Journal of Colloid and Interface Science 362 (2011) 415–422

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Journal of Colloid and Interface Science

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Influence of the Hofmeister anions on self-organization of1-decyl-3-methylimidazolium chloride in aqueous solutions

Justyna Łuczak a, Marta Markiewicz a, Jorg Thöming b, Jan Hupka a, Christian Jungnickel a,⇑a Department of Chemical Technology, Chemical Faculty, Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdansk, Polandb UFT – Centre for Environmental Research and Technology, University of Bremen Leobener Straße, D-28359 Bremen, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 April 2011Accepted 23 June 2011Available online 30 June 2011

Keywords:Self-organizationMicelle formationImidazolium ionic liquidsInorganic salt influenceHofmeister

0021-9797/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.jcis.2011.06.058

⇑ Corresponding author. Fax: +48 58 347 2065.E-mail address: cahj@chem.pg.gda.pl (C. Jungnicke

Inorganic salts usually influence water structure affecting the hydration of the molecules which lead to asalting-in or a salting-out effect of hydrophobic compounds. Specific conductivity and isothermal titra-tion calorimetry have been used to study the effect of inorganic salts on aggregation of the cationic sur-factant 1-decyl-3-methylimidazolium chloride in aqueous solutions. The effect of the concentration, thenature of the anion and temperature on micelle formation were studied.

A decreasing critical micelle concentration (CMC) due to the weakening electrostatic repulsionbetween the headgroups was observed. The salts used in this investigation decreased the CMC and degreeof micelle ionization in the order of Cl� < Br� < NO�3 < I� which parallels the Hofmeister series of ions. Theeffect of the electrolyte concentration on the CMC, and a relationship between the electrolyte counter-ionconcentration and the CMC were also shown.

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1. Introduction

In the last decades interest in ionic liquids (ILs) has continuallyincreased, due to their useful properties e.g. negligible vapor pres-sure and excellent solvation and electrochemical stabilities. Theseorganic salts have been applied as reaction media for synthesis [1–3], separation, catalytic and biocatalytic processes where they havethe potential to replace organic solvents. Ionic liquids consist usu-ally of variable organic cations, and inorganic or organic anions.This variability allows the physicochemical properties of ionic liq-uids to be tailored to specific needs or applications, and thereforeILs are often referred to as designer compounds.

Simple replacement of organic solvents by ILs is not straightfor-ward. One of the obstacles is the amphiphilic nature of some ofthese compounds. The phase behavior of binary and ternary sys-tems will ultimately effect process efficiency e.g. in separationand purification processes. Although the number of ionic liquidsand their applications described in the literature is growing rap-idly, the interactions in multi-component systems is not yet wellunderstood. Despite the emergence of experimental data on phaseequilibria of systems containing ILs, these studies are still rare andnot systematic, and therefore make rational prediction of IL solu-tion behavior difficult. Understanding the molecular interactionsof ionic liquids in ternary and higher solutions is a prerequisite

ll rights reserved.

l).

for sustainably predicting, controlling, and designing their co-sol-vent properties and their application in industrial scale processes.

A number of researchers, including our own group, describedmicelle formation of long-chained ILs in aqueous solutions [1–3].The length of the alkyl chain substituents, the degree of substitu-tion, type of counter ion as well as the temperature were foundto have an effect on the critical micelle concentration (CMC). How-ever, a number of factors, such as the effect of additional com-pounds on the amphiphilic behavior of ILs, have not yet beenfully determined. One of these is the effect of inorganic salts onthe aggregation of ILs in solution [4].

The effect of inorganic salts on protein solubility was first de-scribed by Hofmeisters’ student, Lewith, and was later referred toas the Hofmeister or lyotropic series [5]. This idea was then furtherextended to a variety of physiochemical properties in many fieldsof research ranging from colloidal assembly to protein folding.Even though the idea and nomenclature of Franz Hofmeister isused frequently, 120 years after its conception, the mechanism isstill unclear. Often authors use the term ‘‘Hofmeister series’’ as aphenomenological description of an observation. However, theterm ‘‘Hofmeister series’’ covers a plethora of inter related molec-ular mechanisms.

The goal of this paper is a discussion of the influence of Hofmei-ster anions on the micellisation of a selected ionic liquid. This willbe supported with CMC results and thermodynamic data of theinvestigated process. Salts in solution have a quantifiable influenceon the structure of water surrounding amphiphilic molecules andin consequences the aggregation behavior.

