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Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 278–284

Micelle formation of imidazolium ionic liquids in aqueous solution

Christian Jungnickel a,∗, Justyna Łuczak a,Johannes Ranke b, Jose F. Fernandez b,

Anja Muller b, Jorg Thoming b

a Department of Chemical Technology, Chemical Faculty, Gdansk University of Technology, ul. Narutowicza 11/12, 80-952 Gdansk, Polandb UFT-Centre for Environmental Research and Technology, University of Bremen, Leobener Straße, D-28359 Bremen, Germany

Received 18 May 2007; received in revised form 4 September 2007; accepted 6 September 2007Available online 15 September 2007

bstract

Ionic liquids (ILs) form a new class of compounds with a much varied industrial and technological application. Some research has beenonducted in ecotoxicity of these compounds, but very little is known about their structure in aqueous solutions. It is known that some ioniciquids can form aggregates in aqueous solution. To determine how such structures influence the use, distribution and toxicity of ILs, first theritical micelle concentrations of the ILs need to be determined. Therefore the critical micelle concentration (CMC) for a number of imidazoliumonic liquids was determined in aqueous solution. The expected dependency of the CMC on the length of the alkyl chain of the investigated

-alkyl-3-methylimidazolium ILs was shown. The degree of ionization and the energy of aggregation were calculated and related to the alkyl chainength of the imidazolium. A significant correlation between retention times in reversed phase gradient HPLC, and the pCMC was observed. Itas also shown that the CMC is directly related to the lipophilicity of the cation. 2007 Elsevier B.V. All rights reserved.

ity; S

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eywords: Critical micelle concentration; Imidazolium ionic liquid; Conductiv

. Introduction

Ionic liquids are a new class of compounds which usuallyonsist of an organic cation and one out of a smaller varietyf organic or inorganic anions. This allows this class of com-ounds to have a large spectrum of technical properties, and beynthesised to suit various requirements. By definition, theseompounds are liquid below 100 ◦C, and generally have a neg-igible vapour pressure. They make ideal non-volatile solventsor a variety of chemical syntheses in industrial scale production1]. Due to the wide range of possible applications in industrialrocesses, ionic liquids should be evaluated before they enter thenvironment. As part of a sustainable development of chemicals,he fate of the chemical needs to be assessed and interactions

ere characterized. Much work here has been done recently in

egard to their toxicity and ecotoxicity [2–4]. Some importantssues such as the life cycle of an ionic liquid, purity, stability,

∗ Corresponding author. Tel.: +48 58 347 2334; fax: +48 58 347 2065.E-mail address: cahj@chem.pg.gda.pl (C. Jungnickel).

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927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2007.09.020

urface tension; Lipophilicity; Retention times

iodegradability, and regeneration require more attention [5].he state of knowledge about ILs and their structure in solution

s limited.It has been suggested by some researchers that ILs can form

ggregates [6]. Due to their structure and inherent charge, someLs are structurally similar to ionic surfactants. That is, the ioniciquid anion or cation will consist of a charged hydrophiliceadgroup and one or more hydrophobic ‘tails’. Due to theseharacteristics it is not unreasonable to assume that some or evenany ILs have properties which are comparable to surfactants.hese properties could include micelle formation and surfacectivity in solution. In fact a number of researchers have alreadyound micelle formation of selected ionic liquids in solution toake place [7–12]. Knowledge of the aggregation behavior is aital part of understanding how ILs participate as componentsn a mixed solvent system. The use and application of ILs cane dependent on the formation of self-assembled structures. For

xample, in extraction processes, micelle formation of the ILight reduce the total product extracted [13,14], as this is par-

ially extracted into the micelles. Transport and distribution ofLs in soil at high concentrations may also depend on the micelle

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ormation. The possibilities of micelle formation may have alsoonsequences in synthesis and purification of bulk ILs, the sol-ation properties of the IL molecules by simple solutes, and theormation of dispersed or phase-separated systems.

Therefore the study of the interface and surface interactionss essential in understanding properties, technical applicationsnd environmental fate of ILs.

