ISSN 00360244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 10, pp. 1695–1704. © Pleiades Publishing, Ltd., 2010. Original Russian Text © N.A. Smirnova, E.A. Safonova, 2010, published in Zhurnal Fizicheskoi Khimii, 2010, Vol. 84, No. 10, pp. 1857–1867.

PHYSICAL CHEMISTRY OF SOLUTIONS

Ionic Liquids as Surfactants1 N. A. Smirnova and E. A. Safonova Department of Chemistry, St. Petersburg State University, St. Petersburg, Russia email: [email protected] Received March 24, 2010

Abstract—Problems of selfassembling in systems containing ionic liquids (ILs) are discussed. Main atten tion is paid to micellization in aqueous solutions of dialkylimidazolium ILs and their mixtures with classical surfactants. Literature data are reviewed, the results obtained by the authors and coworkers are presented. Thermodynamic aspects of the studies and problems of molecularthermodynamic modeling receive special emphasis. It is shown that the aggregation behavior of dialkylimidazolium ILs is close to that of alkyltrime thylammonium salts (cationic surfactants) though ILs have a higher ability to selforganize, especially as it concerns longrange ordering. Some aspects of ILs applications are outlined where their common features with classical surfactants and definite specificity are of value. DOI: 10.1134/S0036024410100067

Organic salts having melting temperature below 100°С and named Ionic Liquids (ILs) occupy a spe cial place among solvents. Such properties as a wide liquid range, low volatility, high thermal stability, low toxicity, high ionic conductivity, wide electrochemical window etc. make many ILs perspective for applica tions in various fields of chemistry and technology and determine their importance for “green” chemistry. Since the 1990s researches in the field of ILs are pro gressing intensively. Investigations of chemical pro cesses, catalytic reactions in ILs, the search of their applications in electrochemistry, extraction, chroma tography, for the synthesis of new materials present main directions of the studies [1–3]. Problems of ILs physical chemistry are discussed in one of the last issues of the Physical Chemistry Chemical Physics journal [4]. Like other salts, ILs are formed by ions but at least one of the constituent ions is organic and cation and anion differ significantly in their geometrical charac teristics. As a rule, cation presents massive species. Ionic asymmetry opposes the crystal structure forma tion promoted by strong coulomb interactions, and many organic salts crystallize at temperatures below 100°С, the crystallization temperatures of so called Room Temperature Ionic Liquids (RTILs) are below room temperatures.

Selforganizing of dialkylimidazolium ILs was studied rather extensively – both experimentally and by com puter simulations. Nanostructural organization of individual liquid 1alkyl3methylimidazolium salts [Сnmim]X (X denotes anion) with various alkyl chain length (various n value) was confirmed by molecular dynamic (MD) simulations [5–7]. Formation of two types of domains was revealed: positively charged imi dazolium rings and anions arrange in threedimen sional polar network supported by strong electrostatic interactions, whereas alkyl groups aggregate forming nonpolar domains where shortranged van der Waals interactions are decisive. Thus, microphase separation to hydrophilic and lypophilic regions is observed in ILs. MD simulations of 1octyl3mehylimidazolium nitrate mixtures with water [6] have shown that with the increasing water content the polar network of IL ions is getting ruined, the role of interactions between water molecules and polar IL groups increases; at high dilutions, formation of micelles in aqueous surrounding is registered. It has been confirmed experimentally that in certain con centration range longchain ILs and water form liquid crystal ionomer gels [8, 9]; the structure of lyotropic phases was studied experimentally for concentrated solutions of alkylimidazolium bromides [10].

Many ILs are amphiphilic substances with pro nounced hydrophilic and lipophilic molecular frag ments what determines their surface activity and the ability to selforganize in the individual state and in solutions. Such properties have in particular dialkyl imidazolium, dialkylpyridinium and alkylammonium salts belonging to the most widely spread ILs (Fig. 1). 1

(а)

R

N

+

(b)

N

+

R'

N R

R' Fig. 1. Structural formulas of dialkylimidazolium (a) and dialkylpyridinium (b) cations.

The article was translated by the authors.

1695

1696

SMIRNOVA, SAFONOVA

Atomic force microscopy studies of ILs in contact with solids [11] have shown that the supramolecular structuring is observed not only in layers adjacent to the solid surface but also in the bulk liquid, where the longrange order appears, and this is the specificity of ILs in comparison with classical molecular fluids. Specific behavior of systems containing ILs is depen dent on the complex character of intermolecular interactions incorporating many constituents (ion ion, van der Waals, dispersion interactions, hydrogen bonding, in many systems also n–π and π–πinterac tions). Varying the chemical structure of ions it is pos sible to influence intermolecular interactions and self organization in ILs containing systems, what is impor tant for many applications of ILs. Micellar solutions of classical surfactants in ILs are among the systems of interest [12–15]. In particular, systems of this type containing fluorinated compounds are paid attention [13, 14]. As surfactant aggregates can solubilize many substances not soluble in ILs, adding surfactants makes possible to extend the range of applications of ILs. In particular, surfactant–IL– CO2 mixtures are considered perspective as a reaction medium [16]. Micellization in solutions where ILs act as surface active solutes is studied intensively since 2004 [17– 39]. These are mostly aqueous solutions of alkylimida zolium ILs. The studies permitted to compare ILs with other surfactants, to obtain information on the char acteristics of aggregation in solutions of many individ ual ILs and their mixtures with classical surfactants. The ability to selforganize in solutions opens addi tional perspectives for applications of ILs in chemical synthesis, catalysis, electrochemistry, extraction and chromatography, in the synthesis of polymer materi als, nanomaterials etc., and many results obtained in the last years give evidence of this. We shall give below only several examples. It is shown that chiral and longchain ILs (in par ticular [C16mim]Br) are perspective for the use in micellar electrokinetic capillary chromatography where they can compete successfully with classical surfactants [40–42]. Longchain imidazolium ILs ([C16mim]Cl and others) are superior in comparison with classical sur factants CTAC and CTAB (cetyltrimethylammonium chloride and bromide, respectively) widely applied as templates in solgel technology. In particular, it con cerns the synthesis of porous silica materials with a twodimensional hexagonal mesoporous structure (MCM41 type) [43, 44] and with a cubic gyroid mesostructure (MCM48 type) [45, 46]. A hydrothermal synthesis procedure using [C16mim]Cl as a template gave mesostructural materi als with a high volume fraction of pores and their uni form distribution. A good reproducibility of meso structures was observed over a wide range of IL/SiO2 molar ratios, and the mesophase could be synthesized