416 J. Łuczak et al. / Journal of Colloid and Interface Science 362 (2011) 415–422

Some groups have already investigated ternary IL/inorganic salt/water systems. In 2003 Gutowski et al. presented the ability of akosmotropic salt (water-structure maker), K3PO4 to induce phaseseparation of 1-butyl-3-methylimidazolium chloride ([BMIM][Cl])solutions [6]. Using temperature–composition pseudo-binary andcomposition ternary phase diagrams the same group determined aweaker salting-out effect for octyl- and decylimidazolium deriva-tives. This phenomenon was thought to be due to possible micellaraggregate formation [7].

Blesic et al. determined the effect of salt on micelle formation inaqueous solutions on 1-decyl-3-methylimidazolium chloride ([DMIM][Cl]) and 1-dodecyl-3-methylimidazolium chloride ([DDMI M][Cl]). It was observed that the addition of salts (including both monoand divalent NaCl, and Na2SO4, as well as short chained ILs) de-creased the CMC and enhanced micelle formation [8]. A similarinfluence on the surface activity of three ILs ([DDMIM][Br],1-methyl-3-tetradecylimidazolium bromide [TDMIM][Br] and1-hexadecyl-3-methylimidazolium bromide [HDMIM][Br]) withvarious anions (Cl�, Br� and I�) was described by Dong et al. [9].According to Wang et al. most of the 15 salts used in their studyresulted in a ‘‘salting out effect’’ of [DMIM][Br] [10].

All thermodynamic parameters that describe surfactant solu-tion undergo an alternation during the micellization process.Knowledge of these changes enable understanding/recognition offorces which enable self-association as well as prediction of theinfluence of temperature and pressure on aggregation [11]. Tothe best of our knowledge a thermodynamic discussion of the mic-ellization of ionic liquids in the presence of inorganic salts has notyet been described.

In this publication the micelle formation of [DMIM][Cl] wasinvestigated by the CMC determination in aqueous solutions. Theinfluence of the four salts on the micellization, and their correspond-ing thermodynamic parameters, of [DMIM][Cl] was investigated byspecific conductivity and isothermal titration calorimetry, andthereby provide a thermodynamic interpretation to discuss and ex-plain the behavior of ionic liquids in ternary systems.

2. Experimental

2.1. Materials

The ionic liquid 1-decyl-3-methylimidazolium chloride ([DMIM][Cl]) was obtained from Merck KGaA, Darmstadt, Germany, with apurity of P99%. This IL was degassed and dried under vacuum(Vacucell VUS-B2V/VU 22, Czech Republic) of 0.1 Pa, at the temper-ature of 343 K according to the procedure described by Goodchildet al. [1]. The IL was then stored in a dessicator. The potassium saltsKX (where X is Cl�, Br�, I�, and NO�3 ) were obtained from POCh,Poland. Reverse osmosis purified water was used (HLP smart 2000,Hydrolab, Poland) with an average conductivity of 0.15 lS cm�1.

2.2. Technique: Conductometry

The conductivity measurements were made at 298 ± 0.1 K usinga conductivity meter equipped with an autotitrator (Cerko Lab Sys-tem CLS/M/07/06, Poland) and a microconductivity electrode(Eurosensor, EPST-2ZA, Poland). Stable temperature was main-tained using a thermostatic water bath (PolyScience 9106, USA).The conductivity cell constant k was determined with a range ofaqueous solutions of KCl (0.1–100 mM). Reverse osmosis purifiedwater was progressively added to concentrated ionic liquids solu-tion. Every titration was repeated thrice and the average CMC val-ues were reported.

The breakpoints in the conductivity curves (CMC) weredetermined using Phillips’s definition as the concentration

corresponding to the maximum change in a gradient in the prop-erty versus concentration curve [12] as shown in equation:

d3/

dC3T

!CT¼CMC

¼ 0 ð1:1Þ

where / is a parameter that quantifies the properties that can beused to determine CMC, CT is the total concentration of the amphi-phile. The degree of ionization of the aggregates (b) was calculatedfrom the ratio of the dj/dc slopes of the two linear fragments of theconductivity curves [13,14]. The free energy of the micellizationprocess is given by equation:

DGm ¼ ð2� bÞRT ln CMC ð1:2Þ

where b is the degree of ionization and CMC is the critical micelleconcentration in the mole fraction of surfactant in aqueous solution.

