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CTAB aggregation in aqueous solutions of ammonium based ionic liquids; conductimetric studies
Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 104–108
CTAB aggregation in aqueous solutions of ammonium based ionic liquids; conductimetric studies Ali Modaressi a , Hocine Sifaoui b , Beata Grzesiak c , Roland Solimando d , Urszula Domanska c , Marek Rogalski a,∗ b
a Laboratoire de Chimie et Applications, Universit´ e de Metz, 1, bd Arago, 57078 Metz Cedex 03, France Laboratoire de Thermodynamique et de Mod´elisation Mol´eculaire, Facult´e de Chimie, USTHB, BP 32 El-Alia, 16111 Bab-Ezzouar, Alger, Algeria c Physical Chemistry Division, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland d Laboratoire de Thermodynamique des Milieux Polyphas´ es, Ecole Nationale Sup´erieure des Industries Chimiques, Institut National Polytechnique de Lorraine, 1 rue Grandville, BP 451, F-54001 Nancy Cedex, France
Received 5 May 2006; received in revised form 15 September 2006; accepted 20 September 2006 Available online 24 September 2006
Abstract The electrical conductivity/concentration data of aqueous solutions of n-hexadecyl-trimethylammonium bromide (CTAB), an cationic surfactant, with two ammonium based ionic liquids propyl-(2-hydroxyethyl)-dimethyl-ammonium bromide, (C3Br), butyl-(2-hydroxyethyl)-dimethylammonium bromide, (C4Br) of high melting points (>373 K) were determined in the temperature range 298.15–328.15 K. These data were used to determining the critical micelle concentration (CMC) in function of concentration and temperature. The two ionic liquids and the surfactant have the same anion and the ammonium based cation. The influence of ionic liquids on the micellisation process of the CTAB is discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Ionic liquids; Micelles; Surfactants; Critical micelle concentration; Electrical conductivity
1. Introduction The ability of surfactants to self-aggregate depends on its structure, its concentration, the solubilizing media, and the method used to preparing the self-assemblies [1]. The critical micellisation concentration (CMC) represents the minimum surfactant concentration required for aggregation to occur. Traditionally, the CMC can be determined by observing sharp changes in a number of physical properties such as surface tension, turbidity, UV–vis absorbance, solute solubility, and classically electrical conductivity [2]. Micelles are self-assembled structures formed by surfactant molecules, due to the presence of distinct hydrophobic and hydrophilic within the molecule. Amphiphile molecules may self-assemble into a variety of structures that including micelles, vesicles, liposomes, microtubules, and bilayers, that constitute microphases with oil-like regions and large interfacial areas [3].
∗
Corresponding author. Tel.: +33 387 31 54 34; fax: +33 387 54 74 62. E-mail address: [email protected] (M. Rogalski).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.09.031
Moderately soluble polar and ionic compounds such as ionic liquids may be integrated into the outer region of the micelle [3–4]. Surfactant based processes are found to be very important in pollution control, wastewater purification, elaboration of nanomaterials and in catalytic processes [5–9]. Properties of micelles and the values of the CMC are often tailored in function of given application. Modulation of the CMC of surfactants can be achieved by varying the nature of the surfactant, the net charge of the surfactant, or the nature of the polar head groups and counterions. Another method is to use mixed surfactant systems. There has been recent interest in the formation of mixed micelles formed by room temperature ionic liquids (RTILs) and surfactants [1,10–11]. It has been shown that the CMC values of surfactants may be significantly modified with appropriate RTILs. Thus, mixtures of surfactant with RTILs are an interesting issue for specific applications of surfactants. Room-temperature ionic liquids (RTILs) are a relatively new class of solvents with interesting properties such as a negligible vapor pressure, the stability up to temperatures higher than 200 ◦ C. Room temperature ionic liquids (RTILs) have
A. Modaressi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 104–108