The critical micelle concentration (CMC) is an importantactor in characterising the self-aggregation of amphiphilicompounds. The CMC will provide much information abouthe surface activity, including information about technologicalpplicability. The potential formation of micelles is due to theature of ionic liquid ions, as they often contain both a hydropho-ic and hydrophilic domain. The CMC is also dependent onhe relative sizes of these domains [15]. A larger hydrophobicomain will result in a lower CMC, and a higher hydrophilicomain area will result in a higher CMC. It is thought thathe driving mechanism for the micelle formation in water ishe self-association of the water molecule over associationith other solutes that are not capable entering into hydro-en bonding or dipole interactions with the water molecules.hereby the hydrogen bonding between the water molecules istretched into a ‘water or solvent cage’ around the lipophilic tails15].

In this paper, we report on a study on the micelle forma-ion of a homologous series of 1-alkyl-3-methylimidazoliumerivatives. The scope of this paper is the description of thexperimental methods used to qualify micelle formation of theseLs in aqueous solution and to quantify the critical micelleoncentration, and a number of correlations between variousroperties of ILs and their CMC. The ILs were chosen, suchhat the effect of varying the chain length could be observed,nd thereby keeping the anion, Cl−, constant. The correlationf cation lipophilicity as determined by gradient reversed phasePLC, to the CMC of ILs and its value for predicting CMC forarious ILs is also discussed. A comparison to another IL withdifferent cation, N-octyl-pyridinium chloride, was made.

. Materials and methods

.1. Materials

The ionic liquids 1-butyl-3-methyl-1H-imidazolium chlo-ide ([BMIM] Cl), 1-hexyl-3-methyl-1H-imidazolium chloride[HMIM] Cl), 1-methyl-3-octyl-1H-imidazolium chloride ([O-

IM] Cl), 1-decyl-3-methyl-1H-imidazolium chloride ([D-IM]-10 Cl), 1-tetradecyl-3-methyl-1H-imidazolium chloride

[TDMIM] Cl), 1-hexadecyl-3-methyl-1H-imidazolium chlo-ide ([HDMIM] Cl), 1-octadecyl-3-methyl-1H-imidazoliumhloride ([ODMIM] Cl) and 1-octyl-pyridinium chloride ([OPy]l) were obtained from Merck KGaA, Darmstadt, Germany,ith a purity of ≥98%.

.2. Conductivity

Conductivity measurements were made with a Multi-unction Meter CX-742, Elmetron. The micro-conductivity

amCi

icochem. Eng. Aspects 316 (2008) 278–284 279

lectrode was constructed from two platinum wires (thicknessf 1.2 mm), and coated with platinum black. The cell con-tant k was determined using a range of potassium chlorideolution of known concentrations (0.1–500 mM). All solu-ions were made in glass volumetric flasks cleaned withhromic acid to avoid contamination with other surface-ctive substances. Reverse osmosis water was used to prepareolutions (HLP smart 2000, Hydrolab, Poland) with a con-uctivity between 0.07 and 0.32 �S cm−1. The temperatureas maintained at a constant temperature of 298 ± 0.1 Kith a thermostat bath. The conductivity of each sampleas measured using the micro-conductivity electrode in trip-

icate. An average of the measurements was then made andeported.

.3. Surface tension

The surface tension of the 1-alkyl-3-methylimidazoliumhloride solutions were measured using the drop volume ten-iometer TVT-1, Lauda, Lauda-Konigsberg, Germany, with arecision of ±0.01 mN m−1. The temperature of the fluid in theVT-1 was regulated to 298 ± 0.1 K. The drops were measured

n 10 sets of triplicates for each concentration. A syringe withtotal volume of 5.0 ml and an inner diameter of 1.050 mmas used. The error calculation was performed by the Lauda

oftware.