at relatively low, in comparison with CTAC, IL con centrations. The preferences shown by ILs are due to certain specific features of imidazolium polar groups in comparison with the ammonium ones; these are their stronger interactions with silicate oligomers and a higher tendency to affect long ordering in the system. Disclike imazolium head groups have a tendency to arrange themselves in a parallel fashion, and such ordering is supported by a distinct polarizability of the groups and by the hydrogen bonding. As a result, when ILs are used as templates, mesostructures with a low curvature are formed preferentially, – such as lamellar and bicontinious cubic phases. A method to synthesize zeolite analogues is proposed where imidazolium ILs perform both as a solvent and as a template (ionother mal synthesis) [47]. Applications of ILs in electrochemical systems look promising [48]; in particular, a high selectivity can be attained with sensors containing these surfac tants and nanotubes [49]. Combining ILs and nano tubes is perspective also for other purposes. Thus, using the longchain IL (1hexadecyl3vinylimida zolium bromide), instead of classical surfactants, one can improve the method of preparing aqueous disper sions of singlewalled carbon nanotubes by a noncova lent modification [50], and this is of value for many applications of the dispersions – in particular, in bio medicine. As it concerns the mechanism of IL action, the hydrophobic parts of IL molecules interact with the nanotube surface whereas the hydrophilic parts are turned to the aqueous phase. Gelation of IL contain ing systems is a problem of interest (in particular, for electrochemical needs), and this task can be solved also owing to amphiphilic properties of ILs. In some cases carbon nanotubes are used in the procedure of IL gelation [51, 52]. New results show how the ability of ILs to self organize can affect chemical processes. Thus, the study of organometallic palladium complexes immo bilized in a thin film of alkylimidazolium IL on silica provides evidence for formation of threedimensional solvent “cage” around the complex, what is not observed in the case of classic solvents [53]. This may open new synthetic pathways, in particular for stereo selective reactions. Properties of microemulsions containing ILs are investigated [54], these systems being paid attention as reaction media. A shortchain IL can play the role of a polar component in microemulsions [55–58], a lypo philic IL can perform similarly to a hydrocarbon [59], longchain ILs capable of micelle formation perform as surfactants [60]. Alongside with other systems where formation and breakage of amphiphilic aggregates may be regulated, IL solutions are of special interest for such applica tions as controlled release, drug delivery, nanosynthe sis etc. Regulation is performed through various vari ables (pH, temperature, light etc.). It is known in par

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

Vol. 84

No. 10

2010

IONIC LIQUIDS AS SURFACTANTS

ticular, that the behavior of imidazolium ILs in solutions is greatly affected by the counteranion nature [33, 61]. Thus, aggregates of [C12mim]Br con taining solubilizate can be broken upon exchange of bromideanion for PF6− , and such ionresponsive behavior of IL aggregates may be applied in controlled release and emulsification [62]. A simple and effective method was proposed to obtain stable multilayered vesicles in aqueous solu tions by mixing pyridinium bromide or tetrafluorobo rate ([С6Py]Br, [С6Py]BF4) with sodium dodecylsul fate NaDS [63]. Formation of aggregates of such type is observed usually in solutions of individual surfac tants having a peculiar molecular structure (two hydrocarbon tails attached to one polar head), this occurs also in mixtures of cationic and anionic surfac tants where doubletailed catanionic complexes are formed. ILs may substitute successfully traditional surfactants in preparation of vesicular solutions that are widely applied, in particular for controlled drug delivery. The examples given above illustrate, though not exhaustively, how ILs may be used as surfactants capa ble of selforganizing. Evidently, better understanding of selfassembly phenomena in solutions of ILs pre sents a challenging physicochemical task and helps in many applications. Below we consider micellization in solutions of individual ILs and mixtures of ILs with classical sur factants. Literature data are discussed with an empha sis on the ability of ILs to modulate the aggregation behavior of sodium dodecylsulfate (the data obtained by the authors and coworkers) [39]. Attention is paid to thermodynamic aspects and molecularstatistical thermodynamic modeling helpful in better under standing the observed behavior and in making predic tions. Micellization in ILs Aqueous Solutions Most of the studies of IL aggregation deal with aqueous solutions of 1alkyl3methylimidazolium salts [Cnmim]X, where anion X– is usually Br–, Cl–, BF4− , or PF6− and the number of carbon atoms in the alkyl chain of IL molecule takes values from 2 to 16 [17, 19–37]. The experimental data obtained before 2008 for imidazolium ILs are reviewed in [38]. Aque ous solutions of geminal forms of dialkylimidazolium salts [21, 26, 64], and also of dialkylpyridinium, dialkylpiperidinium and dialkylpyrrolidinium salts etc. [22, 32] were investigated. Formation of small aggregates of [C4mim]+ in water, methanol, 2pro panol, ethyl acetate [65, 30, 31] was registered by massspectrometry and conductometry. There are data on aggregation in mixtures of two ILs: [Cnmim]X (n = 10–16, X = Cl–, BF4− , or Br–) dissolved in ethyl ammonium nitrate (EAN) [66] or 1buthyl3meth RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