been widely used as environmentally benign solvents in chromatographic separations, separation processes, and in organic reactions either as solvents either as heterogeneous catalysts [12–18]. Only few papers reported results on the dependence of CMC of surfactants on the nature of the additive RTILs. Recent work of Anderson et al. [10] and Fletcher and Pandey [1] have shown that anionic and nonionic surfactants can form aggregates with RTILs the nature of which depends on the hydrophobic–hydrophilic properties of the ionic liquids. Beyaz et al. [11] studied the CMC of SDS in the presence of the imidazolium-based RTILs with varying side chains as additives. This study showed that the CMC of sodium dodecylsulfate (SDS) increases from 1.9 to 170 mM going from C8 to C1 side chain in the ionic liquids. These observations are consistent with the expected lowering of CMCs with increasing hydrophobicity of the surfactants [19–22]. Low values of the CMC observed with hydrophobic compounds is probably due to the solubilization of the RTILs in the micellar phase. The higher CMC values corresponding to more hydrophilic RTILs, were attributed to interactions with SDS in the aqueous phase [11]. The same authors observed that the nature of the counterions, Cl− or BF4 − , has no noticeable effect on the observed CMC values. In this study we are concerned with aggregation processes occurring between CTAB, an cationic surfactant, and two ammonium based ionic liquids C3Br and C4Br. As shown in Fig. 1 both salts and the surfactant have the same anion Br− and the ammonium based cation. In spite of the amphiphilic character of both RTIL the formation of mixed micelles is expected. As the melting temperatures of C3Br and C4Br are, respectively, 372.59 and 359.34 K [23] these compounds should be considered as precursors of ionic liquids. Relatively high melting temperatures limit their use as extraction solvents, but allows such applications as the phase transfer catalyst or additives to new processes. Because of their complex interactions with polar solvents, which result from their structure and the presence of the hydroxyl group, they are well soluble in water and alcohols [23].
105
The physical properties and the synthesis of C3Br and C4Br were recently described [23]. It was shown that ammonium cation based ionic liquids can be easily obtained and are cheaper that imidazolium based salts. At room temperature they are solid and hygroscopic but moisture-stable, so they can be easily prepared and stored without a need for special equipment. 2. Materials and methods 2.1. Chemicals Hexadecyl-trimethylammonium bromide (CAS number: 5709-0), was of the highest available purity, from Aldrich. The surfactant was purified by precipitation from anhydrous ethyl alcohol with cold acetone. The ionic liquids propyl(2-hydroxyethyl)-dimethyl-ammonium bromide and butyl-(2hydroxyethyl)-dimethyl-ammonium bromide, were synthesized as was described previously [23]. The filtered precipitate and ionic liquids were vacuum-dried at 60 ◦ C for 36 h. Ionic liquids and the surfactant were stored in a vacuum desiccator. Water was bidistilled, deionized and degassed. 2.2. Measurements of electrical conductivity and determination of the CMC In this work the electrical conductivity of surfactant solutions were used to determining CMC values. The conductivity measurements were carried out with a LR 01/T cell coupled to an Microprocessor Precision Conductivity Meter LF 3000 (WTW) calibrated with KCl solution, before measurements. The temperature of the cell was kept constant to within ±0.01 K by circulating thermostated liquid. Electrical conductivity data were determined by adding to the solution in the cell known amounts of the water. The reproducibility of conductance measurements was estimated to be ±0.5%. The uncertainty in the composition and temperature can be estimated as ±0.0005 mol−1 and ±0.1 K in the molar concentration and temperature, respectively. 3. Results and discussion
Fig. 1. Molecular formulae of: (a) CTAB; (b) C3Br; (c) C4Br.
Plots of specific conductivity, κ, of test solutions against CTAB concentration were obtained for a series of temperatures and are reported in Figs. 2–6. For each temperature, the electrical conductivity increases with concentration with a gradual decrease in the slope. The break in the plot originates from the onset of micellization. The slope change at CMC is due to an effective loss of ionic charges because a fraction of the counterions are confined to the micellar surface. The slope change is sharp with CTAB and more gradual with {CTAB + ionic liquid} mixtures, that can be explained by a modified aggregation mechanism of CTAB. The CMC values were determined using the Phillips criterion [24] (the third derivative of the conductivity in concentration is zero at CMC). Results are listed in Table 1 and in Figs. 7 and 8. As indicated in Table 1 the CMCs of CTAB are in good agreement with literature data [25].
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Fig. 2. Conductivity vs. concentration, Cm of aqueous solutions of CTAB at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328,15 K.
Fig. 4. Conductivity vs. concentration, Cm , of aqueous solutions of {CTAB + C3Br} at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328,15 K; the molar ratio CTAB/C3Br = 0.5.