2.3. Technique: Isothermal titration calorimetry (ITC)

The ITC experiments were performed with the Nano-IsothermalTitration Calorimeter III (N-ITC III, CSC, USA). All solutions were de-gassed (Nuova stirrer, Thermolyne, S18520, Barnstead Interna-tional, USA) for 30 min prior to use. The sample cell was filledwith the 0.1 M solution of inorganic salt, reference cells with waterwhereas the Hamilton gastight syringe was loaded with ternary IL/KX/water solution. IL with a concentration of about ten times high-er than the CMC was applied to observe the demicellization pro-cess at a specific temperature. The titration experiment consistedof 50 injections of 5 lL IL/KX aliquots into the sample cell loadedwith 952 lL KX solution. The solution in the sample cell was con-stantly stirred (250 rpm). The demicellization of the IL was ob-served in the range of temperatures 283–308 K. The injectionsequence was 5 lL aliquots at 300 s intervals between injectionsuntil the region beyond the CMC was reached. The interval be-tween two injections was sufficient for the signal to return to thebaseline. Each measurement was conducted at least four times.The enthalpograms were created using Titration Bindworks soft-ware provided by CSC. From the enthalpograms the enthalpy ofdemicellization and CMC were extracted [15].

3. Results and discussion

The measurements were conducted in order to investigate theaggregate formation of [DMIM][Cl] in ternary IL/KX/water solution.The CMC values of [DMIM][Cl] in water in the presence of selectedinorganic salts with different concentrations were determined byconductometry and ITC. The [DMIM]+ cation was chosen as it is along chained imidazolium compound (liquid at room temperature,m.p. 277 K), for which we have previously determined the CMC in abinary system [3].

3.1. Conductometry

Inorganic salts of the Hofmeister series possessing K+ cation anddifferent anions (Cl�, Br�, I� and NO�3 ) were chosen for the exper-iments. For comparison to ternary systems Fig. 1 presents the var-iation of electrical conductivity of [DMIM][Cl] with concentrationdetermined at 298 K in a binary system (free of inorganic salt).

The increasing specific conductivity with concentration, whichis observed in dilute aqueous systems, is due to the increasingnumber of ions in solution. The two linear regions of specific con-ductivity are observed below and above the CMC. The reducedslope after the breakpoint implies that the formed micelles havelower mobility.

In Figs. 2 and 3 the conductivity changes with various[DMIM][Cl] concentrations in water in the presence of KCl and

0

1

2

3

4

5

6

7

8

c [mM]

κ [m

S/cm

]

0 20 40 60 80 100

Fig. 1. Dependence of the electrical conductivity of the aqueous solutions[DMIM][Cl] on concentration measured at 298 K.

0

2

4

6

8

10

12

14

16

18

0 20 40 60 80 100c [mM]

κ [m

S/cm

]

Fig. 3. Conductivity titration of aqueous solutions of [DMIM][Cl] in presence of 1,10, 25, 50, 100 mM KNO3 at 298 K.

J. Łuczak et al. / Journal of Colloid and Interface Science 362 (2011) 415–422 417

KNO3 are shown. Table 1 presents CMC values, degree of counter-ion binding b as well as DGm calculated using Eq. (1.2).

The results revealed that the addition of all inorganic salts low-ers the CMC of [DMIM][Cl] in comparison to the binary system.Moreover, further increasing the electrolyte concentration addi-tionally favors micellization of the IL. This phenomenon, similarto that of ionic surfactants, depends inversely on the electrostaticrepulsion between ionic groups, which is stronger for completelyionized compounds. The decrease in the CMC values caused bythe inorganic salt is a result of a decrease in the coulombic repul-sion of the headgroups. Additional anions introduced to the systemfurther shield the positive charge on the IL headgroup [16].

The influence of the inorganic salt on the CMC may be corre-lated in the relation Cl� < Br� < NO�3 < I� [17,18]. The screeningability was also confirmed by the strength of counterion bindingto the cationic aggregate. It was found that at higher concentra-tions of KX the degree of ionization was decreasing, followingthe order I� > NO�3 > Br� > Cl�. The degree of ionization was thelowest for KI, at a concentration of 100 mM [19].

