Aggregation behavior of long-chain imidazolium ionic liquids in aqueous solution: Micellization and characterization of micelle microenvironment

Available online at www.sciencedirect.com

Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 666–672

Aggregation behavior of long-chain imidazolium ionic liquids in aqueous solution: Micellization and characterization of micelle microenvironment Bin Dong a , Xueyan Zhao a , Liqiang Zheng a,∗ , Jin Zhang a , Na Li a , Tohru Inoue b a

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, PR China b Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan Received 12 September 2007; received in revised form 3 December 2007; accepted 5 December 2007 Available online 14 December 2007

Abstract Aqueous solutions of long-chain imidazolium ionic liquids have been investigated by surface tension and steady-state fluorescence measurements at room temperature (298 K). The micelle aggregation number (Nagg ) was obtained by pyrene fluorescence quenching method. From the surface tension data, critical micelle concentration (cmc), surface tension at the cmc (γ cmc ), adsorption efficiency (pC20 ), and effectiveness of surface tension reduction (Π cmc ), were determined. Moreover, applying the Gibbs adsorption isotherm, maximum surface excess concentration (Γ max ) and minimum surface area per molecule (Amin ) at the air–water interface were estimated. The effect of sodium halides, NaCl, NaBr, and NaI, on the surface activity was also investigated. The addition of salts decreases significantly both cmc and γ cmc , and the dependence of the salt effect on the anion species is analogous for the case of conventional ionic surfactants. Due to the bulkiness of the imidazolium head group, the microenvironment in long-chain imidazolium ionic liquid micelles exhibits higher polarity compared with the corresponding Cn TAB micelles, and the micelle aggregation numbers (Nagg ) are smaller than those of Cn TAB. © 2007 Elsevier B.V. All rights reserved. Keywords: Long-chain imidazolium ionic liquids; Adsorption efficiency; Effectiveness of surface tension reduction; Critical micelle concentration; Surface tension; Fluorescence

1. Introduction Ionic liquids (ILs) are a class of organic molten electrolytes at or near ambient temperature [1]. Their physical and chemical properties can be tailored by judicious selection of cation, anion, and substituent. They have no significant vapor pressures, outstanding catalytic properties, high ion-conductivity, non-flammability, and are relatively inexpensive to manufacture [2]. Thus ILs have attracted much attention as electrolytes [3] and solvent media for reactions and extractions [4–6]. In recent years, ILs composed of 1-alkyl-3-methylimidazolium cation Cn mim+ have been extensively studied in the field of colloid and interface science. Law and Watson measured the surface tension of a series of Cn mimX [7]. Long-chain ILs have been found to form thermotropic liquid crystal [8]. Bilayer membranes formed from dialkyldimethylammonium bromides in ether-containing ILs were investigated [9]. Dry micelles of

∗

Corresponding author. Tel.: +86 531 88366062; fax: +86 531 88564750. E-mail address: [email protected] (L. Zheng).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.12.001

some traditional surfactants in C4 mimPF6 and C4 mimCl have been reported [10]. Lyotropic liquid crystalline phases of an amphiphilic block copolymer, P123, in C4 mimPF6 were studied [11]. Hexagonal liquid crystalline phases formed in ternary systems of Brij97/water/ILs (C4 mimBF4 and C4 mimPF6 ) were also investigated [12]. In addition, microemulsions including ILs were studied by a number of groups [13–16]. At the same time, the aggregation behaviors of ILs in aqueous solution have also been studied due to the close resemblance of ILs with long-hydrocarbon chain to conventional surfactants in their molecular structure. The behavior of C10 mimBr in aqueous solution has been studied by different groups [17,18], and it has been reported that the lyotropic liquid crystal and micelle were respectively formed at different concentration of C10 mimBr. Butts and co-workers explored the surface, phase and aggregation behaviors of aqueous mixtures of 1-alkyl-3-methylimidazolium halides with a variety of methods [19]. Bowers and co-workers investigated the micelle aggregates of three imidazolium ILs, C4 mimBF4 , C8 mimCl, and C8 mimI, in aqueous solution [20]. The micelle formation of C4 mimC8 SO4 was studied in aqueous solution with con-