Results reported in Table 1 show that the presence of the ionic liquid significantly lowers the value of the CMC. The lower CMC is either due to the increasing concentration of the counterion or to the formation of mixed micelles (cations of the surfactant + cation of the ionic liquid). Increasing the salt concentration reduces the electrostatic repulsion between the charged groups and favours the aggregation. To estimate
the counterion effect in the case of CTAB-ionic liquid solutions we used results of Paredes et al. [26] concerning the CMC of CTAB in water and in 0.05M NaBr solution. Corresponding values (CMC (water) = 0.89 mM dm−3 , CMC (0.05 M NaBr) = 0.12 mM dm−3 ) show that the increase of concentration of Br− ions of 0.001 should shift the CMC of about 0.00002 M. Therefore, the expected decrease of the CMC due to the Br− counterion of the ionic liquid would be of 0.0002–0.0004 M. As
Fig. 3. Conductivity vs. concentration, Cm, of aqueous solutions of {CTAB + C3Br} at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328,15 K; the molar ratio CTAB/C3Br = 1.
Fig. 5. Conductivity vs. concentration, Cm, of aqueous solutions of {CTAB + C4Br} at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328,15 K; the molar ratio CTAB/C4Br = 1.
A. Modaressi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 104–108
Fig. 6. Conductivity vs. concentration, Cm, of aqueous solutions of {CTAB + C4Br} at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328.15 K; the molar ratio CTAB/C4Br = 0.5.
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Fig. 7. Critical micellar concentrations (CMC) as a function of temperature: (䊉) CTAB; (×) {CTAB + C3Br}; the molar ratio CTAB/C3Br = 1; () {CTAB + C4Br}; the molar ratio CTAB/C4Br = 1.
the observed decrease of the CMC was of 0.0034 and 0.0040 with C3Br and C4Br, respectively, we conclude that the organic cation influences the aggregation process and that both ionic liquids contribute to forming mixed micelles with CTAB. Contrary to the usual behaviour of surfactants, the higher CMC is observed with more hydrophobic C4Br. The difference is rather small, that suggests the similar micellization mechanism occurring with two ionic liquids. Therefore, the mechanism of mixed micelle aggregation is based rather on polar/hydrogen bonding than hydrophobic interactions. The influence of temperature on CMC in ionic surfactant solutions is illustrated with Figs. 7 and 8. As expected, CMC increases with rising temperature. However, this increase is significantly lower with the {CTAB + ionic liquid} as compared with the pure CTAB solution. This is probably due to the hydrogen bonding of hydroxyl groups of C3Br and C4Br. Thus, mixed micelles are probably more stable due to new interactions appearing in the polar shell of the micelle.
Table 1 Values of CMC determined with electrical conductivity data; CMC of CTAB are in good agreement with literature data [25]; CMC (298.15 K) = 0.001, CMC (328.15 K) = 0.00133 T (K)
298.15 308.15 318.15 328.15
CMC × 104 (mol dm−3 )
Fig. 8. Critical micellar concentrations (CMC) as a function of temperature: (䊉) CTAB; (×) {CTAB + C3Br}; the molar ratio CTAB/C3Br = 0.5; () {CTAB + C4Br}; the molar ratio CTAB/C4Br = 0.5.
4. Conclusions
CTAB
CTAB/ C3Br = 1
CTAB/ C3Br = 0.5
CTAB/ C4Br = 1
CTAB/ C4Br = 0.5
9.7 11.0 12.1 13.5
6.3 7.3 8.1 9.2
5.7 6.2 6.9 7.2
6.6 7.6 8.5 9.6
6.2 6.8 7.6 8.5
We have studied the aggregation behavior of CTAB with two ionic liquids. Previous studies on the mechanisms of {surfactant + ionic liquid} aggregation were concerned with anionic surfactant. In this case, the organic cation of the ionic liquid was the counterion of the surfactant. In the present study
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two ionic liquids and the surfactant have the same counterion, Br− , and the ammonium based cation. It was shown that the presence in the mixture of the second organic cation strongly influences CMC that can be explained by formation of mixed {CTAB + ionic liquid} micelles. The application of these micelles to solubilization of hydrophobic compounds is under study. References [1] K.A. Fletcher, S. Pandey, Langmuir 20 (2004) 33–36. [2] I. Benito, M.A. Garcıa, C. Monge, J.M. Saz, M.L. Marina, Colloids Surf. A: Physicochem. Eng. Aspects 125 (1997) 221. [3] D.F. Evans, Langmuir 4 (1988) 3. [4] E. Ruckenstein, J.A. Beunen, Langmuir 4 (1988) 77. [5] P.H. Elworthy, A.T. Florence, C.B. Mac Farlane, Solubilization by Surface Active Agents, Chapman and Hall, London, 1968. [6] K.L. Mittal (Ed.), Micellization, Solubilization and Microemulsions, Plenum, New York, 1977. [7] D. Attwood, A.T. Florence, Surfactant Systems. Their Chemistry, Pharmacy and Biology, London, Chapman and Hall, 1983. [8] Y. Moroi, Micelles (Theoretical and Applied Aspects), Plenum, New York, 1992.
