc., 83, 1961, 1733
[11] E. J. Corey, G. T. Kwiatkowski, J. Am. Chem. Soc., 88, 1966, 5654. [12] M.D.M. Gray, D. J.H. Smith, Tetrahedron Lett., 21, 1980, 859-860. [13] Modern Phosphonate Chemistry, Eds. P. Savignac, B. Iorga, 2003, CRC Press
LLC, Boca Raton. [14] K. D. Troev, Chemistry and Application of H-Phosphonates, 2006, Elsevier,
Amsterdam. [15] K. D. Troev, Polyphosphoesters: Chemistry and Application, 2012, Elsevier,
London. [16] Ger. Pat. 1,078,558, 1961; C. A., 55, 14308d, 1961. [17] USSR Pat. 127649, 1960; Ref. Zh. Khim., 13, 1961, 38; K. Petrov, E.
Nifantiev, R. Goltsova, D. Stegolev, B. Bushmin, Zh. Obshch. Khim., 32, 1962, 3723.
195
[18] Brit. Pat. 699,154, 1953; C.A. 49, 6301, 1955; U.S. Pat. 2,670,368, 1954; C. A. 49,2483, 1955; Brit. Pat. 1,298,156, 1972; Ger. Pat. 1,668,031, 1973.
[19] K. Petrov, E. Nifantief, R. Goltsova, M. Belovetsev, S. Korneev, Zh. Obshch. Khim., 32, 1962, 1277.
[20] Ger. Pat. 1,078,136, 1960; C. A., 56, 3423c, 1962; U.S. Pat. 3,036,109, 1962; C. A., 57, 13612g, 1962.
[21] Ger. Pat. DE 4,121,696, 1991; C. A., 118, 234241m, 1993. [22] Brit. Pat. 841671, 1960; C.A., 55, 3433g, 1961; Ger. Pat. 1059425, 1959;
C. A., 56, 11446d, 1962. [23] G. Imaev, V. Masslennikov, V. Gorina, O. Krasheninikova, Zh. Obshch. Khim.,
35, 1965, 75., Ger. Pat. 1,059,425, 1959; C. A., 56, 11446d, 1962; A. DeRose, W. Gerrard, E. Moonly, Chem.Ind., 36, 1961, 1449; K. Troev, G. Borisov, Izv. Otd. Khim Nauki, Bulg. Acad. Sci., 17, 1984, 482.
[24] M. Kluba, A. Zwierzak, Synthesis, 2, 1978, 134-137. [25] A. Zwierzak, M. Kluba, Tetrahedron, 29, 1973, 1089-1094. [26] K. Troev, G. Borissov, Phosphorus & Sulfur, 29, 1987, 129; K. Troev, D. M.
Roundhill, Phosphorus & Sulfur, 37, 1988, 243; K. Troev, D. M. Roundhill, Phosphorus & Sulfur, 37, 1988, 247.
[27] M. Grayson, C. F. Fareley, C. A. Strenli, Tetrahedron, 23, 1967, 1065; G. Doak, L. Freedman, Chem. Rev., 61, 1961, 31.
[28] W. Vogt, S. Balasubramanian, Macromol. Chem., 163, 1973, 111. [29] T. Mastrukova, M. Kabachnik, Uspehi Khimii, 38, 1969, 1751. [30] V. Vassileva, G. Lihina, Izv. Tomsk. Politech. Inst., 1976, 258, 83. [31] I. Ugi, D. Marquarding, H. Kulasacek, P. Gillespie, F. Ramirez, Acc. Chem.
Res., 4, 1971, 288; R. S. Berry, J. Chem. Phys., 32, 1960, 933; K. Mislow, Acc. Chem. Res., 3, 1970, 321.
[32] A. Kong, R. Engel, Bull. Chem. Soc. Jpn., 58, 1985, 3671. [33] K. Troev, E. M. G. Kirilov, D. M. Roundhill, Bull. Chem. Soc. Jpn., 1990, 63,
1284. [34] E. M. Georgiev, J. Kaneti, K. Troev, D. M. Roundhill, J. Am. Chem. Soc.,
1993, 115, 10678. [35] G. Steinberg, J. Org. Chem., 1950, 15, 637. [36] U. S. Pat. 2,815,345, 1957; C. A., 52, 5485a, 1958; U. S. Pat. 2,842,111,
1958; C. A., 52, 11110f, 1958; N. Thoung, Bull. Soc. Chem. France, 1971, 3, 928.
[37] J. Mircheva, D. Trendafilova-Gercheva, M. Georgieva, K. Troev, V. Vassileva, D. Dabov, Pharmacia, 43, 1995, 17; E. M. Georgiev, R. Tzevi, V. Vassileva, K. Troev, D. M. Roundhill, Phosphorus, Sulfur and Silicon, 88, 1994, 139; V. Vassileva, E. M. Georgiev, K. Troev, D. M. Roundhill, Phosphorus, Sulfur and Silicon, 92, 1994, 101.
[38] D. E. Bryant, C. Kilner, T. P. Kee, Inorg. Chim. Acta, 362, 2009, 614–616. [39] D. Trendafilova-Gercheva, K. Troev, M. Georgieva, Bull. Chem. Soc. Jpn., 64,
1991, 2033.
196
[40] K. Troev, D. M. Roundhill, Phosphorus and Sulfur, 36, 1988, 189. [41] E. M. Georgiev, J. Kaneti, K. Troev, D. M. Roundhill, J. Am. Chem. Soc., 115,
1993, 10964. [42] J. C. Brosse, L. Fontaine, D. Derouet, S. Chairatanathvorn, Makromol. Chem.,
190, 1989, 2329. [43] J. Wang, P. C. Zhang, H. F. Lu, N. Ma, S. Wang, H. Q. Mao, K. W. Leong, J.