B. Dong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 666–672

ductivity and turbidity measurements by Bicz´ok et al. [21]. Baker’s group studied the micellization of a homologous series of N-alkyl-N-methylpyrrolidinium bromides in aqueous solution [22]. Aggregation behavior of different ILs in aqueous solutions has been investigated using 1 H NMR, steady-state fluorescence spectroscopy, and refractrometry by Kumar et al. [23]. Jungnickel et al. investigated the micelle formation of 1-alkyl-3methylimidazolium chlorides in aqueous solution by conductivity and surface tension measurements [24]. However, the studies belonging to this category are not so many. Most of all, it is not well known how the special IL structure affects the phase behavior and aggregation behavior of ILs in aqueous solution. Previously, we investigated the surface adsorption and micelle formation in aqueous solution of three long-chain imidazolium ILs, C10 mimBr, C12 mimBr, and C12 mimBF4 [25]. The results demonstrated that the surface activity of the long-chain imidazolium ILs is superior to that of conventional ionic surfactants. As an extension of this study, we report the micelle formation of imidazolium ILs with extended alkyl chains, C14 mimBr and C16 mimBr. Surface tension and steady-state fluorescence measurements were performed at room temperature (298 K). Quite recently, we demonstrated the micelle formation of these long-chain imidazolium ILs by electrical conductivity measurements [26], and Bicz´ok et al. reported a similar study on the same IL species [27]. In the present work, the surface properties of the long-chain imidazolium ILs in aqueous solution in the absence or presence of sodium halides were investigated. A series of useful parameters was obtained to assess their surface activity, and was compared with those for conventional ionic surfactants. 2. Experimental 2.1. Materials The IL samples, C12 mimBr, C14 mimBr, C16 mimBr, were prepared and purified as described elsewhere [25]. Sodium halides, NaCl, NaBr and NaI, purchased from Tianjin Experimental Reagent Co. Ltd., were of analytical grade and were used as received. Pyrene (Fluka) was recrystallized twice from ethanol prior to use. Benzophenone (Beijing Yucai fine chemical plant) was analytical grade reagent. Triply distilled water was used to prepare all the solutions, and the ionic strength was maintained at 0.5 mol/L when the salt effect was investigated. These long-chain imidazolium ILs are miscible with water in the absence or presence of sodium halides within the composition range studied in this work.

667

ness of surface tension reduction, Π cmc , were obtained [28]. The former parameter is defined as pC20 = − log C20

(1)

where C is the surfactant molar concentration, and C20 stands for the concentration required to reduce the surface tension of pure solvent by 20 mN/m. The latter parameter, Π cmc , is the surface pressure at the cmc, given by the following equation: Πcmc = γo − γcmc

(2)

where γ o is the surface tension of pure solvent and γ cmc is the surface tension of the solution at the cmc. 2.3. Steady-state fluorescence measurements The fluorescence emission spectra of pyrene dissolved in long-chain imidazolium ILs aqueous solution were carried out using a PerkinElmer LS-55 spectrofluorometer (PE Company, UK) equipped with a thermostated cell holder at 25 ◦ C. The emission spectra were recorded from 350 to 450 nm after excitation at 335 nm with the slit widths fixed at 2.5 and 10 nm for the emission and the excitation, respectively. Pyrene exhibits fine structure in 370–400 nm region of the steady-state fluorescence emission spectra. The nature and the intensity are extremely dependent on the polarity of the environment [29]. The ratio of the first to the third vibronic peaks, i.e., I1 /I3 , shows the greatest solvent dependency, and hence, can be used to probe the micropolarity of the surfactant aggregates as well as to obtain the cmc of the surfactants in aqueous solution [29]. These two peaks appeared at ca. 373 and 384 nm, respectively. The pyrene concentration was fixed at 1 × 10−7 mol/L in all the measurements. The micelle aggregation number, Nagg , was determined according to the Turro–Yekta method [30]:   Nagg CQ I0 ln = (3) I CILs − cmc where I0 and I are the fluorescence intensities of pyrene in the absence and presence of the quencher (benzophenone in the present case) at a specific wavelength, CQ and CILs are the molar concentration of the quencher and long-chain imidazolium ILs, respectively. Eq. (3) was derived under the assumption that the probe and the quencher are entirely in the micelle phase, their distributions obey Poisson statistics, and the probe fluoresces only without the quencher. The concentration of each IL was less than five times the cmc to avoid the possible formation of polydispersion micelles, which would cause severe errors in the aggregation number determination [31].