.4. Determination of HPLC retention times

For the determination of cation retention times, the procedureescribed in Ranke et al. [16] was applied. Briefly, the HPLCystem employed was a Hewlett Packard system Series 1100,ith gradient pump, online degasser, autosampler and a Bruker

squire ESI-MS ion trap detector. The column was a MetaChemolaris Ether bridged RP-18 column with 150 mm length, 3 mm

nner diameter and 3 �m particle size generously supplied byarian, Inc. A guard column with octadecylsilica material waslso used. The eluent was composed of 0.25% acetic acid (p.a.)btained from Fluka (Buchs, Switzerland), in deionized waterpH of 3.2), mixed with gradient grade acetonitrile also obtainedrom Fluka. A gradient from 0% acetonitrile (100% dilutecetic acid) to 100% acetonitrile within 10 min was used, whichllows for determination of cation lipophilicities over a wideange.

. Results and discussion

.1. Determination of the CMC

The conductivity and surface tension of ILs were measured inrder to investigate surface behavior and CMC of aqueous solu-ions of imidazolium type ILs. Results for a typical measurement

re shown in Fig. 1. The position of the discontinuities was deter-ined using Phillips definition of the CMC. Phillips defined theMC as a concentration corresponding to the maximum change

n a gradient in the property versus concentration curve [17] as

280 C. Jungnickel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 278–284

) of 1

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hown in the following equation:

d3φ

dC3T

)CT=CMC

= 0 (1)

here φ is a parameter that quantifies the properties that cane used to determine CMC, CT is the total concentration of themphiphile or surfactant and:

= a[Cs] + b[Cm] (2)

here a, b are proportionality constants, Cs, Cm is concentra-ion of the monomeric surfactant and micelle, respectively. CMCas determined by computing the second derivative of the localolynomial fit, and the minimum (for conductivity), and max-mum (for surface tension) were recorded. The calculation ofhe local polynomial fit and its derivatives were done with the

ocpoly function from the KernSmooth library of the R statisticaloftware package [18].

The surface tension measurements versus concentration ofhe various ILs are shown in Fig. 2. Discontinuities for [BMIM]

wfc

ig. 2. Surface tension data vs. IL concentration isotherms measured at 298 K for aqul (×), [TDMIM] Cl (♦), [HDMIM] Cl (�) and [ODMIM] Cl ( ).

-tetradecyl-3-methyl-1H-imidazolium chloride ([TDMIM] Cl).

l and [HMIM] Cl were not observed. The surface tensionseasurements of [BMIM] Cl and [HMIM] Cl were conducted

ntil almost pure IL was used. The micelle formation in theseLs cannot be positively confirmed in the current conditions ofeasurements.Increasing concentration of the ILs in solution result in gath-

ring of the molecules at the air/water interface in order to avoidnergetically unfavourable contact of hydrocarbon chains andater (free energy of a amphiphilic molecule located at the

nterface is lower than that of a molecule solubilised in bulkhase). Adsorption at the interface leads to decreasing of theurface energy which is relevant with changing surface tension.dsorption of the amphiphile at the interface is described by theibbs equation:

i = − ai

RT

(∂σ

∂a

)(3)

i T,P

here Γ i is surface excess concentration, ai activity and σ sur-ace tension of component i, respectively, R is the universal gasonstant, T temperature of the measurement. If ∂σ/∂ci in this

eous solutions of [BMIM] Cl (©), [HMIM] Cl (�), [OMIM] Cl (+), [DMIM]

C. Jungnickel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 278–284 281

Fig. 3. Variation of the specific conductivity as a function of ILs concentrationoCr

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Table 1Critical micelle concentration of the substituted imidazolium ionic liquids withdifferent chain length and constant anion Cl− at 298 K determined by differentmethods

Alkyl chainlength of cation

Critical micelle concentration (mM) Retentiontime

Conductivity Surfacetension

AverageCMC

4 – – – 5.826 – – – 7.178 220 234 227 8.21

10 59.9 53.8 56.8 9.0714 3.38 3.15 3.26 11.1411

v(s[

1ddiobCs

ttabsad

f in water solutions at 298 K: [OMIM] Cl ( ), [DMIM] Cl (�), [TDMIM]l (�), [HDMIM] Cl (©). The inset shows a focus of a smaller concentration

ange.