[9] C. Zhang, K.T. Valsaraj, D. Constant, D. Roy, Water Res. 33 (1998) 115–124. [10] J.L. Anderson, V. Pino, E.C. Hagberg, V.V. Sheares, D.W. Armstrong, Chem. Commun. (2003) 2444. [11] A. Beyaz, W.S. Oh, V.P. Reddy, Colloids Surf. B 35 (2004) 119. [12] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [13] A. Berthod, L. He, D.W. Armstrong, Chromatographia 53 (2001) 63–68. [14] J.L. Anderson, J. Ding, T. Welton, D.W. Armstrong, J. Am. Chem. Soc. 124 (2002) 14247. [15] W. Ogihara, J. Sun, M. Forsyth, D.R. MacFarlane, M. Yoshizawa, H. Ohno, Electrochim. Acta 49 (2004) 1797. [16] P. Wasserscheid, W. Keim, Angew. Chem. 112 (2000) 3926. [17] J.S.J. Wilkes, Mol. Catal. A: Chem. 214 (2004) 11. [18] L.A. Blanchard, Z. Gu, J.F. Brennecke, J. Phys. Chem. B 105 (2001) 2437. [19] M.L. Corrin, W.D. Harkins, J. Am. Chem. Soc. 69 (1947) 686. [20] L. Gaillon, J. Lelievre, R. Gaboriaud, J. Colloid Interface Sci. 213 (1999) 287–297. [21] M. Algrem, S. Swarup, J. Colloid Interface Sci. 91 (1983) 256. [22] B. Halle, M. Landgren, B. Jonsson, J. Phys. France 49 (1988) 1235. [23] U. Doma´nska, R. Bogel-Łukasik, J. Phys. Chem. B 109 (2005) 12124. [24] J.N. Phillips, Trans. Faraday Soc. 51 (1955) 561. [25] L.V. Dearden, E.M. Woolley, J. Phys. Chem. 91 (1987) 2404. [26] S. Paredes, L. Sepulveda, M. Tribout, J. Phys. Chem. 88 (1984) 1871.
CTAB aggregation in aqueous solutions of ammonium based ionic liquids; conductimetric studies Ali Modaressi a , Hocine Sifaoui b , Beata Grzesiak c , Roland Solimando d , Urszula Domanska c , Marek Rogalski a,∗ b
a Laboratoire de Chimie et Applications, Universit´ e de Metz, 1, bd Arago, 57078 Metz Cedex 03, France Laboratoire de Thermodynamique et de Mod´elisation Mol´eculaire, Facult´e de Chimie, USTHB, BP 32 El-Alia, 16111 Bab-Ezzouar, Alger, Algeria c Physical Chemistry Division, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland d Laboratoire de Thermodynamique des Milieux Polyphas´ es, Ecole Nationale Sup´erieure des Industries Chimiques, Institut National Polytechnique de Lorraine, 1 rue Grandville, BP 451, F-54001 Nancy Cedex, France
Received 5 May 2006; received in revised form 15 September 2006; accepted 20 September 2006 Available online 24 September 2006
Abstract The electrical conductivity/concentration data of aqueous solutions of n-hexadecyl-trimethylammonium bromide (CTAB), an cationic surfactant, with two ammonium based ionic liquids propyl-(2-hydroxyethyl)-dimethyl-ammonium bromide, (C3Br), butyl-(2-hydroxyethyl)-dimethylammonium bromide, (C4Br) of high melting points (>373 K) were determined in the temperature range 298.15–328.15 K. These data were used to determining the critical micelle concentration (CMC) in function of concentration and temperature. The two ionic liquids and the surfactant have the same anion and the ammonium based cation. The influence of ionic liquids on the micellisation process of the CTAB is discussed. © 2006 Elsevier B.V. All rights reserved. Keywords: Ionic liquids; Micelles; Surfactants; Critical micelle concentration; Electrical conductivity
1. Introduction The ability of surfactants to self-aggregate depends on its structure, its concentration, the solubilizing media, and the method used to preparing the self-assemblies [1]. The critical micellisation concentration (CMC) represents the minimum surfactant concentration required for aggregation to occur. Traditionally, the CMC can be determined by observing sharp changes in a number of physical properties such as surface tension, turbidity, UV–vis absorbance, solute solubility, and classically electrical conductivity [2]. Micelles are self-assembled structures formed by surfactant molecules, due to the presence of distinct hydrophobic and hydrophilic within the molecule. Amphiphile molecules may self-assemble into a variety of structures that including micelles, vesicles, liposomes, microtubules, and bilayers, that constitute microphases with oil-like regions and large interfacial areas [3].