2.2. Surface tension measurements 3. Results and discussion Surface tension measurements were performed with a Kr¨uss K12 tensiometer (Hamburg, Germany, accuracy ±0.01 mN/m). Temperature was controlled at 25 ± 0.1 ◦ C using a HAAKE DC 30 thermostatic bath (Karlsruhe, Germany). The surface tension was determined in single-measurement method. All measurements were repeated at least twice. From surface tension curves (γ − log C), the adsorption efficiency, pC20 , and the effective-

The long-chain imidazolium ILs, Cn mimBr (n = 12, 14, 16), were investigated by surface tension and steady-state fluorescence measurements. Surface tension measurements were performed in the presence of NaCl, NaBr, and NaI with the ionic strength fixed at 0.5 mol/L as well as under the salt-free condition, whereas steady-state fluorescence measurements were

668

B. Dong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 666–672

Fig. 1. Surface tension as a function of concentration for long-chain imidazolium ILs at 298 K: () C12 mimBr; () C14 mimBr; () C16 mimBr. The straight lines are drawn to determine the cmc values.

carried out only in aqueous solution without added sodium halides. 3.1. Surface tension measurements The surface tension was measured to evaluate the surface activity of the aqueous IL solutions. Fig. 1 represents the surface tension (γ) versus concentration (C) plot for the aqueous solution of the three long-chain imidazolium ILs at 298 K. For each IL, the surface tension progressively decreases with the increase in the IL concentration up to a plateau region, above which a nearly constant value (γ cmc ) is obtained. It is worth mentioning that the absence of a minimum around the breakpoint confirms the high purities of these long-chain imidazolium ILs; the fact that sharp endothermic peaks appeared in our differential scanning calorimetry (DSC) experiments also indicates the high purity of the sample we prepared [26,32]. It is evident that the γ cmc values of these long-chain imidazolium ILs are essentially the same, which is reasonable because they are homologues with only a slight difference in the hydrocarbon chain length [28]. In general, the effect of the hydrocarbon chain length on the γ cmc is weak. From the surface tension plot, two additional parameters, i.e., the adsorption efficiency, pC20 , and the effectiveness of surface tension reduction, Π cmc , are also obtained. It is well known that C20 is the minimum concentration needed to lead to a saturation of the surface adsorption. Thus, C20 can be a measure of the efficiency of the adsorption of surfactant molecules at the air–water interface [28]. Usually, the logarithm of the reciprocal C20 is used instead of C20 itself as shown in Eq. (1). Therefore, the greater the pC20 value, the higher the adsorption efficiency of the surfactant is. Another parameter Π cmc indicates the max-

imum reduction of surface tension caused by the dissolution of surfactant molecules, and hence, becomes a measure for the effectiveness of the surfactant to lower the surface tension of the solvent [28]. The values of these two parameters obtained for the three long-chain imidazolium ILs are summarized in Table 1. One can see that both the pC20 and the Π cmc increase in the order C12 mimBr < C14 mimBr < C16 mimBr, although the increase is not so great for the Π cmc values. Furthermore, the values of pC20 and Π cmc for the present ILs are compared with those for some conventional ionic surfactants in Figs. 2(a) and (b), respectively. On the basis of these limited data, both parameters are larger for ILs than for conventional ionic surfactants when compared at the same hydrocarbon chain length, although the difference is not so great. This indicates that the surface activity of the long-chain imidazolium ILs is superior to that of conventional ionic surfactants. The values of cmc determined from the surface tension data are listed in Table 1. They are in accordance with those reported in Refs. [24,25], which were determined by electrical conductivity measurements (see Table 3). The cmc values decrease in the order C12 mimBr > C14 mimBr > C16 mimBr as expected from the increased hydrophobicity due to the elongation of hydrocarbon chain. Thus, on the cmc, unlike γ cmc , the effect of the length of the hydrocarbon chain is great. Actually, compared with the cmc values reported for imidazolium ILs with shorter hydrocarbon chain, i.e., C4 mimBF4 (cmc = 0.8 M [20]), C8 mimCl (cmc = 0.1 M [20] and 0.2 M [24]), C8 mimI (cmc = 0.1 M [20]), C9 mimBr (cmc = 0.074 M [27]), C10 mimBr (cmc = 0.029 M [25] and 0.041 M [27]), the investigated ILs with longer hydrocarbon chain have significantly lower cmc values, which is a common trend to single-tail ionic surfactants. Jungnickel et al. obtained the cmc values for C14 mimCl of 3.26 mM and for C16 mimCl of 1.21 mM [24], which were higher than our results for C14 mimBr and C16 mimBr, respectively. The difference is due to the weaker hydration of the larger Br− , which more effectively decreases electrostatic repulsion and in this way facilitates micelle aggregation. The cmc values for the present ILs are compared with those for conventional ionic surfactants in Fig. 2(c). When compared at the same hydrocarbon chain length, the cmc values of these longchain imidazolium ILs are smaller than those of typical cationic surfactants, alkyl trimethylammonium bromides (Cn TAB), and slightly larger than those of anionic surfactants, sodium alkyl sulfates. These results indicate that the long-chain imidazolium ILs are superior to, or at least comparable to, traditional ionic surfactants in their capability for micelle formation. By applying the Gibbs adsorption isotherm to the surface tension versus concentration plot in the concentration range below and close to the cmc, the maximum surface excess concentration,