The anions used in this study posses a number of characteristicswhich include different sizes in solution, different number of water[10] molecules in the hydration layer, variable polarizability, andmolar surface tension increment (MSTI) [20]. As shown in Table2 a chloride anion is smaller and more strongly hydrated, has ahigher MSTI and is less polarizable in comparison to a bromide

0

2

4

6

8

10

12

14

16

18

20

c [mM]

κ [m

S/cm

]

0 20 40 60 80 100

Fig. 2. Conductivity titration of aqueous solution of [DMIM][Cl] in the presence of 1,10, 25, 50 and 100 mM KCl at 298 K.

anion [21]. Thus less polarizable ions (strongly hydrated) are lessable to neutralize the charge on the surface of the micelle andthereby contributing to formation of small and spherical micelles.On the contrary, highly polarizable (weakly hydrated) ions, aremore effective in limiting the electrostatic repulsion, thus facilitat-ing micelle formation at a lower CMC with more complex shapes,including vesicles, bilayers, etc. [22,23]. As can be seen in Fig. 4,the dependence of the polarizability and radius of the anions onthe CMC of the IL/KX/water system, presents a linear relationship.A good fit was achieved for anion radius versus CMC graph (Fig. 4B)using the equatorial radius of NO�3 instead of the axial radius [24].All of the parameters listed in Table 2 can be considered factorsdetermining the CMC. In addition, in Fig. 4 we can also observe thatthe CMC decreases with increasing salt concentration.

The relationship between inorganic salt and the CMC, describedpreviously, inversely follows the well known Hofmeister series.Transferring the phenomenological description of protein folding,made by Hofmeister, to surfactant water systems is difficult. Termssuch a kosmotropes, chaotropes, salting-out and salting-in havearisen, and have been used frequently. Kosmotropes, also knownas water-structure makers, are salts which enhance the H-bondformation, due to their high charge density. These ions are stronglyhydrated. Chaotropes also known as water-structure breakers aresalts which inhibit the H-bond formation due to their low chargedensity. These ions are usually weakly hydrated in solution. Theproblem with this terminology is related to the terms salting-outand salting-in. The salting-out effect refers to a decrease in thesolubility of macormolecules as a result of the addition of a saltof the Hofmeister series. This is often justified in literature thatthe salt promotes aggregation of the compound by the competitionfor the water molecules. However, it should be mentioned that thesalting-out effect can also be observed for salting-in salts at higherconcentrations [6].

In a number of papers, the addition of salt and consequentdecrease of the CMC of the surfactant is classified as a salting-outeffect, however this classification is questionable. The decrease ofthe CMC is usually described as a promotion of the aggregationof the surfactant and in consequence a decrease in solubility, andis therefore characteristic for kosmotropes. However, decreasingthe CMC is a result of an increased solubility of the surfactant inwater [7].

For the IL using the CMC and b we were able to calculate theGibbs free energy of the micellization process – the energyrequired to transfer one mole of dispersed IL to a micelle. The re-sults of the DGm are summarized in Table 1. The micelle formationof the ionic liquid in aqueous solution is a spontaneous process

Table 1Summary of CMC data of [DMIM][Cl] in presence of potassium salts measured at 298 K including degree of ionization and free energy of micellization.

KX (mM) KCl KBr KNO3 KI

CMC (mM) b DGm (kJ/mol) CMC (mM) b DGm (kJ/mol) CMC (mM) b DGm (kJ/mol) CMC (mM) b DGm (kJ/mol)

0 57.2 ± 0.3 0.53 �25.0 57.2 ± 0.3 0.53 �25.0 57.2 ± 0.3 0.53 �25.0 57.2 ± 0.3 0.53 �25.01 52.8 ± 0.4 0.51 �25.7 52.9 ± 0.7 0.51 �25.7 53.7 ± 0.9 0.51 �25.6 52.0 ± 1.1 0.60 �24.2

10 51.1 ± 0.8 0.51 �25.8 49.6 ± 0.5 0.50 �26.1 44.1 ± 0.4 0.52 �26.2 33.5 ± 0.7 0.60 �25.725 47.2 ± 0.7 0.50 �26.3 41.8 ± 0.8 0.50 �26.7 37.2 ± 0.8 0.52 �26.6 16.8 ± 0.9 0.54 �29.350 42.0 ± 1.0 0.50 �26.7 34.0 ± 0.7 0.47 �28.0 28.0 ± 1.0 0.51 �27.8 10.5 ± 0.5 0.36 �34.6

100 35.7 ± 0.7 0.48 �27.6 25.6 ± 0.9 0.46 �29.3 19.2 ± 0.9 0.46 �30.4 8.2 ± 0.8 0.26 �38.1

Table 2Anion radius – r; polarizability – a, molar surface tension increment – r, partial molar volume – v of anions used in this study.