∗
Corresponding author. Tel.: +33 387 31 54 34; fax: +33 387 54 74 62. E-mail address: [email protected] (M. Rogalski).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.09.031
Moderately soluble polar and ionic compounds such as ionic liquids may be integrated into the outer region of the micelle [3–4]. Surfactant based processes are found to be very important in pollution control, wastewater purification, elaboration of nanomaterials and in catalytic processes [5–9]. Properties of micelles and the values of the CMC are often tailored in function of given application. Modulation of the CMC of surfactants can be achieved by varying the nature of the surfactant, the net charge of the surfactant, or the nature of the polar head groups and counterions. Another method is to use mixed surfactant systems. There has been recent interest in the formation of mixed micelles formed by room temperature ionic liquids (RTILs) and surfactants [1,10–11]. It has been shown that the CMC values of surfactants may be significantly modified with appropriate RTILs. Thus, mixtures of surfactant with RTILs are an interesting issue for specific applications of surfactants. Room-temperature ionic liquids (RTILs) are a relatively new class of solvents with interesting properties such as a negligible vapor pressure, the stability up to temperatures higher than 200 ◦ C. Room temperature ionic liquids (RTILs) have
A. Modaressi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 104–108
been widely used as environmentally benign solvents in chromatographic separations, separation processes, and in organic reactions either as solvents either as heterogeneous catalysts [12–18]. Only few papers reported results on the dependence of CMC of surfactants on the nature of the additive RTILs. Recent work of Anderson et al. [10] and Fletcher and Pandey [1] have shown that anionic and nonionic surfactants can form aggregates with RTILs the nature of which depends on the hydrophobic–hydrophilic properties of the ionic liquids. Beyaz et al. [11] studied the CMC of SDS in the presence of the imidazolium-based RTILs with varying side chains as additives. This study showed that the CMC of sodium dodecylsulfate (SDS) increases from 1.9 to 170 mM going from C8 to C1 side chain in the ionic liquids. These observations are consistent with the expected lowering of CMCs with increasing hydrophobicity of the surfactants [19–22]. Low values of the CMC observed with hydrophobic compounds is probably due to the solubilization of the RTILs in the micellar phase. The higher CMC values corresponding to more hydrophilic RTILs, were attributed to interactions with SDS in the aqueous phase [11]. The same authors observed that the nature of the counterions, Cl− or BF4 − , has no noticeable effect on the observed CMC values. In this study we are concerned with aggregation processes occurring between CTAB, an cationic surfactant, and two ammonium based ionic liquids C3Br and C4Br. As shown in Fig. 1 both salts and the surfactant have the same anion Br− and the ammonium based cation. In spite of the amphiphilic character of both RTIL the formation of mixed micelles is expected. As the melting temperatures of C3Br and C4Br are, respectively, 372.59 and 359.34 K [23] these compounds should be considered as precursors of ionic liquids. Relatively high melting temperatures limit their use as extraction solvents, but allows such applications as the phase transfer catalyst or additives to new processes. Because of their complex interactions with polar solvents, which result from their structure and the presence of the hydroxyl group, they are well soluble in water and alcohols [23].