Controlled Release, 83, 2002, 157. [44] S. Penczek, T. Biela, P. Klosinski, G. Lapienis, Makromol. Chem., Macromol.
Symp., 6, 1986, 123. [45] E. Bezdushna, H. Ritter, K. Troev, Macromol. Rapid Comm., 26, 2005, 471. [46] S. W. Kim, Y. H. Bac, T. Okano, Pharmacol. Res., 81, 1992, 85; J. Wang, R.
Zhuo, Eur. Polym. J., 35, 1999, 491. [47] P. Walden, Bull. Acad. Imper. Sci. St. Petersbourg, 8, 1914, 405. [48] J. v. Braun, Ber. Dt. Chem. Ges., 33, 1900, 1438. [49] D. J. Adams, P. J. Dyson, S. J. Tavener, Chemistry in Alternative Reaction
Media; 2004, Wiley: Chicester, U.K., F. M. Kerton, Alternative Solvents for Green Chemistry; 2009, Royal Society of Chemistry: Cambridge, U.K., W. Leitner, P. G. Jessop, C.-J. Li in Handbook of Green Chemistry — Green Solvents; Eds. P. Wasserscheid, A. Stark, 2010, Wiley-VCH: Weinheim, Germany, Vol. 6.
[50] http://ilthermo.boulder.nist.gov/ILThermo/ [51] http://www.basf.com [52] R. Wagner, N. Preschitschek, S. Passerini, J. Leker, M. Winter, J Appl.
Electrochem., 43, 2013,481–496. [53] S. A. Shamsi, N. D. Danielson, J. Sep. Sci., 30, 2007, 1729; D. Bankmann,
R. Giernoth, Prog. Nucl. Magn. Reson. Spectrosc., 51, 2007, 63; S. Pandey, Anal. Chim. Acta, 556, 2006, 38; M. Koel, Crit. Rev. Anal. Chem., 35, 2005, 177.
[54] F. van Rantwijk, R. A. Sheldon, Chem. Rev., 107, 2007, 2757; U. Kragl, M. Eckstein, N. Kaftzik, Curr. Opin. Biotechnol., 13, 2002, 565.
[55] H. Zhao, Chem. Eng. Commun., 193, 2006, 1660; A. Riisager, R. Fehrmann, M. Haumann, P. Wasserscheid, Top. Catal., 40, 2006, 91; S. Werner, M. Haumann, P. Wasserscheid, Annu. Rev. Chem. Biomol. Eng., 1, 2010, 203–230.
[56] D. Zhao, Z. Fei, C. A. Ohlin, G. Laurenczy, P. J. Dyson, Chem. Commun., 2004, 2500.
[57] S. Himmler, S. Hoermann, R. van Hal, P. S. Schulz P. Wasserscheid, Green Chem., 8, 2006, 887.
[58] R. E. Del Sesto, C. Corley, A. Robertson, J. S. Wilkes, J. Organomet. Chem., 690, 2005, 2536.
[59] E. Kuhlmann, S. Himmler, H. Giebelhaus, P. Wasserscheid, Green Chem., 9, 2007, 233.
[60] H.-P. Nguyen and M. Baboulene, US Patent, 2010/0121075 A1, 2010.
197
[61] H. Ohno, Y. Fukaya, Chem. Lett., 1, 2009, 2; M. Abe, Y. Fukaya, H. Ohno, Green Chem., 12, 2010, 1274.
[62] Y. Fukaya, K. Hayashi, M. Wada, H. Ohno, Green Chem., 10, 2008, 44; Y. Fukaya, A. Tsukamoto, K. Koruda, H. Ohno, Chem. Commun., 47, 2011, 1994.
[63] J. R. Morrow, L. A. Buttrey, K. A. Berback, Inorg. Chem. 31, 1992, 16-20. [64] B. Zhao, L. Greiner and W. Leitner, Chem. Commun., 47, 2011, 2973. [65] Lehrbuch der Physikalischen Chemie, Ed. G. Wedler, 5. Edition, 2004, Wiley-
VCH. [66] Organische Chemie, Eds. H. Butenschoen, K. P. C. Vollhardt, N. E. Schore, 5.
Edition, 2011, Wiley-VCH. [67] C. M. Jaeger, T. Schmaltz, Michael Novak, A. Khassanov, A. Vorobiev,M.
Hennemann, A. Krause, H. Dietrich, D. Zahn, A. Hirsch, M. Halik, T. Clark, J. Am. Chem. Soc., 135, 2013, 4893-4900.
[68] N. Taccardi, D. Assenbaum, M. E. M. Berger, A. Boesmann, F. Enzenberger, R. Woelfel, S. Neuendorf, V. Goeke, N. Schoedel, H.-J. Maass, H. Kistenmacher, P. Wasserscheid, Green Chem., 12, 2010, 1150.
[69] A. C. Cole, J. L. Jensen, J. Ntai, K. L. T. Tran, K. J. Weaver, D. C. Forbes, J. H. Davis, J. Am. Chem. Soc., 124, 2002, 5962; G. Y. Zhu, R. Wang, G. Hua Liu, L. Q. Xu, B. Zhang, X. Q. Wu, Chin. Chem. Lett., 18, 2007, 633.
[70] N. Paape, W. Wei, A. Boesmann, C. Kolbeck, F. Maier, H.-P. Steinrueck, P. Wasserscheid, P. S. Schulz, Chem. Commun., 2008, 3867.