Anion r (nm) [24] a (Å3) [24] r (mN L m�1 mol�1) [20] v (cm3 mol�1) [20]

Cl� 0.181 3.76 1.63 16.5Br� 0.196 5.08 1.31 23.5NO�3 0.206 4.47 1.18 27.8I� 0.220 7.41 1.02 34.8

0

10

20

30

40

50

60

polarizability [A3]

CM

C [

mM

]

0

10

20

30

40

50

60

53 4 6 7 8 0.17 0.18 0.19 0.20 0.21 0.22 0.23

anion radius [nm]

CM

C [

mM

]A B

Fig. 4. Dependence between the polarizability (A) and anion radius (B) with the CMC of the IL/KX/water systems at 298 K with 1 (}), 10 (�), and 25 mM (⁄) of added KX. Thesequence of salts is KCl, KBr, KNO3 and KI from left to right, in both graphs.

418 J. Łuczak et al. / Journal of Colloid and Interface Science 362 (2011) 415–422

indicated by a negative free energy. It was observed that withincreasing concentration of inorganic salts DGm decreased; therebyindicating that miellization process becomes more spontaneous.This follows also from the observed decrease of the CMC. Compar-isons of CMC and DGm values with previously published surfactantdata (restricted to chain length of ten carbons) is shown in Table 3.As shown surfactants with the same chain length generally have ahigher CMC than comparable ionic liquids.

3.2. Isothermal titration calorimetry

To further investigate the thermodynamics of the micellizationprocess of [DMIM][Cl] in water/inorganic salt systems, we appliedisothermal titration calorimetry to obtain a direct measure of theenthalpy of demicellization (DHdm), and thereby the enthalpy ofmicellization (DHm) [16].

Fig. 5A presents a typical titration curve obtained by the addi-tion of a concentrated [DMIM][Cl]–KCl solution to a 0.1 M KCl solu-tion at 298 K. The typical sigmoidal shape enthalpogram may bedivided in three parts. The first part is a result of the titration ofthe concentrated IL solution to the sample cell during which thedemicellization process takes place. Thereby, the IL concentrationremains below the CMC in the sample cell. It is observed as a majorenthalpic effect consisting of heat of micelle dilution, demicelliza-tion process and dilution of monomers in the solvent.

The second part of the enthalpogram, is observed as a sharpdecrease or increase of the heat effect due to reaching CMC by IL.

The last region is connected with the titration of the concen-trated IL solution to the micellar solution. The heat effect resultsfrom dilution of the micellar aggregates [32]. To determine the en-thalpy of demicellization the injection peaks were integrated andnormalized with respect to the injected number of moles, as shownin Fig. 5B. The heat of micellization was determined from the dif-ference between the observed enthalpies of the two linear partsof the sigmoidal curves at the CMC. To determine the CMC by ITCtitration, the first derivative of the integrated peaks versus the totalconcentration of IL was calculated. The extrema of the differentialcurves were taken as the CMCs [14]. A minima or maxima of thefirst derivative curve indicate also a positive or negative enthalpyof demicellization, respectively. The method of the DHm and CMCdetermination was presented in Fig. 5.

Figs. 5 and 6 present exemplary results of the heat of demicel-lization of [DMIM][Cl] in presence of KCl and KI, respectively. TheCMC values measured by ITC and conductivity titration methodsat 298 K are in good agreement. For the DGm calculation, using datafrom the ITC, a corresponding b value was taken from conductivitymeasurements, following the procedure described previously forexample Jiang et al. [14]. The entropy of micellization DSm wascalculated from DHm and DGm.

The ITC results confirmed that the addition of salts to theIL–water system decreases CMC values. For comparison, the

Table 3Comparison of CMC and DGm determined by conductometric and calorimetric methods for [DMIM][Cl] in inorganic salts systems. Literature comparison for common surfactantsincluding decyl-trimethylammonium chloride (C10TAC), decyl-trimethylammonium bromide (C10TAB), sodium decylsulfate (C10DS), decylammonium bromide (C10AB),decyltrimethylammonium bromide (C10TMAB), decylpyridinium chloride (C10PyCl), and decylmethylpyrrolydinium bromide (C10C1PyrrBr) is also given. The salt NaSal is sodiumsalicylate.