quation is <0 it means solute accommodation at the surfaceausing increasing area of this surface. The effect of this phe-omenon is observable as a surface tension decreasing as shownn Fig. 2. When the interface becomes saturated increasing ofhe concentration results in dissolution of the IL ions in watereading to an increase in free energy of the system due to thencreasing number of unfavourable alkyl chains in water. AfterMC ions cluster to form micelles with the lipophilic hydro-arbon chains oriented towards the interior of the micelle whichtop the free energy from increasing further. Micelles are noturface active, so they maintain a surface tension of the solutionhich is approximately constant after CMC. Sometimes it is pos-

ible to observe a characteristic minimum in the area of CMCn the curve, as shown in Fig. 2. In the case of [OMIM] Cl andDMIM] Cl characteristic minima were observed. It could sug-est desorption over a small concentration range where ∂σ/∂ci

s positive and transport of the molecules to the bulk solution. Itrobably results from trace amounts of the surface-active impu-

ities which are usually present in commercial materials. Forxample in [OMIM] Cl or [DMIM] Cl, the impurities may bedsorbed onto the interface below the CMC and then solubilisedn the micelle above the CMC. Theoretical deliberations for

sosS

Fig. 4. Comparative conductivity grap

6 1.26 1.14 1.21 12.088 0.40 0.45 0.43 13.92

ery pure amphiphiles suggest approximately zero adsorption∂σ/∂ci ∼ 0), however experimental data indicate that ∂σ/∂ci istill negative, what means that σ changes a little above the CMC19,20].

Fig. 3 shows the conductivity data versus concentration for-alkyl-3-methylimidazolium solutions. Increasing of the con-uctivity with concentration, which is generally observed foriluted solutions, is relevant with increasing number of ionsn solutions. Two linear regions for specific conductivity arebserved below and above the CMC. The lower slope after thereakpoint implies that the formed micelles have lower mobility.ounter ions become dynamic parts of the micelle causing high

urface charge and reducing the number of current carriers.As it was expected, elongation of the chain length decreases

he CMC as shown in Table 1. A decrease of the CMC from 227o 0.43 mM was found with increasing the number of the carbonstoms from 8 to 18. This means that increasing the hydropho-ic part of the IL cations favours micelle formation as has beenhown in the surfactant literature [21,22]. Generally, a reason-ble agreement between the two techniques can be stated. Theifferences are related to the different physical properties mea-

ured and which are influenced by the micelle formation. Somef the variation may be also result of the presence of impurities,mall amount of which may have a significant effect on the CMC.ome variability may also result from the method of discontinu-

hs of [OMIM] Cl and [OPy] Cl.

2 A: Physicochem. Eng. Aspects 316 (2008) 278–284

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ty determination. As can be seen in Fig. 4, both [OMIM] Cl andOPy] Cl showed two distinct minima using the Phillips methodo determine the CMC. These can be tentatively explained byypothesising that two aggregate regimes are formed. Howeverue to the small concentration range in which these two regimesppear it is more likely that the two minima are a result of experi-ental variation, and consequent amplification in the variability

n the calculation of the second derivative.Some CMC values have already been measured in liter-

ture. Our results compare well with experiments done byther researchers with similar compounds. The most thor-ugh investigation was done by Blesic et al. [11], and theybtained CMC values of 220, 55, 15, and 4 mM for 1-alkyl-3-ethylimidazolium derivatives with alkyl chain lengths of 8, 10,

2, and 14. These values are almost identical to our results. Bow-rs et al. [7] obtained results for [OMIM] I and [OMIM] Cl ofbout 100 ± 10 mM. This value indicates first breakpoints wereeasured. Siriex-Plenet et al. [23] obtained results for [DMIM]r at 40 ◦C of 40 mM which is lower than our results of [DMIM]l. The difference of our result can be explained by the largernion Br which generally lowers the CMC because of weakerydration. Weaker hydrated anions are easily absorbed on theurface of the micelles which decreases electrostatic repulsionnd in this way facilitates aggregation. This effect is also seen inhe results of Dong et al. [10] who obtained a result of 29.3 mMor [DMIM] Br. Modaressi et al. [9] obtained the results of 60nd 40 mM for [DMIM] Cl and 1-dodecyl-3-methylimidazoliumhloride, respectively.