[71] D. W. Roberts, D. L. Williams, Tetrahedron, 43, 1987, 1027-1062. [72] P. Wasserscheid, B. Drießen-Hoelscher, R. van Hal, H. C. Steffens, J.
Zimmermann, Chem. Commun., 2003, 2038-2039. [73] K. Fukumoto, M. Yoshizawa, H. Ohno, J. Am. Chem. Soc., 127, 2005, 2398. [74] J. H. Davis, Jr. in Ionic Liquids in Synthesis, Eds. P. Wasserscheid and T.
Welton, 2007, Wiley, Weinheim. [75] Ionic Liquids UnCOILed: Critical Expert Overviews, Eds. K. R. Seddon and N.
V. Plechkova, 2013, John Wiley & Sons. [76] Green solvents II, Properties and Applications of Ionic Liquids, Eds. A.
Mohammad, Inamuddin, 2012, Springer. [77] A. W. Taylor, K. R. J. Lovelock, A. Deyko, P. Licence, R. G. Jones, Phys.
Chem. Chem. Phys., 12, 2010, 1772-1783. [78] H. Olivier-Bourbigou, L. Magna, D. Morvan, Appl. Catalysis A: General, 373,
2010, 1–56. [79] A. E. Visser, R. P. Swatloski, R. D. Rogers, Green Chem., 2, 2000, 1-4; G. Cui,
C. Wang, J. Zheng, Y. Guo, X. Luo, H. Li, Chem. Commun., 48, 2012, 2633–2635.
[80] B. Li, L. Wang, B. Kang, P. Wang, Y. Qiu, Solar Energy Mat. Solar Cells, 90, 2006, 549–573.
[81] Z.-B. Zhou, H. Matsumoto, K. Tatsumi, Chem. Eur. J., 10, 2004, 6581; Z.-B. Zhou, H. Matsumoto, K. Tatsumi, Chem. Eur. J., 11, 2005, 752.
198
[82] C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki, J. F. Brennecke, J. Chem. Eng. Data, 49, 2004, 954–964.
[83] S. Wilkes, J. A. Levisky, R. A. Wilson, C. L. Hussey, Inorg. Chem., 21, 1982, 1263; A. A. Fannin Jr., D. A. Floreani, L. A. King, J. S. Landers, B. J. Piersma, D. J. Stech, R. J. Vaughn, J. S. Wilkes, J. L. Williams, J. Phys. Chem., 88, 1984, 2614.
[84] H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan, M. Watanabe, J. Phys. Chem. B, 109, 2005, 6103–6110.
[85] A. P. Froeba, H. Kremer, A. J. Leipertz, Phys. Chem. B, 112, 2008, 12420–12430.
[86] B. Hasse, J. Lehmann, D. Assenbaum, P. Wasserscheid, A. Leipertz, A. P. Froeba, J. Chem. Eng. Data, 54, 2009, 2576–2583.
[87] K. R. Seddon, A. Stark, M. J. Torres, Pure Appl. Chem., 72, 2000, 2275; J. A. Widegren, A. Laesecke, J. W. Magee, Chem. Comm., 2005, 1610.
[88] W. Lu, A. G. Fadeev, B. Qi, E. Smela, B. R. Mattes, J. Ding, G. M. Spinks, J. Mazurkiewicz, D. Zhou, G. G. Wallace, Science, 297, 2002, 983-987; A. Fernicola, B. Scrosati, H. Ohno, Ionics, 12, 2006, 95-102.
[89] J. Le Bideau, L. Viau, A. Vioux, Chem. Soc. Rev., 40, 2011, 907–925. [90] P. Vidinha, N. M. T. Lourenco, C. Pinheiro, A. R. Bras, T. Carvalho, T. Santos-
Silva, A. Mukhopadhyay, M. J. Romao, J. Parola, M. Dionisio, J. M. S. Cabral, C. A. M. Afonso, S. Barreiros, Chem. Commun., 2008, 5842; T. Carvalho, V. Augusto, A. R. Bras, N. M. T. Lourenco, C. A. M. Afonso, S. Barreiros, N. T. Correia, P. Vidinha, E. J. Cabrita, C. J. Dias, M. Dionisio, B. Roling, J. Phys. Chem. B., 116, 2012, 2664-2676.
[91] K. Hanabusa, H. Fukui, M. Suzuki, H. Shirai, Langmuir, 21, 2005, 10383; W. Kubo, S. Kambe, S. Nakade, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, J. Phys. Chem. B, 107, 2003, 4374; K. Hanabusa, K. Hiratsuka, M. Kimura, H. Shirai, Chem. Mater., 11, 1999, 649; N. Mohmeyer, P. Wang, H. W. Schmidt, S. M. Zakeerudin, M. Graetzel, J. Mater. Chem., 14, 2004, 1905.
[92] N. Winterton, J. Mater. Chem., 16, 2006, 4281; M. A. B. H. Susan, T. Kaneko, A. Noda, M. Watanabe, J. Am. Chem. Soc., 127, 2005, 4976.
[93] P. Izak, S. Hovorka, T. Bartovsky, L. Bartovska, J. G. Crespo, J. Membr. Sci., 296, 2007, 131.
[94] B. Ziolkowski, K. Bleek, B. Twamley, .K. J. Fraser, R. Byrne, D. Diamond, A. Taubert, Eur. J. Inorg. Chem., 32, 2012, 5245–5251.
[95] S. Shimano, H. Zhou, I. Honma, Chem Mater., 19, 2007, 5216; Y. Hanyu, I. Honma, Sci. Rep. 2, 2012, Article number: 453.