Table 1 Surface properties of C12 mimBr, C14 mimBr, and C16 mimBr at 298 K ILs

cmc (mmol/L)

γ cmc (mN/m)

Π cmc (mN/m)

pC20

Γ max (␮mol/m2 )

˚ 2) Amin (A

C12 mimBr C14 mimBr C16 mimBr

10.9 2.8 0.55

39.4 39.2 39.1

33.6 33.8 33.9

2.67 3.33 3.78

1.91 1.96 2.03

86.8 84.7 81.6

B. Dong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 666–672

669

relation: Amin =

1 ˚ ) (×1023 A NA · Γmax

(5)

where NA is the Avogadro constant (6.022 × 1023 mol−1 ). The values of Γ max are 1.91, 1.96, and 2.03 ␮mol/m2 for C12 mimBr, C14 mimBr, and C16 mimBr, respectively. In regard to the available data for typical cationic surfactants [34], these values are somewhat smaller than that for tetradecyl trimethylammonium bromide (Γ max = 2.7 ␮mol/m2 ), but close to those for tetradecyl tripropylammonium bromide (Γ max = 1.9 ␮mol/m2 ) and hexadecyl tripropylammonium bromide (Γ max = 1.8 ␮mol/m2 ). It is well established that the size of hydrophilic head group is a dominant factor to determine Amin value, and hence, Γ max value of surfactants [28]. Thus, the results that Γ max values for the present ILs are similar to those of alkyl tripropylammonium bromides rather than alkyl trimethylammonium bromides suggest that the bulkiness of both imidazolium group and tripropylammonium group are close to each other. Furthermore, the biggest Γ max of C16 mimBr among the present ILs could be attributed to the increased hydrophobicity resulting from the elongation of hydrocarbon chain. 3.2. Effect of sodium halides on the surface activity of ILs The surface tension of aqueous solution of these long-chain imidazolium ILs was measured in the presence of NaCl, NaBr, and NaI. As a representative example, Fig. 3 shows the results obtained for C16 mimBr. The surface tension as well as the cmc of C16 mimBr is reduced by all the sodium halides. The analogous trend has been observed for cetylpyridinium chloride (CPyCl) in the presence of NaCl and NaBr [35]. The cmc values of these long-chain imidazolium ILs in the presence of NaCl, NaBr, and NaI are summarized in Table 2. The decrease in the cmc values caused by the salt addition may be interpreted as follows. The added halogen anions compress the electric double layer surrounding the micelles, and consequently induce the screening of the electrostatic repulsion among the polar head groups,

Fig. 2. Comparison of pC20 (a), Π cmc (b), and cmc (c) for long-chain imidazolium ILs with those for conventional ionic surfactants. Nc is the number of carbon atoms in the hydrocarbon chain included in each surfactant: () long-chain imidazolium ILs (this work); (♦) sodium alkyl sulfates; () alkyl pyridinium bromides; () alkyl trimethylammonium bromides. The data for conventional ionic surfactants were taken from Refs. [26,29].