1697

ylimidazolium tetrafluoroborate ([C4mim]BF4) [67]. Aggregates in these solvents have appeared larger than in the case of aqueous solutions and the critical micelle concentrations (cmc) were approximately one order of magnitude higher. Cmc values and structural characteristics of aggre gates were determined applying various experimental techniques (tensiometry, conductometry, potentiome try, fluorimetry, NMR spectroscopy, light scattering, SmallAngle Neutron Scattering (SANS), titration calorimetry). Although there are some discrepancies in the quantitative estimations, the general regularities in the dependence of the characteristics on IL struc ture, temperature and solution concentration are well seen. The data show that in aqueous solutions of [Cnmim]X the cmc is distinctly revealed at n ≥ 8 but the aggregation occurs also in systems with lower n val ues. In particular, small polydisperse spherical aggre gates (with aggregation numbers around 11) were reg istered by SANS for [C4mim]BF4 solutions at concen trations above 800 mM [17, 31]. The studies permit to conclude that [Cnmim]X salts behave like typical cat ionic surfactants. Their aggregation behavior is similar to that of alkyltrimethylammonium salts (CnTAX) but the cmc values for [Cnmim]X salts are some lower than for CnTAX with the same n value. The higher IL tendency to aggregation in compar ison with CnTАХ may be explained by specific features of imidazolium head group, such as the ability of hydrogen bond formation, what was mentioned ear lier. Both for classical surfactants and for ILs the dependence of log cmc on n (the number of carbon atoms in alkyl chain) is close to linear (Fig. 2). There is a similarity also in the character of the cmc temper ature dependence. In case of aqueous solutions of classical ionic surfactants the cmc initially decreases with temperature but than starts to raise, the tempera ture corresponding to the cmc(T) minimum is close to room temperature [68]. The similar behavior is observed in the case of ILs [28, 37, 69]. The observed cmc minimum is a consequence of two main factors: (1) the decreasing degree of hydration of ionic groups with temperature results in the increase of surfactant hydrophobicity, and this favours the aggregation, (2) structured water clusters surrounding hydrophobic domains break up with temperature, and this results in the growth of entropy, that is in reducing the hydro phobic effect which is the main driving force of micel lization. The cmc minimum for more hydrophobic ILs is observed at lower temperatures but this changing is inconsiderable. In case of alkylmethylimidazolium [Cnmim]X and tetraalkylammonium CnTAX halides the effect of anion nature on the cmc and the degree of counter ion binding is similar: cmc values for bromides are lower (Table 1) and the degree of counter ion binding is somewhat higher than for chlorides. The conductivity measurements gave for [Cnmim]Br at n = 8–16 the Vol. 84

No. 10

2010

1698

SMIRNOVA, SAFONOVA

logcmc 100

10 1 2 3

1

8

10

12

14

16 n

Fig. 2. The log cmc [mM] values versus the number of car bon atoms n for aqueous [Cnmim]Br (1) [33], CnTAB (2) and sodium alkylsulfate (3) solutions: points represent the experimental cmc values; the line shows the results of pre dictions by quasichemical model for [Cnmim]Br, T = 298.15 K [39].

degree of counter ion binding β from 0.66 to 0.78 (the degree of micelle ionization is α = 1 – β) [19, 24, 28, 34, 35]. The β values for chlorides are slightly lower than for bromides; though there are some discrepan cies in the data a tendency for β values to increase with n is seen [20, 27–29]. A higher degree of counterion binding in the case of bromides may be explained by weaker hydration of bromide ions in comparison with chloride ions and by their more intensive dispersion interactions with IL polar group. Due to this bromide Table 1. cmc data for aqueous solutions of alkylmethylim idazolium chlorides and bromides from the tensiometric measurements at T = 298.15 K n

[Cnmim]Cl

[Cnmim]Br

8 9 10

234 [29]

150а [10] 73b [33] 43 [19] 29.3 [24] 41c [39] 10.9 [26]

12 14

16

18 Note:

55 [23] 53.8 [29] 39.9 [27] 15 [23] 13.2 [27] 4 [23] 3.15 [29] 3.0 [27] 1.14 [29] 0.89 [66] 0.87 [27] 0.45 [29]

2.8 [26]

0.8 [21] 0.55 [26]

a At 293 K, b at 295 K, c data from the conductometry.

ions are more easily adsorbed on the micelle surface and more effectively decrease the electrostatic repul sion between the surfactant cations in the micellar “crown” what favors the aggregation. The cmc value is correspondingly lower. Whereas the effect of the anion nature on the cmc and on the degree of coun terion binding for halides is rather well studied, the information about ILs with other anions is not sys tematic. It was found that in some cases the cmc val ues for [Cnmim]Х salts with distinct X are close. The cmc values for aqueous solutions of dicationic (gem inal) imidazolium ILs are lower in comparison with those of monocationic having the same alkyl chain length [21, 64]. The average aggregation numbers Nagg for micellar aqueous solutions of [Cnmim]Х were determined by fluorimetry, SANS and NMR [10, 17, 26, 27, 32, 34– 36]. It was shown that the micelle shape in the region of the cmc is close to spherical and the aggregation number increases with n; in case of n = 10 the elonga tion of micelles with the concentration was observed [10]. Thermodynamic functions of micellization in aqueous solutions are determined for several ILs, the most complete are the data for alkylmethylimidazo lium chlorides and bromides [15, 20, 32, 69]. The Gibbs energy of micellization can be estimated from the cmc and degree of counterion binding data:

ΔG mic = (1 + β )RT ln X cmc , where Xcmc is the surfactant mole fraction at the cmc. The direct method to determinate the enthalpy of micellization is titration calorimetry, this method was applied in [15, 20, 32, 39, 69]. For alkylmethylimida zolium bromides the enthalpy was estimated using the cmc data in dependence on the temperature [28]:

∂ ( Δ G mic/ T ) . ∂ (1 / T ) The enthalpy of micellization ΔHmic for all studied systems at temperatures above 290 K is negative and has a small absolute value (Table 2), what corresponds to the usual temperature dependence of the cmc val ues in this region (weak growth with temperature). For [Cnmim]Br aqueous solutions (n = 12, 14, or 16) inves tigated in the temperature range from 278.15 K to 328.15 K, ΔHmic changes with temperature from posi tive to negative [69]. The entropy term TΔSmic = ΔHmic – ΔGmic for the studied systems at 308 K or below is positive and has a higher value than |ΔHmic|. It means that ΔGmic is essentially determined by the entropy contribution, i.e. the hydrophobic effect is the most important driving force of micellization, like in the case of other surfactants. As the temperature rises, the enthalpy contribution becomes higher. The abso lute values of the Gibbs energy and the entropy of micellization grow with n. ΔHmic and ΔGmic values for alkylmethylimidazolium bromides are more negative Δ H mic =

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

Vol. 84

No. 10

2010

IONIC LIQUIDS AS SURFACTANTS

than those for alkyltrimethylammonium chlorides with the same alkyl chain length.

Table 2. cmc values and thermodynamic functions of mi cellization (kJ/mol) for aqueous solutions of [Cnmim]Cl (n = 8–14) at 308.15 K [20]

Effect of IL Addition on Micellization of Classical Surfactants The ability of ILs to affect micellization character istics of classical surfactants was studied for various ILsurfactant combinations, including mixtures of several ILs with ionic [18, 23, 38, 39, 70–73], non ionic [15, 74] and zwitterionic surfactants [61, 75]. The effects on addition of alkylmethylimidazolium salts to sodium dodecylsulfate solutions (anionic sur factant) were investigated the most thoroughly. Mea surements for NaDS solutions containing [Cnmim]Cl (n = 2–10, the interval of IL concentrations 5– 100 mM) have shown that the IL addition favors NaDS micellization. The concentration of NaDS answering the transition from the solution of mono mers to the micellar solution (cmcNaDS) decreases with the increase of the IL content, and the effect becomes more pronounced with the growth of the IL alkyl chain length (at higher n values) [23]. The data for aqueous NaDS–[C4mim]BF4 solutions at IL concen trations up to ~1.3 M have shown that at around 100 mM there is a change in the character of the IL effect on the cmcNaDS [71, 72]. The further addition of [C4mim]BF4 results not in the decrease of the cmcNaDS but in its increase. Evidently in this composition range the concentration of [C4mim]BF4 in the aqueous sur roundings of micelles becomes significant, and one observes the growing role of IL as a cosolvent decreas ing the absolute value of the Gibbs energy of NaDS micellization. Effects of very small additions of IL to surfactant solutions are of particular interest. In [70] these effects were investigated for [C4mim]PF6, in our studies [39] mixtures of NaDS with [C4mim]PF6, [C6mim]X (X = Br, Cl, BF4) and [C10mim]Br were under consider ation. The cmcNaDS values were determined applying conductivity measurements, potentiometry and titra tion calorimetry. Examples of the curves obtained in the potentiometry measurements with DSselective electrode and in the titration calorimetry experiments are presented in Fig. 3. As is seen in Fig. 4 even very small additions of IL significantly decrease the cmcNaDS, and ILs act more effectively than inorganic salts, the effect growing with the increase of the IL alkyl chain length. This is due to the combination of the electrostatic cationanion interactions with the hydrophobic effect what results in the formation of mixed micelles. Alongside with the cmcNaDS versus CIL dependences we have considered the cmcNaDS dependence on the relative mole fraction of IL ( x IL ' ) in NaDS–IL mixture (the solventfree based mole fraction). Such a description is especially useful in the case when the IL forms micelles in its individual aqueous solutions (n ≥ 8) as it helps to com RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

1699

n

cmc, mM

–ΔGmic

–ΔHmic

TΔSmic

8 10 12 14

263 66 13.8 3.3

18.4 24.8 33.2 39.6

1.3 2.9 6.3

23.5 30.3 32.5

pare the systems under study with mixed solutions of classical surfactants. In this case cmc = CNaDS + CIL is the total concentration of NaDS and IL in the range of transition from the nonaggregated solution to the micellar one. The ccm( x IL ' ) dependence for NaDS–[C10mim]Br solutions (Fig. 5) is typical for mixtures of anionic and cationic surfactants: the cmc decreases sharply on small additions of one component to the other, whereas over the wide range of x IL ' values between 0.1 and 0.9 the cmc changes are insignificant; at x IL ' = 0.5 cmc = 0.16 mM, CNaDS = CIL = 0.08 mM. Evi dently a similar character of the cmc( x IL ' ) depen dences should be observed for all NaDS–[Cnmim]X mixtures at n ≥ 8, and the IL concentration along the whole cmc line is lower than the cmc value for the individual IL in water. Let us mention that in the liter ature some cmcNaDS estimations were made by extrap olation of the cmcNaDS versus CIL data to IL concentra tions higher than the cmc of the individual IL [23]. Evidently such estimations are not correct. The enthalpy of micellization (ΔHmic) measured at 308.15 and 318.15 K is negative for all studied mix tures of NaDS with [C4mim]PF6, [C6mim]X (X = Br, Cl, BF4) and [C10mim]Br; in this range the cmc increases with temperature [39]. The absolute ΔHmic values per a mole of NaDS at NaDS : IL =1 : 1 are higher than for solutions where the relative IL content is smaller. The lowest micellization enthalpy was observed for the equimolar solution of NaDS and [C10mim]Br where, as one can suppose, micelles are formed by the catanionic complex [C10mim]+DS–, so they are similar to micelles of a nonionic surfactant having a polar “head” with two attached alkyl tails. Thermodynamic characteristics of micellization for equimolar NaDSIL solutions were calculated applying the pseudophase separation model. It has been found that the entropy contribution and the absolute value of the Gibbs energy of micellization increase with the alkyl chain length in IL. The data presented in [39] demonstrate the effect of the IL anion nature on the aggregation characteristics. It is shown in particular that changing 1hexyl3meth ylimidazolium bromide for the chloride has small effect on the thermodynamic functions of micelliza tion. Results of calorimetric measurements for solu Vol. 84