Ionic liquid + Salt T (K) Conductivity Calorimetry Literature

CMC (mM) DGm (kJ/mol) CMC (mM) DGm (kJ/mol) DH (kJ/mol) �TDS (kJ/mol)

[DMIM][Cl] only 298 57.2 ± 0.3 �25.0 58.5 ± 1.3 �25.0 ± 0.1 3.5 ± 0.2 �28.5 ± 0.2 [15]298 40.47 �31.14 66 �24.8 [25]

62.7 �29.0 0.0 �29.0 [26]C10TAC 298 94.7 �23.3 [27]

68.0 �11.65 [28]C10AB 303 60.2 50.8 �30.9 �1.59 �29.4 [29]C10DMAM 303 77.7 61.7 �29.1 �1.05 �28.2 [29]C10TAB 298 66.9 �11.25 �26.7 [30]

303 60.2 �28.7 �1.59 �27.3 [33]C10DS 303 42.6 34.2 �32.4 �1.22 �31.2 [29]C10PyCl 303 19.8 �15.7 19.2 �31.9 �1.93 �30.0 [29]C10C1PyrrBr 55.5 �28.7 [31][DMIM][Cl] + 0 mM NaCl 298 55 [8][DMIM][Cl] + 50 mM NaCl 298 40 [8][DMIM][Cl] + 150 mM NaCl 298 25 [8][DMIM][Cl] + 0 mM Na2SO4 298 55 [8][DMIM][Cl] + 50 mM Na2SO4 298 40 [8][DMIM][Cl] + 150 mM Na2SO4 298 22 [8]C10TAB + 300 mM NaF 303 41.7 1.65 [29]C10TAB + 300 mM NaCl 303 30.2 1.74 [29]C10TAB + 300 mM NaBr 303 28.4 �1.40 [29]C10TAB + 300 mM NaI 303 8.0 �1.5 [29]C10TAB + 300 mM Na2SO4 303 31.3 3.21 [29]C10TAB + 300 mM NaSal 303 1.5 �1.41 [29]C10TAB + 100 mM NaBr 303 41.7 �1.56 [29]

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

Time [s]

Hea

t [μ

W]

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

c [mM]

Q [k

J/m

ol]

-50

50

150

250

350

450

dQ/d

c0 3000 6000 9000 12000 15000 0 10 20 30 40 50 60 70

ΔHdm CMC

A B

Fig. 5. (A) An exemplary ITC titration heat-flow results of titration of [DMIM][Cl] (0.3 M) into KCl (0.1 M) at 298 K. (B) Enthalpogram of titration, first derivative indicating theCMC, and DHdm of demicellization are indicated.

-140

-130

-120

-110

-100

-90

-80

-70

-60

Time [s]

Hea

t [m

W]

-3-2

-1012

345

67

c [mM]

Q [k

J/m

ol]

-1200

-800

-400

0

dQ/d

c

0 4000 8000 12000 16000 20000 24000 0 5 10 15 20 25

A B

Fig. 6. (A) An exemplary ITC titration heat-flow result of the titration of [DMIM][Cl] (0.1 M) into KI (0.1 M) at 308 K. (B) Enthalpogram of titration, first derivative indicatingthe CMC.

J. Łuczak et al. / Journal of Colloid and Interface Science 362 (2011) 415–422 419

-4

-2

0

2

4

6

8

0 5 10 15 20 25c [mM]

Q [k

J/m

ol]

308 K303 K298 K

293 K288 K

Fig. 9. Heat change versus concentration of IL from demicellization process of[DMIM][Cl] 0.1 M to 0.1 M KI at various temperatures.

420 J. Łuczak et al. / Journal of Colloid and Interface Science 362 (2011) 415–422

micellization of [DMIM][Cl] in deionized water was also measured.The experiments carried out in the binary system revealed that themicellization of [DMIM][Cl] in deionized water is an endothermicprocess at all temperatures measured. With the addition of KCl,the micellization process was also found to be endothermic(DHm > 0), however the DHm values were lower. For KNO3 andKBr an exothermic process (DHm < 0) was observed at higher tem-peratures above 293 K and 298 K, respectively. KI salt additionresulted in an exothermic process for all measured temperatures.Therefore, the introduction of these inorganic salts affects adecrease in DHm, where the trend is described by the Hofmeisterseries of anions.