Γ max , and the area occupied by a single surfactant molecule at the air–water interface, Amin , can be estimated [33]. The Gibbs equation for monovalent ionic surfactants is given by   ∂γ 1 (mmol/m2 ) (4) Γmax = − 2RT ∂ ln C T where R is the gas constant (8.314 J/(mol K)), T is the absolute temperature, and C is the surfactant concentration in bulk solution. When Γ max is obtained, Amin value is estimated from the

Fig. 3. Surface tension as a function of concentration of C16 mimBr in the absence or presence of sodium halides at 298 K: () in water; () in 0.5 mol/L NaCl; () in 0.5 mol/L NaBr; () in 0.5 mol/L NaI.

670

B. Dong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 666–672

Table 2 Critical micelle concentration of C12 mimBr, C14 mimBr, and C16 mimBr in sodium halide solutions at 298 K Salt

NaCl NaBr NaI

Concentration (M)

0.5 0.5 0.5

cmc (mmol/L) C12 mimBr

C14 mimBr

C16 mimBr

1.4 0.71 0.19

0.18 0.086 0.037

0.052 0.024 0.011

leading to remarkably lower cmc in comparison with the salt-free system. The specificity of the ion interactions must be responsible for the difference among anion species in their effect on the cmc of cationic surfactants [36]. The specific interactions are involved in the binding of anions to cationic micelles and are presently under intensive investigation [37]. Although the origin of these interactions in the micelle formation phenomenon is not well understood [38], it is expected that the effect of anions increases in the order Cl− < Br− < I− , since the hydration of these ions decreases in the order Cl− > Br− > I− , and the ion being less hydrated is more effective in neutralizing the charges on the micelle surface [31]. This expectation is in accordance with the order of the effectiveness of halide anions on the depression of the cmc of long-chain imidazolium ILs.

Fig. 4. Representative pyrene fluorescence emission spectra in aqueous C16 mimBr solution at the concentrations below and above the cmc. Excitation wavelength: 335 nm; [C16 mimBr]: 8 × 10−5 , 2 × 10−3 mol/L.

3.3. Steady-state fluorescence measurements We employed another technique, steady-state fluorescence measurements using pyrene as the probe, to study the micelle aggregation behavior of these long-chain imidazolium ILs in aqueous solution. Pyrene is a strongly hydrophobic probe and its fluorescence emission spectrum exhibits characteristic five bands in the region of 370–400 nm (Fig. 4). In polar media, there is an enhancement in the intensity of the 0–0 band (peak 1) at the expense of others, which is due to vibronic coupling, being similar to the Ham effect in the absorption spectra of benzene [39]. Thus, the intensity ratio of the first to the third vibronic peaks, i.e., I1 /I3 , can be taken as a measure for the polarity of the environment. In the presence of micelles, pyrene solubilized in water would be preferentially incorporated into the interior hydrophobic region of micelles, resulting in a steep change of I1 /I3 values [29]. Fig. 5 represents the variation of the I1 /I3 ratio with the concentration of long-chain imidazolium ILs, which is utilized to estimate the cmc values. The cmc values obtained from Fig. 5 are listed in Table 3 together with those determined

Fig. 5. I1 /I3 ratio of pyrene as a function of concentration for long-chain imidazolium ILs in aqueous solution at 298 K: () C12 mimBr; () C14 mimBr; () C16 mimBr.

by other methods. It is clearly seen that these cmc values are rather in good agreement with each other. Note here that the absorption and fluorescence of each IL had no effect on the steady-state fluorescence of pyrene when pyrene was excited at 335 nm. As seen in Fig. 5, above the cmc, the I1 /I3 values are independent of each IL concentration and are nearly the same for these three long-chain imidazolium ILs. These low I1 /I3 values indicate that pyrene is solubilized in the palisade layer near the polar headgroups in all the tested ILs micelles [29]. Furthermore, the I1 /I3 values are larger than those in Cn TAB

Table 3 Critical micelle concentration, I1 /I3 values of pyrene solubilized in micelles, and micelle aggregation number (Nagg ) for C12 mimBr, C14 mimBr, and C16 mimBr in aqueous solution at 298 K ILs

cmca (mmol/L)

cmcb (mmol/L)

cmcc (mmol/L)

I1 /I3 b

Nagg

C12 mimBr C14 mimBr C16 mimBr

10.9 2.8 0.55

11.2 2.9 0.84

9.5 2.6 0.65

1.28 1.25 1.24

37b (at 20 mM) 48b (at 10 mM) 64b (at 4 mM)

a b c d

From surface tension measurements (this work). From steady-state fluorescence measurements (this work). From electrical conductivity measurements (Ref. [24]). Reported in Ref. [25].