No. 10

2010

1700

SMIRNOVA, SAFONOVA

ΔH, kJ/mol

cmcNaDS, mМ (a)

20

1 2 3 4 5 6 7

9

15 6 10 5

3

1 2

0 −5

0 emf, mV 280

260

1

2

3 4 CNaDS × 103, M

(b)

0

2

4

6

8

10 Cadditive, mМ

2

Fig. 4. cmcNaDS dependence on molar concentration of additives for aqueous solutions of NaDS containing [C4mim]PF6 (1), [C6mim]BF4 (2), [C6mim]Br (3), [C6mim]Cl (4), and NaCl (5). The data were obtained by conductometry at T = 298.15 K (1–5) and by calorimetry for [C4mim]PF6 (7) and [C6mim]Br (6), T = 308.15 K.

1

NaDS solutions, a significant cmc decrease being attained already at very low IL concentrations.

240

220

200

0

Molecular Thermodynamic Modeling of Micellization in Aqueous ILs Solutions and Their Mixtures with NaDS

Fig. 3. Titration curves for aqueous solutions of NaDS– [C6mim]Br at x IL ' = 0.5 (1) and x IL ' = 0.79 (2), T = 308.15 K (a); the emf values of DSselective electrode ver sus – log m NaDS (m is molality) for aqueous solutions of

Approximate models aimed at the search of rela tionships between molecular properties of compounds and their aggregation behavior are intensively devel oping since the 1970s [78–86]. According to the so called quasichemical approach, formation of mixed micelles in dilute solu tions of two surfactants (A and B) is described like chemical reaction:

NaDS–[C6mim]Br at x IL ' = 0.7 (1) and x IL ' = 0.8 (2), T = 298.15 K (b).

nA A 1 + nBB1 → A nAB nB ,

2.5

3.0

3.5

4.0

4.5

−logmNaDS

tions containing [C6mim]BF4 are greatly affected by –

the hydrolytic instability of BF 4 ions. A number of lit erature data confirm such instability [76, 77]. There is a substantial difference in the titration curves for the systems containing [C6mim]BF4 and [C6mim]Br (or [C6mim]Cl), and this difference grows with tempera ture. The temperature change from 308.15 to 318.15 K does not affect markedly the cmc value and the Gibbs micellization energy for NaDS–[C6mim]BF4 solu tions, whereas the enthalpy changes significantly, especially at x IL ' > 0.5. In total the studies have shown that alkylimidazo lium ILs are effective modulators of micellization in

where A1, B1 are the monomeric forms of surfactants and nА, nВ are the numbers of molecules. The follow ing expression can be derived from the thermody namic equilibrium condition for the process:

⎡ ⎛ N agg g mic ⎞⎤ x nα = x1nAA x1nBB exp ⎢− ⎜ ⎟⎥ , ⎣ ⎝ kT ⎠⎦ where xi, are the mole fractions of mixed micelles (i = nα) and surfactant monomers (i = 1A, 1B), the aggre gation number is Nagg = nA + nB. The Gibbs energy of micellization gmic depends on the shape of micelles, their composition and size (Nagg). Equilibrium charac teristics of the micellar system correspond to the Gibbs energy minimum. The quasichemical model of aggregation was successfully applied to many solutions

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

Vol. 84

No. 10

2010

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

cmc, mМ 50 1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

40

αmic

of classical surfactant and their mixtures (see e.g., the reviews [87] and [88]). We used this approach for the first time to model micellization in aqueous ILs solutions and their mix tures with inorganic salts and classical ionic surfac tants [39]. We applied the version by Nagarajan and Ruckenstein [86], where the Gibbs energy of micelli zation is presented as the sum of the following contri butions: —transfer of the hydrophobic tails of amphiphilic molecules from water to the hydrocarbon core of the aggregate, —deformation of the surfactant tails at the forma tion of the micelle core, —formation of the “aggregate core–solvent” interface, —steric repulsions of polar head groups on the micelle surface, —mixing of A and B species in the micelle, —head groups dipoledipole interactions in the micellar crown, —electrostatic interactions of charged head groups with surrounding ions. We have used the expressions for the contributions given in [86] but the electrostatic contribution was estimated applying the Oshima approximation [89, 90]. The Gibbs energy of mixing was calculated using the Hildebrand solubility parameters. The following molecular model parameters are needed for the calculations: n is the number carbon atoms in the hydrophobic tail of surfactant molecule, ap is the effective crosssectional area of the head group of surfactant molecule, δ is the distance from the hydrophobic core surface to the center of the counterion. The parameter ap = 0.46 nm2 for [Cnmim]Br was estimated by fitting the experimental cmc data for [C12mim]Br solutions. This value is a bit lower than ap for alkyltrimethylammonium bromides, and this reflects a higher tendency of [Cnmim]Br to selfaggre gate. The calculated cmc values for [Cnmim]Br homo logues (8 < n < 16) agree satisfactorily with the experi mental data although at n = 8 and 10 they are some thing higher (Fig. 2). In the case of n = 4, 6 both the experimental data and the model calculations give no cmc for IL in aqueous solution. The agreement between the experimental and calculated aggregation numbers is also satisfactory, taking into account sim plifications of the model approach and experimental inaccuracies. The results obtained for the IL–NaDS mixtures show that the model reproduces in main features the effect of small additions of IL with different chain length (as well as sodium chloride) on the cmcNaDS. Thus, for mixed aqueous solutions of [C10mim]Br– NaDS the model predicts well the shape of the cmc curve in the whole range of relative concentrations of