The value of the CMC is determined mainly by the nature ofhe amphiphile and by the length of its alkyl chain. The size ofhe head group will influence the stability of the micelle. Due tohe delocalization of the positive charge on the imidazolium headroup, as compared to the point charge on a quaternary ammo-ium surfactant, the steric hindrance, and Columbic repulsionould indicate higher CMC values.For a homologous series of linear single-chain amphiphiles

he CMC follows the empirical Stauff–Klevens rule, which indi-ates a logarithmic relationship between CMC and the numberf carbon atoms in the alkyl chain [21] as shown in the followingquation:

og CMC = A − Bx (4)

here A, B are constants for a particular homologous series andemperature and x is number of carbon atoms in the hydrocar-on chain. The constant A varies with the nature and numberf hydrophilic groups while B is a constant which measures theffect of each additional methylene group on the CMC. B repre-ents the energetics (free energy) of the transfer a methylene unitrom aqueous background to the micellar structure. For nonionicmphiphiles B is approximately 0.5, for paraffin chain salts withsingle ionic head group 0.29–0.30, 0.29 for potassium carboxy-

ates and 0.25 for divalent ionic surfactants [15,24]. For different

omologues the values A are varying considerably whereas B islose to log 2 which means that addition of a methylene groupo a hydrocarbon chain decreases the CMC of these compoundsy approximately a factor of 2 [25]. Knowing this rule the CMC

bapt

ig. 5. Chain length relationship between CMC and alkyl chain length of themidazolium cation and constant Cl− anion.

an be estimated from CMC values for various chain lengths26].

Applying the Stauff–Klevens rule, the chain length depen-ence on the CMC was determined for the 1-alkyl-3-methylmidazolium cations and is shown in Eq. (5) and graphicallyepresented in Fig. 5:

= 4.51 − 0.28x (5)

The observed value of the B is similar to the value for n-alkylonic surfactants with a single ionic head group and correspondso a �GCH2 of −1.57 kJ mol−1 at 298 K [25]. From this Eq. (5)t can also be seen why micelle formation for the IL with chainength 4 and 6 was not observed. The calculated CMC is closeo the concentration of the IL at zero dilution.

The micelle formation of the ionic liquid in aqueous solutions a spontaneous process and it means that the free energy ofhange of this phenomenon is negative. There are two modelsescribing the aggregation process. The phase separation modelreats micelles in solution as a separate phase from the solvent. Inomparison, the mass action model treats the amphiphilic salts a solute forming dimers, trimers, etc. in aqueous solutions19,27]. Whether a reaction equilibrium or phase equilibriumpproach is adopted depends inter alia on the size of the micellesormed. In aqueous systems the phase separation model is gen-rally used [25].

Applying the phase separation model to the monomer–icelle equilibrium for cationic amphiphile as shown where C+

nd A− represent molar concentration of cations and anions,espectively,

C+ + (n − p)A−salt(aq)

� [CnA(n−p)]p+

micelle(6)

he free energy of aggregation �G◦agg is given by the following

quation:

G◦agg = RT (2 − β) ln CMC (7)

here β is the degree of ionization and the CMC is given asole fraction. �G◦

agg indicates free energy difference per mole

etween IL molecules in water and in micelles. �G◦

agg meanslso the free energy of transfer of 1 mol IL from the aqueoushase to the micellar pseudophase. The degree of ionization inhis equation is the fraction of charges of amphiphile ions in the

C. Jungnickel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 278–284 283

Table 2Summary of data for imidazolium ionic liquids with corresponding degree ofionization and free energy

Alkyl chain length CMC (M) β �G (kJ/mol)

8 0.22 0.45 −21.2410 0.06 0.51 −25.2214 0.003 0.42 −38.0011

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icelle neutralized by micelle-bound counterions. Monomersf ionic amphiphiles are entirely dissociated in solutions butn their micelles they are partially associated with counterions.he degree of ionization of the micelles was calculated from the

atio of the slopes of the two linear fragments of the conductivityurves [28].