44d (at 55 mM) 59d (at 25 mM) 66d (at 10 mM)

B. Dong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 666–672

671

Cn mimBr micelles than that for Cn TAB micelles owing to the bulkiness of the imidazolium head group. The micelle aggregation numbers (Nagg ) of Cn mimBr are smaller than those of the corresponding Cn TAB, and the values of Nagg of Cn mimBr are practically independent of the concentration. This work clarified the aggregation behavior of long-chain imidazolium ILs in dilute aqueous solution and would serve for potential application of ILs in colloidal systems. Acknowledgments

Fig. 6. ln(I0 /I) of pyrene as a function of concentration of the quencher benzophenone in long-chain imidazolium ILs aqueous solution at 298 K: () 20 mmol/L C12 mimBr; () 10 mmol/L C14 mimBr; () 4 mmol/L C16 mimBr.

micelles when compared at the same hydrocarbon chain length [40]. This suggests that the palisade layer of long-chain imidazolium ILs is packed less tightly than that of Cn TAB. There are more water molecules in the palisade layer of the ILs micelles than Cn TAB micelles, leading pyrene molecules to sense more polar microenvironment. The bulkiness of the imidazolium head group may be responsible for this loose packing of the ILs molecules in their micelles, just like the case of Γ max in surface adsorption. Fig. 6 displays the logarithm of the pyrene intensity ratio (I0 /I) as a function of concentration of the quencher benzophenone in aqueous solutions of long-chain imidazolium ILs. As can be seen in this figure, good linear correlations appear for each IL. The relevant Nagg values were obtained from the linear relationship by applying Eq. (3), and they are listed in Table 3. The Nagg values increase on going from C12 mimBr to C16 mimBr. Compared with those reported in Ref. [25], although the tested concentration of each IL is different, the Nagg values are comparable, implying that the values of Nagg for long-chain imidazolium ILs are practically independent of the concentration. The aggregation numbers reported for Cn TAB micelles at 20 ◦ C are 55 (C12 TAB), 70 (C14 TAB), and 89 (C16 TAB) [41]. The Nagg values of long-chain imidazolium ILs are considerably smaller than those of Cn TAB. This difference may be attributed again to the difference in the head group size between ILs and Cn TAB molecules. 4. Conclusions In this work we studied the micelle formation of 1-alkyl3-methylimidazolium bromides with long-hydrocarbon chain in aqueous solution. It is concluded from the surface tension data that both the adsorption efficiency (pC20 ) and the effectiveness of surface tension reduction (Π cmc ) are rather higher than those for the corresponding traditional ionic surfactants. The cmc values of Cn mimBr are obviously lower than those for typical cationic surfactants Cn TAB, indicating the higher aggregation tendency of the IL molecules. The fluorescence pyrene probe sensed slightly higher micropolarity of the Stern layer for