1701

30

0 20

0

αmon

IONIC LIQUIDS AS SURFACTANTS

0 0.2 0.4 0.6 0.8 1.0 x'[Cnmim]Br

10

0 0

0.2

0.4

0.6

0.8 1.0 x'[C10mim]Br

Fig. 5. The experimental cmc values (points) and model predictions (line) versus x IL ' for aqueous solutions of [C10mim]Br–NaDS; cmc = cmcNaDS + CIL. The insert shows model predictions of IL mole fractions in mixed micelles (αmic) and in monomer population (αmon) for [C10mim]Br–NaDS (solid line) and [C12mim]Br–NaDS (dashed line) at total solution concentration CNaDS + IL = 12 mM, T = 298.15 K.

two amphiphilies; in agreements with the experiment the calculations give low cmc values at 0.1 < x IL ' < 0.9 (Fig. 5). The calculated cmc values are somewhat higher than the experimental ones (e.g., at x [' C10mim ]Br = 0.5 the corresponding values are 0.47 and 0.16 mM) but the discrepancies become smaller when the ар value for [C10mim]Br is taken lower. For NaDS mix tures with [C4mim]PF6 and [C6mim]BF4 the model predictions repeat basically the shape of the experi mental cmcNaDS curves and their relative position at 0 < x IL ' < 0.5 but without a quantitative agreement. The model description becomes better if ар parameter for these systems is decreased [39]. The approximation of the constant ap parameter (the effective crosssection area per a surfactant head group) is evidently rather crude when it is applied at different lengths of IL’s hydrocarbon radical and different compositions of mixed micelles. There is no doubt that due to strong cationanion interactions in mixed micelles this parameter should become smaller. In the case of NaDS mixtures with shortchain ILs another model restriction can also play role. This concerns the description of the deformation contribution, the approximation used doesn’t work well for mixed micelles where the sizes of alkyl chains for two amphiphilies are markedly different. Calculations of the aggregation numbers for [Cnmim]Br–NaDS solutions (n = 10, 12) with total NaDS+IL concentration 12 mM give approximately Vol. 84

No. 10

2010

1702

SMIRNOVA, SAFONOVA

ρ, nm−3 40

electric charge of micelles and formation of the catan ionic complex [Cnmim]+DS–.

(a)

1

The calculations have shown that the quasichem ical aggregation model can be quite useful for predic tions of aggregative behavior in the case of longchain ILs and their mixtures with classical surfactants.

2 30

20 3 10 4 0 40

2

1 1

3 (b) 2

30

20 5

3

10 4 6 0

1

2

3 r, nm

Fig. 6. Local density profiles for forcefield centers of CH2 or CH3groups of hydrocarbon micelle core (1), H2O (2), polar heads of [Cnmim]+ (3) and DS– (5), Br– (4) and Na+ ions (6); r = 0 is the center of the micelle. Sys tems: aqueous solutions of [C12mim]Br (a) and [C12mim]Br + NaDS (1 : 1) (b); T = 298.15 K.

constant Nagg values at small relative content of IL or NaDS whereas in the vicinity of the equimolar range the Nagg values increase significantly. The curve pre senting Nagg versus x IL ' for the mixture of [C10mim]Br and NaDS is asymmetric. The calculations of the composition of mixed micelles and that of monomer population in the aqueous surroundings (Fig. 5) indi cate that in the concentration range x IL ' < 0.4 the IL monomer fraction is very low, nearly all [Cnmim]+ ions are located in micelles and the monomer population consists in fact only of dodecylsulfate ions. When x IL ' > 0.6 the roles of DS– and [Cnmim]+ ions interchange. This is due to a tendency for the compensation of the

The results presented in [39] include structural characteristics of spherical micelles for individual [C12mim]Br and NaDS + [Cnmim]X mixtures (n = 4, 12, X = Br, PF6) in aqueous medium obtained by MD simulations. The influence of the IL cation alkyl chain length, the anion nature and NaDS/IL molar ratio on the local structure of aggregates at 298.15 K were investigated applying the unitedatom approach. The aggregate structure was described by the local density profiles calculated for all forcefield centers (atoms or atom groups). The simulations show a typical micellar structures where the aggregate has a hydrocarbon core surrounded by a polar crown. A substantial penetra tion of water molecules in the local range of polar heads is observed. For [C12mim]+ micelles, the absorption of bromide anions is pronounced. In the case of mixed micelles (NaDS + [Cnmim]X) the polar crown is composed of the polar groups of [Cnmim]+ cations and DS– anions, and when the ratio is 1 : 1 their charges are totally compensated, no charge sep aration occurs along the radial direction of the micelle (Fig. 6). Inorganic counterions are displaced to the outside of the micelle though some small absorption of Na+ ions in the micellar crown is seen. The micellar crown in mixed micelles NaDS : [C4mim]Br = 3 : 1 bears a negative charge and therefore it absorbs Na+ cations considerably. The micellar surface area per a surfactant molecule in the case of 1 : 1 NaDS + [C4mim]Br mixture is significantly lower than for their 3 : 1 mixture and than for NaDS : [C12mim]Br = 1 : 1 mixture. For mixed micelles this parameter is much lower than for micelles of individual IL [C12mim]Br. The calculated structural characteristics of micelles for the ILs differing only by anions are quite close. Thus, MD simulations confirmed main results given by the quasichemical aggregation model: the penetration of IL cations inside dodecylsulfate micelles and strong “cation–anion” interactions in the polar crown. They support the earlier made con clusion that the parameter ap of the quasichemical model should not be taken constant in the case of short alkylchain ILs and their mixtures with NaDS. ACKNOWLEDGMENTS The work was financially supported by Russian Foundation for Basic Research (project no. 0903 00746) and program of the President “Leading scien tific schools of Russian Federation” (project no. NSh 165.2008.3).