[96] T. Fukushima, A. Kosaka, Y. Ishimura, T. Yamamoto, T. Takigawa, N. Ishii, T. Aida, Science, 300, 2003, 2072-2074; T. Fukushima, K. Asaka, A. Kosaka,T. Aida, Angew. Chem. Int. Ed., 44, 2005, 2410–2413; T. Fukushima, A. Kosaka, Y. Yamamoto, T. Aimiya, S. Notazawa, T. Takigawa, T. Inabe, T. Aida, Small, 2, 2006, 554–560; J. Lee, T. Aida, Chem Comm, 47, 2011, 6757-6762.
199
[97] M. P. Scott, M. Rahman, C. S. Brazel, Eur. Polym. J., 39, 2003, 1947; P. G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem. Int. Ed., 47, 2008, 2930; J. Lu, F. Yan, J. Texter, Prog. Polym. Sci., 34, 2009, 431.
[98] Y. He, T. P. Lodge, Macromolecules, 41, 2008, 167. [99] T. P. Lodge, Science, 321, 2008, 50. [100] J. H. Cho, J. Lee, Y. He, B. Kim, T. P. Lodge, C. D. Frisbie, Adv. Mater., 20,
2008, 686; S. Zhang, K. H. Lee, C. D. Frisbie, T. P. Lodge, Macromolecules, 44, 2011, 940-949.
[101] K. H. Lee, S. Zhang, T. P. Lodge, C. D. Frisbie, J. Phys. Chem. B, 115, 2011, 3315–3321.
[102] F. W. Lichtenthaler, In Methods and Reagents for Green Chemistry: An Introduction; P. Tundo, A. Perosa, F. Zecchini, Eds.; John Wiley and Sons, Inc., New York, 2007.
[103] R. P. Swatloski, S. K. Spear, J. D. Holbrey, R. D. Rogers, J. Am. Chem. Soc., 124, 2002, 4974–4975.
[104] R. C. Remsing, R. P. Swatloski, R. D. Rogers, G.Moyna, Chem. Comm., 2006, 1271–1273.
[105] Y. Fukaya, A. Sugimoto, H. Ohno, Biomacromolecules, 7, 2006, 3295–3297. [106] D. Zhaoa, H. Li, J. Zhang, L. Fu, M, Liu, J. Fu, P. Ren, Carbohydrate
Polymers, 87, 2012, 1490–1494. [107] C. Froschauer, K. Wurst, G. Laus, H. K. Weber, H. Z. Schottenberger,
Kristallogr., 227, 2012, 21−23; M. Hummel, C. Froschauer, G.Laus, T. Roeder, H. Kopacka, L. Hauru, H. K. Weber, H Sixta, H. Schottenberger, Green Chem., 13, 2011, 2507−2517; C. Froschauer, M. Hummel, G. Laus, H. Schottenberger, H. Sixta, H. K. Weber, G. Zuckerstaetter, Biomacro-molecules, 13, 2012, 1973–1980.
[108] H. Zhang, J. Wu, J. Zhang, J. He, Macromolecules, 38, 2005, 8272; L. Feng, Z.-l. Chen, J. Mol. Liq., 142, 2008, 1.
[109] H. Du, X. Qian, Carbohydrate Research, 346, 2011, 1985–1990. [110] A. Pinkert, K. N. Marsh, S. Pang, Ind. Eng. Chem. Res., 49, 2010, 11121–
11130; A. Pinkert, K. N. Marsh, S. Pang, Ind. Eng. Chem. Res., 49, 2010, 11809-11813.
[111] H. Zhao, G. A. Baker, Z. Y. Song, O. Olubajo, T. Crittle, D. Peters, Green Chem., 10, 2008, 696–705.
[112] N. Kimizuka, T. Nakashima, Langmuir, 17, 2001, 6759. [113] Q. Liu, M. H. A. Janssen, F. van Rantwijk, R. A. Sheldon, Green. Chem., 7,
2005, 39–42. [114] M. Gericke, K. Schlufter, T. Liebert, T. Heinze, T. Budtova, Biomacro-
molecules, 10, 2009, 1188–1194. [115] S. J. Haward, V. Sharma, C. P. Butts, G. H. McKinley, S. S. Rahatekar, Bio-
macromolecules, 13, 2012, 1688−1699. [116] X. Duan, J. Xu, B. He, J. Li, Y. Sun, BioResources, 6, 2011, 4640-4651. [117] J. Kadokawa, M. Murakami, Y. Kaneko, Carbohydr. Res., 343, 2008, 769–
772.
200
[118] V. Finkenstadt, J. L. Willett, Macromol. Symp., 227, 2005, 367–371. [119] W. Ning, Z. Xingxiang, L. Haihui, H. Benqiao, Carbohydrate Polymers, 76,
2009, 482–484. [120] P. K. Singh, B. Bhattacharya, R. K. Nagarale, K.-W. Kim, H.-W. Rhee,
Synthetic Metals, 160, 2010, 139–142. [121] T. Singh, T. J. Trivedi, A. Kumar, Green Chem., 12, 2010, 1029–1035. [122] M. Mazza, D.-A. Catana, C. Vaca-Garcia, C. Cecutti, Cellulose, 16, 2009,
207-215. [123] E. Gileadi, Physical Electrochemistry, Fundamentals, Techniques and
Applications, 2011, Wiley-VCH, Weinheim. [124] B. Roling, M. Drüschler, Electrochim. Acta, 76, 2012, 526–528. [125] A. J. Bard, L. R. Faulkner, Electrochemical methods: fundamentals and
applications, 2nd ed., 2001, John Wiley. [126] C. H. Hamann, A. Hamnett, W. Vielstich, Electrochemistry, 2nd ed., 2007,
Wiley-VCH, Weinheim. [127] J. P. Zheng, C. M. Pettit, P. C. Goonetilleke, G. M. Zenger, D. Roy, Talanta,
78, 2009, 1056–1062. [128] A. A. J. Torriero, A. I. Siriwardana, A. M. Bond, I. M. Burgar, N. F. Dunlop,
G. B. Deacon, D. R. MacFarlane, J. Phys. Chem. B, 113, 2009, 11222–11231.
[129] E. Barsoukov, J. R. Macdonald, Impedance Spectroscopy: Theory, Experiment, and Applications, 2nd ed., 2005, Wiley.