This work was supported by Natural Scientific Foundation of China (Grant nos. 50472069, 20773081), the key scientific project from Education Ministry (106100) and the National Basic Research Program (2007CB808004). References [1] R.X. Li, Green Solvent: Synthesis and Application of Ionic Liquids, Chemistry Technology Press, Beijing, 2004. [2] D.R. Robin, R.S. Kenneth, Ionic liquids—solvents of the future, Science 302 (2003) 792–793. [3] A.B. McEwen, H.L. Ngo, K. Lecompte, J.L. Goldman, Electrochemical properties of imidazolium salt electrolytes for electrochemical capacitor applications, J. Electrochem. Soc. 146 (1999) 1687–1695. [4] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2084. [5] L.A. Blanchard, A. Hancu, E.J. Bechman, J.F. Brennecke, Green processing using ionic liquids and CO2 , Nature 399 (1999) 28–29. [6] J.L. Anthony, E.J. Maginn, J.F. Brennecke, Solubilities and thermodynamic properties of gases in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate, J. Phys. Chem. B 106 (2002) 7315–7320. [7] G. Law, P.R. Watson, Surface tension measurements of N-alkylimidazolium ionic liquids, Langmuir 17 (2001) 6138–6141. [8] A.E. Bradley, C. Hardacre, J.D. Holbrey, S. Johnston, S.E.J. McMath, M. Nieuwenhuyzen, Small-angle X-ray scattering studies of liquid crystalline 1-alkyl-3-methylimidazolium salts, Chem. Mater. 14 (2002) 629–635. [9] T. Nakashima, N. Kimizuka, Vesicles in salt: formation of bilayer membranes from dialkyldimethylammonium bromides in ether-containing ionic liquids, Chem. Lett. 31 (2002) 1018–1019. [10] J.L. Anderson, V. Pino, E.C. Hagberg, V.V. Sheares, D.W. Armstrong, Surfactant solvation effects and micelle formation in ionic liquids, Chem. Commun. 19 (2003) 2444–2445. [11] L.Y. Wang, X. Chen, Y.C. Chai, J.C. Hao, Z.M. Sui, W.C. Zhuang, Z.W. Sun, Lyotropic liquid crystalline phases formed in an ionic liquid, Chem. Commun. 24 (2004) 2840–2841. [12] Z.N. Wang, F. Liu, Y.A. Gao, W.C. Zhuang, L.M. Xu, B.X. Han, G.Z. Li, G.Y. Zhang, Hexagonal liquid crystalline phases formed in ternary systems of Brij97-water-ionic liquids, Langmuir 21 (2005) 4931–4937. [13] Y.A. Gao, S.B. Han, B.X. Han, G.Z. Li, D. Shen, Z.H. Li, J.M. Du, W.G. Hou, G.Y. Zhang, TX-100/water/1-butyl-3-methylimidazolium hexafluorophosphate microemulsions, Langmuir 21 (2005) 5681–5684. [14] J. Eastoe, S. Gold, S.E. Rogers, A. Paul, T. Welton, R.K. Heenan, I. Grillo, Ionic liquid-in-oil microemulsions, J. Am. Chem. Soc. 127 (2005) 7302–7303. [15] N. Li, Y.A. Gao, L.Q. Zheng, J. Zhang, L. Yu, X.W. Li, Studies on the micropolarities of BmimBF4 /TX-100/toluene ionic liquid microemulsions and their behaviors characterized by UV–visible spectroscopy, Langmuir 23 (2007) 1091–1097. [16] D. Seth, A. Chakraborty, P. Setua, N. Sarkar, Interaction of ionic liquid with water in ternary microemulsions (Triton X-100/water/1-butyl-3methylimidazolium hexafluorophosphate) probed by solvent and rotational relaxation of Coumarin 153 and Coumarin 151, Langmuir 22 (2006) 7768–7775.

672

B. Dong et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 317 (2008) 666–672

[17] M.A. Firestone, J.A. Dzielawa, P. Zapol, L.A. Curtiss, S. Seifert, M.L. Dietz, Lyotropic liquid-crystalline gel formation in a room-temperature ionic liquid, Langmuir 18 (2002) 7258–7260. [18] J. Sirieix-Pl´enet, L. Gaillon, P. Letellier, Behaviour of a binary solvent mixture constituted by an amphiphilic ionic liquid, 1-decyl-3methylimidazolium bromide and water: potentiometric and conductimetric studies, Talanta 63 (2004) 979–986. [19] I. Goodchild, L. Collier, S.L. Millar, I. Prokeˇs, J.C.D. Lord, C.P. Butts, J. Bowers, J.R.P. Webster, R.K. Heenan, Structural studies of the phase, aggregation and surface behaviour of 1-alkyl-3-methylimidazolium halide + water mixtures, J. Colloid Interf. Sci. 307 (2007) 455–468. [20] J. Bowers, C.P. Butts, P.J. Martin, M.C. Vergara-Gutierrez, Aggregation behavior of aqueous solutions of ionic liquids, Langmuir 20 (2004) 2191–2198. [21] Z. Miskolczy, K. Seb˝ok-Nagy, L. Bicz´ok, S. G¨okt¨urk, Aggregation and micelle formation of ionic liquids in aqueous solution, Chem. Phys. Lett. 400 (2004) 296–300. [22] G.A. Baker, S. Pandey, S. Pandey, S.N. Baker, A new class of cationic surfactants inspired by N-alkyl-N-methyl pyrrolidinium ionic liquids, Analyst 129 (2004) 890–892. [23] T. Singh, A. Kumar, Aggregation behavior of ionic liquids in aqueous solutions: effect of alkyl chain length, cations, and anions, J. Phys. Chem. B 111 (2007) 7843–7851. [24] C. Jungnickel, J. Łuczak, J. Ranke, J.F. Fern´andez, A. M¨uller, J. Th¨oming, Micelle formation of imidazolium ionic liquid in aqueous solution, Colloids Surf. A 316 (2008) 278–284. [25] B. Dong, N. Li, L.Q. Zheng, L. Yu, T. Inoue, Surface adsorption and micelle formation of surface active ionic liquids in aqueous solution, Langmuir 23 (2007) 4178–4182. [26] T. Inoue, H. Ebina, B. Dong, L.Q. Zheng, Electrical conductivity study on micelle formation of long-chain imidazolium ionic liquids in aqueous solution, J. Colloid Interf. Sci. 314 (2007) 236–241. [27] R. Vany´ur, L. Bicz´ok, Z. Miskolczy, Micelle formation of 1-alkyl-3methylimidazolium bromide ionic liquids in aqueous solution, Colloids Surf. A 299 (2007) 256–261. [28] M.J. Rosen, Surfactants and Interfacial Phenomena, 2nd ed., Wiley, New York, 1989. [29] K. Kalyanasundaram, J.K. Thomas, Environmental effects on vibronic band intensities in pyrene monomer fluorescence and their application in studies of micellar systems, J. Am. Chem. Soc. 99 (1977) 2039–2044.