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

Vol. 84

No. 10

2010

IONIC LIQUIDS AS SURFACTANTS

1. Ionic Liquids in Synthesis, 2d ed., Ed. by P. Wassersc heid and T. Welton (Weinheim, WileyVCH, 2008). 2. Ionic Liquids: Industrial Applications to Green Chemis try, ACS Symp. Ser., Vol. 818, Ed. by R. D. Rogers and K. R. Seddon (ACS, Washington, 2002). 3. L. A. Aslanov, M. A. Zakharov, and N. L. Abramycheva, Ionic Liquids among Other Solvents (Mosk. Gos. Univ., Moscow, 2005) [in Russian]. 4. “Physical Chemistry of Ionic Liquids,” Phys. Chem. Chem. Phys. 12, 1648 (2010). 5. J. N. Canongia Lopes and A. A. H. Padua, J. Phys. Chem. B 110, 3330 (2006). 6. W. Jiang, Y. Wang, and G. A. Voth, J. Phys. Chem. B 111, 4812 (2007). 7. Y. Wang and G. A. Voth, J. Am. Chem. Soc. 127, 12192 (2005). 8. M. A. Firestone, J. A. Dzielawa, P. Zapol, et al., Lang muir 18, 7258 (2002). 9. M. A. Firestone, P. G. Rickert, S. Seifert, et al., Inorg. Chim. Acta 357, 3991 (2004). 10. I. Goodchild, L. Collier, S. L. Millar, et al., J. Colloid Interface Sci. 307, 455 (2007). 11. R. Hayes, G. G. Warr, and R. Atkin, Phys. Chem. Chem. Phys. 12, 1709 (2010). 12. D. Fennell Evans, A. Yamauchi, G. Jason Wel, et al., J. Phys. Chem. 87, 3537 (1983). 13. N. Li, S. Zhang, L. Zheng, et al., Langmuir 25, 10473 (2009). 14. N. Li, S. Zhang, L. Zheng, et al., J. Phys. Chem. B 112, 12453 (2008). 15. T. Inoue, Y. Higuchi, and T. Misono, J. Colloid Inter face Sci. 338, 308 (2009). 16. M. M. Hoffmann, M. P. Heitz, and J. B. Carr, J. Dis pers. Sci. Technol 24, 155 (2003). 17. J. Bowers, C. P. Butts, P. J. Martin, et al., Langmuir 20, 2191 (2004). 18. Z. Miskolczy, K. SebökNagy, L. Biczók, et al., Chem. Phys. Lett. 400, 296 (2004). 19. J. SirieixPlénet, L. Gaillon, and P. Letellier, Talanta 63, 979 (2004). 20. G. Bai, A. Lopes, and M. Bastos, J. Chem. Thermo dyn. 40, 1509 (2008). 21. Q. Q. Baltazar, J. Chandawalla, K. Sawyer, et al., Col loids Surf. A 302, 150 (2007). 22. M. Blesic, A. Lopes, E. Melo, et al., J. Phys. Chem. B 112, 8645 (2008). 23. M. Blesic, M. H. Marques, N. V. Plechkova, et al., Green Chem. 9, 481 (2007). 24. B. Dong, N. Li, L. Zheng, et al., Langmuir 23, 4178 (2007). 25. B. Dong, J. Zhang, L. Q. Zheng, et al., J. Colloid Inter face Sci. 319, 338 (2008). 26. B. Dong, X. Y. Zhao, L. Q. Zheng, et al., Colloids Surf. A 317, 666 (2008). 27. O. A. El Seoud, P. A. R. Pires, T. AbdelMoghny, et al., J. Colloid Interface Sci. 313, 296 (2007). 28. T. Inoue, H. Ebina, B. Dong, et al., J. Colloid Interface Sci. 314, 236 (2007). RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