[130] NOVA Impedance spectroscopy tutorial, http://www.metrohm-autolab.com/export/Homepages/Autolab/download/NovaTutorials/Impedance_measurements_tutorial.pdf, downloaded on 13.02.2013.
[131] A. Lasia, Modern Aspects of Electrochemisty (Eds. B. E. Conway, J. O’M. Bockris, R. E. White), 32, Plenum Press, New York, 1999.
[132] G. J. Brug, A. L. G. Van Den Eeden, M. Sluyters-Rehbach, J. H. Sluyters, J. Electroanal. Chem. 176, 1984, 275–295.
[133] T. Pajkossy, J. Electroanal. Chem, 364, 1994, 111-125; T.Pajkossy, Solid State Ionics, 176, 2005, 1997.
[134] J. R. Macdonald, Solid State Ionics, 13, 1984, 147-149. [135] M. E. Orazem, B. Tribollet, Electrochemical Impedance Spectroscopy, Wiley,
2008. [136] P. Zoltowski, J. Electroanal. Chem., 443, 1998, 149–154. [137] F. Silva, C. Gomes, M. Figueiredo, R. Costa, A. Martins, C. M. Pereira, J.
Electroanal. Chem., 622, 2008, 153–160. [138] S. Chechirlian, P. Eichner, M. Keddam, H. Takenouti, H. Mazille, Elecfrochim.
Acta, Vol. 35, No. 7, 1990, 1125-1131. [139] L. G. Lin, Y. Wang, J. W. Yan, Y. Z. Yuan, J. Xiang, B. W. Mao, Electrochem.
Commun., 2003, 5, 995. [140] R. Atkin, S. Z. El Abedin, R. Hayes, L. H. S. Gasparotto, N. Borisenko, F.
Endres, J. Phys. Chem. C, 113, 2009, 13266.
201
[141] Y. Z. Su, Y. C. Fu, J. W. Yan, Z. B. Chen, B. W. Mao, Angew. Chem., 121, 2009, 5250.
[142] R. Atkin, N. Borisenko, M. Drüschler, S. Z. El Abedin, F. Endres, R. Hayes, B. Huber, B. Roling, Phys. Chem. Chem. Phys., 13, 2011, 6849.
[143] V. O. Santos Jr., M. B. Alves, M. S. Carvalho, P. A. Z. Suarez, J. C. Rubim, J. Phys. Chem. B, 110, 2006, 20379.
[144] Y. X. Yuan, T. C. Niu, M. M. Xu, J. L. Yao, R. A. Gu, J. Raman Spectrosc., 41, 2009, 516.
[145] V. Lockett, R. Sedev, J. Ralston, M. Horne, T. Rodopoulos, J. Phys. Chem. C, 112, 2008, 7486.
[146] J. P. Zheng, P. C. Goonetilleke, C. M. Pettit, D. Roy, Talanta, 81, 2010, 1045.
[147] M. Gnahm, T. Pajkossy, D. M. Kolb, Electrochim. Acta, 55, 2010, 6212. [148] M. Drüschler, B. Huber, S. Passerini, B. Roling, J. Phys. Chem. C, 114,
2010, 3614. [149] L. Siinor, K. Lust, E. Lust, J. Electrochem. Soc., 157, 2010, F83. [150] R. Costa, C. M. Pereira, F. Silva, Phys. Chem. Chem. Phys., 12, 2010,
11125. [151] N. Ignat’ev, U. Welz-Biermann, A. Kucheryna, G. Bissky, H. Willner, J.
Fluorine Chem., 126, 2005, 1150 –1159. [152] R. T. Gore, T. Bond, W. Zhang, R. W. J. Scott, I. Burgess, Electrochem.
Commun., 12, 2010, 1340. [153] V. Lockett, M. Horne, R. Sedev, T. Rodopoulos, J. Ralston, Phys. Chem.
Chem. Phys., 12, 2010, 12499. [154] T. Pajkossy, Pure Appl. Chem., 83, 2011, 259. [155] M. Drüschler, B. Huber, B. Roling, J. Phys. Chem. C, 115, 2011, 6802. [156] T. Pajkossy, D. M. Kolb, Electrochem. Commun., 13, 2011, 284. [157] M. Gnahm, C. Mueller, R. Répánszki, T. Pajkossy, D. M. Kolb, Phys. Chem.
Chem. Phys., 13, 2011, 11627. [158] B. Roling, M. Drüschler, B. Huber, Faraday Discuss., 154, 2012, 303. [159] Y. Z. Su, Y. C. Fu, Y. M. Wie, J. W. Yan, B. W. Mao, ChemPhysChem, 11,
2010, 2764. [160] M. V. Fedorov, A. A. Kornyshev, Chem. Rev., 114, 2014, 2978 – 3036. [161] M. V. Fedorov, A. A. Kornyshev, Electrochim. Acta, 53, 2008, 6835. [162] M. V. Fedorov, A. A. Kornyshev, J. Phys. Chem. B Lett., 112, 2008, 11868. [163] M. V. Fedorov, N. Georgi and A. A. Kornyshev, Electrochem. Commun., 12,
2010, 296. [164] J. Vatamanu, O. Borodin, G. D. Smith, J. Am. Chem. Soc., 132, 2010,
14825. [165] N. Georgi, A. A. Kornyshev, M. V. Fedorov, J. Electroanal. Chem., 649,
2010, 261. [166] C. Nanjundiah, S. F. McDevitt, V. R. Koch, J. Electrochem. Soc., 144, 1997,
3392.