The results are summarized in Table 2. A lower β indicatesbetter packed micelle. Usually slightly increasing values of β

ith elongation of the chain length are observed. However in ourase the β values are too variable to obtain a clear correlation.

.2. Correlation between CMC and lipophilicity

The lipophilicity of ionic surfactants if often modified byhanging the number of the methylene units in hydrocarbonhains (extensive mode of lipophilicity variation). Anotherethod is based on variation of the intrinsic nature of the

ipophilic groups (intensive mode). Most of the commerciallytilized surfactants include alkyl or aryl lipophilic groups [29].ipophilicity due to the interaction between the alkyl chain of

he amphiphile and water plays an important role in micelleormation.

The quantification of lipophilicity is most often based on theartitioning coefficient log Kow between water and 1-octanol,hich can be directly measured. Indirect determination of thisarameter can be done experimentally by reversed phase HPLCr estimated by various computational methods. In surfactantcience [30], and recently also in the ionic liquids literature [31],og Kow values are often predicted from their structure, based onhe group contribution method of Hansch et al. [32]. This worksell within homologue groups of ionic liquid or surfactant ions

haring the same basic structure and the same counter ion (e.g.-alkyl-3-methylimidazolium chlorides).

Different methods have been proposed to calculate lipophilic-ty parameters from a single gradient HPLC run [33–35]. Thepproaches of Krass et al. [34] and Valko et al. [35] are designedo predict log Kow values. Here, results from a recently developeduick chromatographic method for characterizing the lipophilic-ty of cations was employed [16].

As it was mentioned before, the critical micelle concentra-ion of amphiphiles can be correlated with the chain lengthf the alkyl chains on the ionic molecules, as indicated by

tauff–Klevens rule in Eq. (4). One of the ways to achievenother relevant correlation is to compare the lipophilicity tohe CMC. The correlation of retention times tR and lipophilicitys not exactly linear as shown previously [16]. However, reten-

SeSD

ig. 6. CMC dependence of retention time of cation variation with constantnion Cl−.

ion times can be used as a measure of lipophilicity as has alsoeen proposed earlier by Valko et al. [35].

The results shown in Table 2 indicate an approximate linearorrelation between retention time and critical micelle concen-ration, as is shown in Fig. 6. That is because of increasing chainength of the cation the retention time increases linearly, there-ore the same correlation can be expected with the pCMC. Aomparison to a different cation was also made. The retentionime of N-octyl-pyridinium was measured to be 8.425. Using theorrelation as shown in Fig. 6. A CMC of N-octyl-pyridiniumhloride of 108 mM was predicted, as compared to the mea-ured CMC of 274 mM. The retention time–CMC relationshipppears to the suitable to indicate the order of magnitude athich micelle formation takes place. Therefore until studies

omprising various head groups have been performed, the esti-ation of CMC from retention times is limited to the order ofagnitude.

. Conclusion

The micelle formation of ILs in water was investigated. TheMC of 1-alkyl-3-methylimidazolium chlorides was systemati-ally, successfully and reproducibly determined by conductivitynd surface tension measurements. The results suggest thaticelle formation of 1-alkyl-3-methylimidazolium chlorides

akes place with longer (larger than hexyl) alkyl chains. Theetention times of the cations were determined, and the cationetention times were correlated with the CMC of the ILs. It washown that the lipophilicity of the ions dictates, and will allowor the prediction of the likelihood of micelle formation.

cknowledgements

Financial support was provided by the Polish Ministry ofcience and Higher Education with grant N205 041 32/2340.

chool of the Central University of Venezuela (UCV) acknowl-dges the financial support of the German Academic Exchangeervice (DAAD). We would also like to thank Elke Obert andr. Holger Wessolowski for their support.

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