The micellization enthalpy of all ionic liquids is strongly depen-dant on the temperature. It was observed that with increasing tem-perature the DHm decreased, as shown in Figs. 7–9. For example,for the [DMIM][Cl]–KI system the heat of micellization at 288 Kis �4.3 kJ/mol whereas it is �7.4 kJ/mol at 308 K. Similar tenden-cies were observed in literature [29]. The measured CMC andDHm of the samples, as well as the calculated DGm and TDSm arelisted in Table 4.

The enthalpy of micellization is comprised of at least twoopposing phenomena. The positive contribution to DHm is attrib-uted to the energy required to release the structural water fromthe hydration layer around the hydrophilic domain as well as thewater cage around the hydrophobic moiety of the IL molecule[15]. To form a micelle, a corresponding cavity in the water struc-

-5

-4

-3

-2

-1

0

1

2

c [mM]

Q [k

J/m

ol]

303K298K293K

288K283K

0 20 40 60 80 100

Fig. 7. Heat change versus concentration of IL of demicellization process of[DMIM][Cl] 0.5 M to water at various temperatures.

-6

-5

-4

-3

-2

-1

0

1

2

c [mM]

Q [k

J/m

ol]

303 C

298 C293 C

288 C283 C

0 10 20 30 40 50 60 70

Fig. 8. Heat change versus concentration of IL of demicellization process of[DMIM][Cl] 0.3 M to 0.1 M KCl at various temperatures.

ture should be created. If the water is highly structured, as a resultof the addition of a kosmotrope for example, it is energeticallymore difficult to break the network. On the contrary, the additionof a chaotrope enhances the cavity formation, as it will require lessenergy to create the same size cavity. In our work, we observedthat the presence of the inorganic salts in the aqueous phase de-creased DHm. This is probably a result of a decrease of the energyrequirement to break the structural water, and increase the nega-tive contribution connected with the transfer of monomers into themicelle, as well as the decreasing repulsion between the head-groups. In Table 3 of the CMC and DHm reductions of C10TAB inthe presence of various salts are shown. Similarly to our IL results,the addition of NaBr and NaI the DHm was also positive, and forbromides and iodides negative.

Generally, the entropy is the principle driving force towardsmicellization, which is initiated by the highly ordered structureof the water around the hydrophobic domain of the IL. During mic-ellization, water molecules in the hydration cage around thehydrophobic parts of the IL monomers are reduced. This is reflectedby the increase of entropy during micellization and decrease of freeenergy [16]. This increase in entropy is determined by various inor-ganic salts; as part of the Hofmeister series. Kosmotropes willimpede micelle formation as the solution entropy is low and inconsequence the entropic driving force is also lower. When chao-tropes are added, the entropy difference is high – micelle forma-tion is enhanced as a result of a significant increase in theentropy difference of the system. As can be seen in Table 4, addi-tion of salts increases TDSm from 28.4 kJ/mol in binary systemand from 29.9 kJ/mol for KCl to 32.3 kJ/mol for KI at 298 K.

The variations of the entropy are positive over the whole tem-perature range measured and decrease with temperature. Withincreasing temperature the contribution of entropy to the micelleformation become less, as at higher temperatures water moleculesaround the hydrophobic and hydrophilic domain of the moleculebecome less ordered. Thus, in high temperatures the major contri-bution in DGm comes from DHm, which becomes more negative[16]. In all systems �DHm < TDSm indicates that the micellizationprocess in mainly entropy-driven [14].

4. Conclusions

The first step of characterization and implementation of neo-teric chemicals is the determination of physicochemical parame-ters in simplified (but well known) systems. To this date anumber of publications, including our own, revealed that ionic liq-uids in binary systems posses surface activity. However, the second

Table 4Summary of thermodynamic parameters of micellization of [DMIM][Cl] with KX salts at various temperatures. As a comparison the results of [DMIM][Cl] in a binary system wereincluded.