202
[167] M. T. Alam, M. M. Islam, T. Okajima, T. Ohsaka, Electrochem. Commun., 9, 2007, 2370.
[168] M. T. Alam, M. M. Islam, T. Okajima, T. Ohsaka, J. Phys. Chem. C, 111, 2007, 18326.
[169] M. M. Islam, M. T. Alam and T. Ohsaka, J. Phys. Chem. C, 112, 2008, 16568.
[170] M.T. Alam, J. Masud, M.M. Islam, T. Okajima, T. Ohsaka, J. Phys. Chem. C, 115, 2011, 19797–19804.
[171] S. Baldelli, J. Phys. Chem. B, 109, 2005, 13049. [172] M. Galinski, S. R. Kraewski, Bulg. Chem. Commun., 38, 2006, 192. [173] R. J. Gale and R. A. Osteryoung, Electrochim. Acta, 25, 1980, 1527. [174] M. Drüschler, N. Borisenko, J. Wallauer, C. Winter, B. Huber, F. Endres, B.
Roling, Phys. Chem. Chem. Phys. 14, 2012, 5090–5099. [175] L. Siinor, C. Siimenson, V. Ivaništšev, K. Lust, E. Lust, J. Electroanal. Chem.
668, 2012, 30–36. [176] L. Siinor, K. Lust, E. Lust, Electrochem. Commun. 12, 2010, 1058–1061. [177] L. Siinor, K. Lust, E. Lust, ECS Trans. 16, 2009, 559–567. [178] L. Siinor, R. Arendi, K. Lust, E. Lust, J. Electroanal. Chem., 689, 2013, 51–
56. [179] M. Zistler, P. Wachter, C. Schreiner, M. Fleischmann, D. Gerhard, P.
Wasserscheid, A. Hinsch, H. J. Gores, J. Electrochem. Soc., 154, 2007, B925.
[180] A. D. Graves, D. Inman, Nature 208, 1965, 481; A. Graves, J. Electroanal. Chem., 25, 1970, 349; A. Graves, D. Inman, J. Electroanal. Chem., 25, 1970, 3.
[181] A. A. Kornyshev, J. Phys. Chem. B, 111, 2007, 5545–5557. [182] D. Boda, K.-Y. Chan, D. Henderson, J. Chem. Phys., 109, 1998, 7362; D.
Boda, D. Henderson, K.-Y. Chan, D. T. Wasan, Chem. Phys. Lett., 308, 1999, 473.
[183] M. Holovko, V. Kapko, D. Henderson, D. Boda, Chem. Phys. Lett., 341, 2001, 363.
[184] H. Weingaertner, Angew. Chem. Int. Ed. 47, 2008, 654 – 670. [185] R. Atkin, G. G. Warr, J. Phys. Chem. C, 111, 2007, 5162. [186] M. Mezger, H. Schroder, H. Reichert, S. Schramm, J. S. Okasinki, S.
Schroder, V. Honkimaki, M. Deutsch, B. M. Ocko, J. Ralston, M. Rohwerder, M. Stratmann, H. Dosch, Science, 322, 2008, 424.
[187] R. G. Horn, D. F. Evans, B. W. Ninham, J. Phys. Chem., 92, 1988, 3531. [188] G.-B. Pan, W. Freyland, Chem. Phys. Lett., 427, 2006, 96. [189] W. Zhou, S. Inoue, T. Iwahashi, K. Kanai, K. Seki, T. Miyamae, D. Kim, Y.
Katayama, Y. Ouchi, Electrochem. Commun., 2010, 12, 672. [190] H. Sun, B. Qiao, D. Zhang, C. Liu, J. Phys. Chem. A, 114, 2010, 3990. [191] D. Bedrov, O. Borodin, Z. Li, G. D. Smith, J. Phys. Chem. B, 114, 2010,
4984.
203
[192] K. Fumino, T. Peppel, M. Geppert-Rybczynska, D. H. Zaitsau, J. K. Lehmann, S. P. Verevkin, M. Köckerling, R. Ludwig, Phys. Chem. Chem. Phys., 13, 2011, 14064.
[193] R. Parsons, Chem. Rev., 90, 1990, 813; F. Illas, J. Rubio, J. Ricard, J. A. Garrido, J. Electroanal. Chem., 200, 1986, 47.
[194] B. E. Conway, Chem. Soc. Rev., 21, 1992, 253. [195] C. O. Ania, J. Pernak, F. Stefaniak, E. Raymundo-Pinero, F. Beguin, Carbon,
44, 2006, 3113. [196] H. Matsumoto in Electrochemical Aspects of Ionic Liquids, Ed. H. Ohno,
2005, Wiley, Hoboken, New Jersey. [197] T. Isono, J. Chem. Eng. Data, 29, 1984, 45-52. [198] H. Vogel, Phys. Z., 22, 1921, 645; G. S. Fulcher, J. Am. Ceram. Soc., 8,
1925, 339; G. Tammann, W. Hesse, Z. Anorg. Allg. Chem., 156, 1926, 245.
[199] Molten Salts and Ionic Liquids: Never the Twain, Eds. M. Gaune-Escard, K. R. Seddon,2009, Wiley.