[30] N.J. Turro, A. Yekta, Luminescent probes for detergent solutions. A simple procedure for determination of the mean aggregation number of micelles, J. Am. Chem. Soc. 100 (1978) 5951–5952. [31] G.X. Zhao, B.Y. Zhu, Principles of Surfactant Action, China Light Industry Press, Beijing, 2003. [32] T. Inoue, B. Dong, L.Q. Zheng, Phase behavior of binary mixture of 1dodecyl-3-methylimidazolium bromide and water revealed by differential scanning calorimetry and polarized optical microscopy, J. Colloid Interf. Sci. 307 (2007) 578–581. [33] M.J. Jaycock, G.D. Parfitt, Chemistry of Interfaces, John Wiley and Sons, New York, 1981. [34] R.L. Venable, R.V. Nauman, Micellar weights of and solubilization of benzene by a series of tetradecylammonium bromides. The effect of the size of the charged head, J. Phys. Chem. 68 (1964) 3498–3503. [35] D. Varade, T. Joshi, V.K. Aswal, P.S. Goyal, P.A. Hassan, P. Bahadur, Effect of salt on the micelles of cetylpyridinium chloride, Colloids Surf. A 259 (2005) 95–101. [36] E. Feitosa, M.R.S. Brazolin, R.M.Z.G. Naal, M.P.F. de Morais Del Lama, J.R. Lopes, W. Loh, M. Vasilescu, Structural organization of cetyltrimethylammonium sulfate in aqueous solution: the effect of Na2 SO4 , J. Colloid Interf. Sci. 299 (2006) 883–889. [37] M. Benrraou, B.L. Bales, R. Zana, Effect of the nature of the counterion on the properties of anionic surfactant. 1. cmc, ionization degree at the cmc and aggregation number of micelles of sodium, cesium, tetrabutylammonium dodecyl sulfates, J. Phys. Chem. B 107 (2003) 13432–13440. [38] V.K. Aswal, P.S. Goyal, Counterions in the growth of ionic micelles in aqueous electrolyte solutions: a small-angle neutron scattering study, Phys. Rev. E 61 (2000) 2947–2953. [39] M. Koyanagi, Effect of dispersion forces of solvents. II. On the O–O band of the near ultraviolet absorption spectrum of benzene in fluid solutions, J. Mol. Spectrosc. 25 (1968) 273–290. [40] G.B. Ray, I. Chakraborty, S. Ghosh, S.P. Moulik, R. Palepu, Selfaggregation of alkyltrimethylammonium bromides (C10 -, C12 -, C14 -, and C16 TAB) and their binary mixtures in aqueous medium: a critical and comprehensive assessment of interfacial behavior and bulk properties with reference to two types of micelle formation, Langmuir 21 (2005) 10958–10967. [41] P. Lianos, R. Zana, Fluorescence probe studies of the effect of concentration on the state of aggregation of surfactants in aqueous solution, J. Colloid Interf. Sci. 84 (1981) 100–107.