29. C. Jungnickel, J. Luczak, J. Ranke, et al., Colloids Surf. A 316, 278 (2008). 30. A. Modaressi, H. Sifaoui, M. Mielcarz, et al., Colloids Surf. A 302, 181 (2007). 31. T. Singh and A. Kumar, J. Phys. Chem. B 111, 7843 (2007). 32. T. Singh and A. Kumar, Colloids Surf. A 318, 263 (2008). 33. S. L. I. Toh, J. McFarlane, C. Tsouris, et al., Solvent Extract. Ion Exchange 24, 33 (2006). 34. R. Vanyur, L. Biczók, and Z. Miskolczy, Colloids Surf. A 299, 256 (2007). 35. J. J. Wang, H. Y. Wang, S. L. Zhang, et al., J. Phys. Chem. B 111, 6181 (2007). 36. Y. Zhao, S. J. Gao, J. J. Wang, et al., J. Phys. Chem. B 112, 2031 (2008). 37. J. L – uczak, C. Jungnickel, M. Joskowska, et al., J. Col loid Interface Sci. 336, 111 (2009). 38. J. L – uczak, J. Hupka, J. Thöming, and C. Jungnickel, Colloids Surf. A 329, 125 (2008). 39. N. A. Smirnova, A. A. Vanin, E. A. Safonova, et al., J. Colloid Interface Sci. 336, 793 (2009). 40. J. Niu, H. Qiu, J. Li, et al., Chromatografia 69, 1093 (2009). 41. S. A. A. Rizvi and S. A. Shamsi, Anal. Chem. 78, 7061 (2006). 42. M. Borissova, K. Palk, and M. Koel, J. Chromatogr. A 1183, 192 (2008). 43. C. J. Adams, A. E. Bradley, and K. R. Seddon, Aust. J. Chem. 54, 679 (2001). 44. A. Zukal, M. Thommes, and J. Cejka, Microporous Mesoporous Mater. 104, 52 (2007). 45. T. Wang, H. Kaper, M. Antonietti, et al., Langmuir 23, 1489 (2007). 46. H. Kaper and B. Smarsly, Z. Phys. Chem. 220, 1455 (2006). 47. E. R. Cooper, C. D. Andrews, P. S. Wheatley, et al., Nature 430, 1012 (2004). 48. D. Wei and A. Avaska, Anal. Chim. Acta 607, 126 (2008). 49. H. Xu, H.Y. Xiong, Q.X. Zeng, et al., Electrochem. Commun. 11, 286 (2009). 50. A. Di Crescenzo, D. Demurtas, A. Renzetti, et al., Soft Matter 5, 62 (2009). 51. T. Fukushima and T. Aida, Chem.Eur. J. 13, 5048 (2007). 52. P. Yu, J. Yan, H. Zhao, et al., J. Phys. Chem. C 112, 2177 (2008). 53. C. Sievers, O. Jimenez, T. E. Müller, et al., J. Am. Chem. Soc. 128, 13990 (2006). 54. Z. Qiu and J. Texter, Curr. Opin. Coll. Interf. Sci. 12, 129 (2007). 55. R. Atkin and G. G. Warr, J. Phys. Chem. B 111, 9309 (2007). 56. O. Zech, S. Thomaier, P. Bauduin, et al., J. Phys. Chem. B 113, 465 (2009). 57. J. Eastoe, S. Gold, S. E. Rogers, et al., J. Am. Chem. Soc. 127, 7302 (2005). ˆ

REFERENCES

1703

Vol. 84

No. 10

2010

1704

SMIRNOVA, SAFONOVA

58. Y. Gao, N. Li, L. Q. Zheng, et al., J. Phys. Chem. B 111, 2506 (2007). 59. N. Anjum, M.A. GuedeauBoudeville, C. Stuben rauch, et al., J. Phys. Chem. B 113, 239 (2009). 60. C. Qin, J. Chai, J. Chen, et al., Colloid Polym. Sci. 286, 579 (2008). 61. K. Behera and S. Pandey, J. Colloid Interface Sci. 331, 196 (2009). 62. Y. Shen, Y. Zhang, D. Kuehner, et al., Chem. Phys. Chem. 9, 2198 (2008). 63. K. Singh, D. G. Marangoni, J. G. Quinn, et al., J. Col loid Interface Sci. 335, 105 (2009). 64. M. Ao, G. Xu, and Y. Zhu, J. Colloid Interface Sci. 326, 490 (2008). 65. S. Dorbritz, W. Ruth, and U. Kragl, Adv. Synth. Catal. 347, 1273 (2005). 66. S. Thomaier and W. Kunz, J. Mol. Liq. 130, 104 (2007). 67. N. Li, S. H. Zhang, L. Q. Zheng, et al., Phys. Chem. Chem. Phys. 10, 4375 (2008). 68. S. K. Mehta, K. K. Bhasin, R. Chauhan, et al., Colloids Surf. A 255, 153 (2005). 69. F. Geng, J. Liu, L. Zheng, et al., J. Chem. Eng. Data 55, 147 (2010). 70. K. Behera and S. Pandey, J. Colloid Interface Sci. 316, 803 (2007). 71. K. Behera and S. Pandey, J. Phys. Chem. B 111, 13307 (2007). 72. S. Lei, J. Zhang, and J. B. Huang, Acta Phys. Chim. Sin. 23, 1657 (2007). 73. K. Behera and S. Pandey, Langmuir 24, 6462 (2008). 74. K. Behera, M. D. Pandey, M. Porel, et al., J. Chem. Phys. 127, 184501 (2007).

75. A. Beyaz, W. S. Oh, and V. P. Reddy, Colloids Surf. B 35, 119 (2004). 76. D. G. Archer, J. A. Widegren, D. R. Kirklin, et al., J. Chem. Eng. Data 50, 1484 (2005). 77. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (Wiley, New York, 1988). 78. C. Tanford, The Hydrophobic Effect, 2nd ed. (Wiley, New York, 1980). 79. Solvent Properties of Surfactant Solutions, Ed. by K. Shi noda (Marcel Dekker, New York, 1967). 80. A. I. Rusanov, Micelle Formation in Solutions of Surfac tants (Khimiya, St.Petersburg, 1992) [in Russian]. 81. P. Mukerjee, J. Phys. Chem. 76, 565 (1972). 82. S. Puvvada and D. Blankschtein, J. Chem. Phys. 92, 3710 (1990). 83. S. Puvvada and D. Blankschtein, J. Phys. Chem. 96, 5567 (1992). 84. D. Blankschtein and S. Puvvada, MRS Symp. Proc. 177, 129 (1990). 85. J. N. Israelachvili, D. J. Mitchell, and B. W. Ninham, J. Chem. Soc., Faraday Trans. 2 72, 1525 (1976). 86. R. Nagarajan and E. Ruckenstein, Langmuir 7, 2934 (1991). 87. N. A. Smirnova, Usp. Khim. 74, 129 (2005). 88. E. A. Safonova, M. V. Alekseeva, and N. A. Smirnova, Kolloidn. Zh. 71, 704 (2009) [Colloid. J. 66, 717 (2009)]. 89. H. Ohshima, T. W. Healy, and L. R. White, J. Colloid Interface Sci. 90, 17 (1982). 90. V. A. Andreev, A. Yu. Vlasov, and N. A. Smirnova, Zh. Fiz. Khim. 80, 39 (2006) [Russ. J. Phys. Chem. A 80, 31 (2006)].

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

Vol. 84

No. 10

2010