[200] M. Drüschler, B. Huber, B. Roling, GIT Labor-Fachzeitschrift 12, 2012, 867. [201] A. S. Best, A. I. Bhatt, A. F. Hollenkamp, J. Electrochem. Soc., 157 (8),
2010, A903 [202] R. K. Donato, M. V. Migliorini, M. A. Benvegnu, J. Dupont, R. S. Goncalves,
H. S. Schrekker, J. Solid State Electrochem. 11, 2007, 1481 – 1487. [203] L. E. Barrosse-Antle, A. M. Bond, R. G. Compton, A. M. O’Mahony, E. I.
Rogers, D. S. Silvester, Chem. Asian J., 5, 2010, 202 – 230. [204] P. Bonhôte, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M.Graetzel,
Inorg. Chem. 35, 1996, 1168–1178. [205] D. R. MacFarlane, P. Meakin, J. Sun, N. Amini, J. Phys. Chem. B, 103, 1999,
4164–4170. [206] J. Sun, M. Forsyth, D. R. MacFarlane, J. Phys. Chem. B, 102, 1998, 8858 –
8864. [207] D. R. MacFarlane, J. Sun, P. Golding, P. Meakin, M. Forsyth, Electrochim.
Acta, 45, 2000, 1271–1278. [208] P. C. Trulove, R. A. Mantz in Ionic Liquids in Synthesis, Eds. P. Wasserscheid
and T. Welton, 2007, Wiley, Weinheim. [209] V. A. Cocalia, A. E. Visser, R. D. Rogers, J. D. Holbrey in Ionic Liquids in
Synthesis, Eds. P. Wasserscheid and T. Welton, 2007, Wiley, Weinheim. [210] M. C. Buzzeo, R. G. Evans, R. G. Compton, ChemPhysChem, 5, 2004, 1106–
1120. [211] G. E. Gray, J. Winnick, P. A. Kohl, J. Electrochem. Soc., 143, 1996, 3820. [212] J. A. Widegren, E. M. Saurer, K. N. Marsh, J. W. Magee, J. Chem. Thermo-
dyn., 37, 2005, 569–575. [213] B. D. Fitchett, T. N. Knepp, J. C. Conboy, J. Electrochem. Soc., 151, 2004,
E219. [214] M. C. Buzzeo, C. Hardacre, R. G. Compton, ChemPhysChem, 7, 2006, 176–
180.
204
[215] S. Randstroem, G. B. Appetecchi, C. Lagergren, A. Moreno, S. Passerini, Electrochim. Acta, 53, 2007, 1837–1842; S. Randstroem, M. Montanino, G. B. Appetecchi, C. Lagergren, A. Moreno, S. Passerini, Electrochim. Acta, 53, 2008, 6397-6401.
[216] M. C. Buzzeo, O. V. Klymenko, J. D. Wadhawan, C. Hardacre, K. R. Seddon, R. G. Compton, J. Phys. Chem. A, 107, 2003, 8872-8878;
[217] A. M. O’Mahony, D. S. Silvester, L. Aldous, C. Hardacre, R. G. Compton, J. Chem. Eng. Data, 53, 2008, 2884–2891.
[218] H. Kato, K. Nishikawa, Y. Koga, J. Phys. Chem. B, 112 (9), 2008, 2655–2660.
[219] P. Walden, Z. Phys. Chem. 55, 1906, 207 and 246. [220] E. I. Izgorodina, R. Maganti, V. Armel, P. M. Dean, J. M. Pringle, K. R.
Seddon, D. R. MacFarlane, J. Phys. Chem. B, 115, 2011, 14688–14697. [221] K. J. Fraser, E. I. Izgorodina, M. Forsyth, J. L. Scott, D. R. MacFarlane,
Chem. Commun. 2007, 3817–3819; M. Yoshizawa, W. Xu, C. A. Angell, J. Am. Chem. Soc., 125, 2003, 15411–15419; D. R. MacFarlane, M. Forsyth, E. I. Izgorodina, A. P. Abbott, G. Annat, K. Fraser, Phys. Chem. Chem. Phys., 11, 2009, 4962–4967.
[222] K. S. Cole, R. H. Cole, J. Chem. Phys., 9, 1941, 341; Broadband Dielectric Spectroscopy; Eds. F. Kremer, A. Schoenhals, 2003, Springer, Berlin,; C. J. F. Boettcher, P. Bordewijk, Theory of Electric Polarization: Dielectrics in Time-Dependent Fields, 2nd ed.; 1992, Elsevier: Amsterdam,; Vol. II, Chapters XIII and IX.
[223] A. Lewandowski, M. Galinski, J. Phys. Chem. Solids, 65, 2003, 281. [224] S. I. Dokashenko, V. P. Stepanov, Russ. J. Electrochem. 29, 1993, 1297. [225] M. Drüschler, Dissertation, 2013. [226] M. Anouti, M. Caillon-Caravanier, C. Le Floch, D. Lemordant, J. Phys. Chem.
B, 112, 2008, 9412. [227] I. M. Hodge, J. Non-Cryst. Solids, 202, 1996, 164; R. Boehmer, K. L. Ngai,
C. A. Angell, D. J. Plazek, J. Chem. Phys. 99, 1993, 4201. [228] E. Donth, J. Non-Cryst. Solids, 53, 1982, 325. [229] I. Bandrés, D. F. Montano, I. Gascón, P. Cea, C. Lafuente, Electrochim. Acta,
55, 2010, 2252–2257. [230] J. Leys, M. Wuebbenhorst, C. P. Menon, R. Rajesh, J. Thoen, C. Glorieux, P.
Nockemann, B. Thijs, K. Binnemans, S. Longuemart, J. Chem. Phys., 128, 2008, 064509.