T (K) CMC (mM) DGm (kJ/mol) DHm (kJ/mol) �TDSm (kJ/mol)

[DMIM][Cl]binary 283 70.8 ± 1.1 �23.0 5.2 ± 0.5 �28.2288 65.6 ± 0.9 �23.7 4.6 ± 0.3 �28.3293 62.2 ± 1.0 �24.3 3.3 ± 0.2 �27.6298 58.5 ± 1.3 �24.9 3.5 ± 0.2 �28.4303 54.8 ± 1.4 �25.6 2.2 ± 0.3 �27.8308 58.1 ± 1.3 �25.8 1.4 ± 0.2 �27.2

[DMIM][Cl] + KCl 283 46.6 ± 1.1 �24.7 5.0 ± 0.2 �29.6288 44.5 ± 1.2 �25.3 4.8 ± 0.1 �30.0293 39.6 ± 0.6 �25.9 4.2 ± 0.2 �30.1298 38.0 ± 1.1 �26.5 3.4 ± 0.1 �29.9303 37.2 ± 0.2 �27.2 2.5 ± 0.5 �29.7

[DMIM][Cl] + KBr 283 31.7 ± 0.7 �27.5 2.5 ± 0.1 �30.0288 30.3 ± 1.2 �28.2 1.4 ± 0.2 �29.5293 29.5 ± 0.4 �28.8 0.5 ± 0.1 �29.2298 28.3 ± 1.0 �29.4 �1.5 ± 0.3 �27.9303 25.5 ± 0.9 �30.3 �2.5 ± 0.3 �27.8

[DMIM][Cl] + KNO3 288 23.0 ± 0.7 �28.7 0.5 ± 0.1 �29.2293 22.1 ± 0.5 �29.4 �0.8 ± 0.4 �28.6298 21.4 ± 1.0 �30.0 �1.2 ± 0.2 �28.8303 20.7 ± 0.4 �30.6 �2.3 ± 0.3 �28.3308 20.9 ± 0.5 �31.1 �3.7 ± 0.5 �27.4

[DMIM][Cl] + KI 288 7.4 ± 0.4 �37.2 �4.3 ± 0.3 �32.9293 7.1 ± 0.4 �38.0 �4.7 ± 0.4 �33.3298 7.2 ± 0.7 �38.6 �6.2 ± 0.7 �32.3303 8.7 ± 1.5 �38.4 �6.8 ± 0.3 �31.6308 8.9 ± 0.7 �38.9 �7.4 ± 1.5 �31.5

J. Łuczak et al. / Journal of Colloid and Interface Science 362 (2011) 415–422 421

step is the understanding of behavior of molecules in moreadvanced systems – such as ternary (or higher) systems. Knowl-edge of the aggregation phenomena of surface active compoundsis important to understand how they behave such systems as thesecan condition or influence the efficiency of many unit operationsand processes.

At this stage of ILs development, measuring and comparing thevarious properties is important yet challenging as number of theILs may be enormous. The work reported here has demonstratedthat liquid [DMIM][Cl] acts as a cationic surfactant in aqueoussolution. The formation of micelles is affected by the electrolyteadded to the system. The salt influence on the aggregation behav-ior on [DMIM][Cl] between 283 and 308 K was studied for the firsttime. A decrease of the CMC as a result of screening the electro-static repulsion between the heads groups was observed. The saltsused in this study decrease CMC following the trend which paral-lels Hofmeister series of ions.

ITC method was used to obtain a detailed thermodynamicdescription of the micellization of cationic IL in aqueous solutionin presence of salts at various temperatures. As the anion of theadded inorganic electrolyte changes from Cl� to I� following theHofmeister series, the micellization process became more exother-mic, thus the DH was found to decrease. This is accompanied by anincrease of entropy and decrease of free energy of micellization.

In prior publications, few authors have described the effect ofinorganic salts on IL/water systems [8–10]. However, this is thefirst time that a thermodynamic description with direct measure-ment has been given for ionic liquids. The present work hasclarified the energetic behavior of ternary ionic liquid/water/inor-ganic salt systems. Our results assist in the understanding of theinfluence of salts on the water structure and thereby changes inits solvating ability of our ionic liquid.

Acknowledgments

Financial support was provided by the Polish Ministry ofScience and Higher Education in the years 2007–2010 Grant No.:

N205 041 32/2340, and years 2010–2011 Grant No.: N305359138. This research was also supported by the European Unionwithin the European Social Fund in the framework of the Project‘‘InnoDoktorant – Scholarships for PhD students 2nd edition’’.

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