[231] S. V. Dzyuba, R. A. Bartsch, Tetrahedron Lett., 43, 2002, 4657. [232] J. Fuller, R. T. Carlin, and R. A. Osyteryoung, J. Electrochem. Soc., 144,
1997, 3881; A. B. McEwen, H. L. Ngo, K. LeCompte, J. L. Goldman, J. Electro-chem. Soc., 146, 1999, 1687; H. L. Ngo, K. LeCompte, L. Hargens, A. B. McEwen, Thermochim. Acta 357, 2000, 97.
[233] S. Zhang, N. Sun, X. He, X. Lu, X. Zhang, J. Phys. Chem. Ref. Data, Vol. 35, No. 4, 2006, 1475–1517.
205
[234] C.-M. Jin, C. Ye, B. S. Phillips, J. S. Zabinski, X. Liu, W. Liu, J. M. Shreeve, J. Mater. Chem., 16, 2006, 1529–1535; Z. Zeng, B. S. Phillips, J.-C. Xiao, J. M. Shreeve, Chem. Mater., 20, 2008, 2719–2726; H. Shirota, T. Mandai, H. Fukazawa, T. Kato, J. Chem. Eng. Data. 56, 2011, 2453–2459.
[235] M. K. Kaernae, M. K. Lahtinen, J. U. Valkonen, J. Chem. Eng. Data, 58, 2013, 1893−1908.
[236] S. J. Sachnov, P. S. Schulz, P. Wasserscheid, Chem. Commun., 47, 2011, 11234–11236.
[237] S. Thiemann, S. J. Sachnov, F. Pettersson, R. Bollström, R. Österbacka, P. Wasserscheid, J. Zaumseil, Adv. Funct. Mater., 2013, doi: 10.1002/adfm.201302026; S. Thiemann, Dissertation, 2014.
[238] N. Mechin, J. Plomley, R. E. March, T. Blasco, J.-C. Tabet, Rapid Commun. Mass Spectr., 9, 1995, 5-8.
8. Abbreviations
ROMAN SYMBOLS Symbol Meaning Usual Units A -Mean area of the working electrode
-VFT fit parameter cm-2 S cm-1
B VFT fit parameter K С capacitance F cm-2 Cdl double-layer capacitance F cm-2 Cs bulk capacitance F cm-2
Ctot total capacitance F cm-2 D diffusion m2 s-1 E potential V EA activation energy J mol-1 f frequency of the alternating voltage Hz j current density A cm-2 jdl double-layer current density A cm-2 jct faradaic (= charge transfer) current density A cm-2 k -cell constant
-reaction rate constant - min-1
kB Boltzman constant J K-1 k0 pre-exponential factor min-1 l Distance between the electrodes cm L inductance H m fragility - M molar mass g mol-1
Q double-layer capacitance of CPE -r rate of conversion mol2 L-2min-1
R -gas constant -resistance
J mol-1 K-1
206
Rad adsorption resistance Rct charge transfer resistance RS solution (= bulk) resistance t time s; min; h T temperature °C; K T0 Vogel temperature K Tdec decomposition temperature °C Tg glass transition temperature K Z impedance
Z‘ real part of impedance Z“ imaginary part of impedance W Warburg impedance
207
GREEK SYMBOLS Symbol Meaning Usual Units exponent - -chemical shift
-Nernst-diffusion layer thickness ppm cm
viscosity Pa s
molar conductivity S cm2 mol-1
stoichiometric coefficient - density g cm-3
conductivity S cm-1 phase angle ° angular frequency s-1 STANDARD ABBREVIATIONS Abbreviation Meaning ac alternating current ad adsorption Bun n-butyl But tert-butyl CE counter electrode CPE Constant-phase Element ct charge transfer d doublet DE distributed element DSSC dye sensitized solar cell ec equivalent circuit EIS Electrochemical Impedance Spectroscopy Et ethyl EW electrochemical window FET field-effect transistor GC glassy carbon HPLC High Performance Liquid Chromatography IL ionic liquid i-Pr iso-propyl IR Infrared Spectroscopy LMWG Low molecular weight gelator m multiplet Me methyl MP Methylphosphonate m.p. melting point NMR Nuclear Magnetic Resonance Spectroscopy Nu nucleophile ocp open cicuit potential PE Polyethylene
208
PEG Polyethyleneglycol PEO Polyethylene oxide PMMA polymethyl methacrylate PP Polypropylene RE reference electrode REp pseudo reference electrode s singlet SFG sum frequency generation vibrational spectroscopy SN1 substitution nucleophilic uni-molecular SN2 substitution nucleophilic bi-molecular STM scanning tunneling microscope t triplet TSIL task-specific ionic liquid VFT Vogel-Fulcher-Tamman WE working electrode [BBIM]+ 1,3-Dibutylimidazolium [BMIM]+ 1-butyl-3-methylimidazolium [BMPyr]+ Butyl methyl pyridinium [BMPyrr]+ Butyl methyl pyrrolidinium [EMIM]+ 1-ethyl-3-methylimidazolium [HMIM]+ 1-hexyl-3-methylimidazolium [NBu4]+ Tetrabutylammonium [OMA]+ Trioctylmethylammonium [BF4]- Tetrafluoroborat [dmpSe]- O,Se-dimethyl phosphoroselenoate [dmpt]- O,S-dimethyl phosphorothioate [EtSO4]- Ethysulfate [FAP]- Tris-(pentafluoroethyl)trifluorophosphate [N(CN)2]- Dicyanamide [NTf2]- Bis(trifluoromethylsulfonyl)imide [OcSO4]- Octylsulfate [PF6]- Hexafluorophosphate TCB- Tetracyanoborate
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