105
The physical properties and the synthesis of C3Br and C4Br were recently described [23]. It was shown that ammonium cation based ionic liquids can be easily obtained and are cheaper that imidazolium based salts. At room temperature they are solid and hygroscopic but moisture-stable, so they can be easily prepared and stored without a need for special equipment. 2. Materials and methods 2.1. Chemicals Hexadecyl-trimethylammonium bromide (CAS number: 5709-0), was of the highest available purity, from Aldrich. The surfactant was purified by precipitation from anhydrous ethyl alcohol with cold acetone. The ionic liquids propyl(2-hydroxyethyl)-dimethyl-ammonium bromide and butyl-(2hydroxyethyl)-dimethyl-ammonium bromide, were synthesized as was described previously [23]. The filtered precipitate and ionic liquids were vacuum-dried at 60 ◦ C for 36 h. Ionic liquids and the surfactant were stored in a vacuum desiccator. Water was bidistilled, deionized and degassed. 2.2. Measurements of electrical conductivity and determination of the CMC In this work the electrical conductivity of surfactant solutions were used to determining CMC values. The conductivity measurements were carried out with a LR 01/T cell coupled to an Microprocessor Precision Conductivity Meter LF 3000 (WTW) calibrated with KCl solution, before measurements. The temperature of the cell was kept constant to within ±0.01 K by circulating thermostated liquid. Electrical conductivity data were determined by adding to the solution in the cell known amounts of the water. The reproducibility of conductance measurements was estimated to be ±0.5%. The uncertainty in the composition and temperature can be estimated as ±0.0005 mol−1 and ±0.1 K in the molar concentration and temperature, respectively. 3. Results and discussion
Fig. 1. Molecular formulae of: (a) CTAB; (b) C3Br; (c) C4Br.
Plots of specific conductivity, κ, of test solutions against CTAB concentration were obtained for a series of temperatures and are reported in Figs. 2–6. For each temperature, the electrical conductivity increases with concentration with a gradual decrease in the slope. The break in the plot originates from the onset of micellization. The slope change at CMC is due to an effective loss of ionic charges because a fraction of the counterions are confined to the micellar surface. The slope change is sharp with CTAB and more gradual with {CTAB + ionic liquid} mixtures, that can be explained by a modified aggregation mechanism of CTAB. The CMC values were determined using the Phillips criterion [24] (the third derivative of the conductivity in concentration is zero at CMC). Results are listed in Table 1 and in Figs. 7 and 8. As indicated in Table 1 the CMCs of CTAB are in good agreement with literature data [25].
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Fig. 2. Conductivity vs. concentration, Cm of aqueous solutions of CTAB at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328,15 K.
Fig. 4. Conductivity vs. concentration, Cm , of aqueous solutions of {CTAB + C3Br} at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328,15 K; the molar ratio CTAB/C3Br = 0.5.
Results reported in Table 1 show that the presence of the ionic liquid significantly lowers the value of the CMC. The lower CMC is either due to the increasing concentration of the counterion or to the formation of mixed micelles (cations of the surfactant + cation of the ionic liquid). Increasing the salt concentration reduces the electrostatic repulsion between the charged groups and favours the aggregation. To estimate
the counterion effect in the case of CTAB-ionic liquid solutions we used results of Paredes et al. [26] concerning the CMC of CTAB in water and in 0.05M NaBr solution. Corresponding values (CMC (water) = 0.89 mM dm−3 , CMC (0.05 M NaBr) = 0.12 mM dm−3 ) show that the increase of concentration of Br− ions of 0.001 should shift the CMC of about 0.00002 M. Therefore, the expected decrease of the CMC due to the Br− counterion of the ionic liquid would be of 0.0002–0.0004 M. As
Fig. 3. Conductivity vs. concentration, Cm, of aqueous solutions of {CTAB + C3Br} at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328,15 K; the molar ratio CTAB/C3Br = 1.
Fig. 5. Conductivity vs. concentration, Cm, of aqueous solutions of {CTAB + C4Br} at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328,15 K; the molar ratio CTAB/C4Br = 1.
A. Modaressi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 296 (2007) 104–108
Fig. 6. Conductivity vs. concentration, Cm, of aqueous solutions of {CTAB + C4Br} at different temperatures: (䊉) T = 298.15 K; () T = 308.15 K; () T = 318.15 K; (×) T = 328.15 K; the molar ratio CTAB/C4Br = 0.5.
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Fig. 7. Critical micellar concentrations (CMC) as a function of temperature: (䊉) CTAB; (×) {CTAB + C3Br}; the molar ratio CTAB/C3Br = 1; () {CTAB + C4Br}; the molar ratio CTAB/C4Br = 1.
the observed decrease of the CMC was of 0.0034 and 0.0040 with C3Br and C4Br, respectively, we conclude that the organic cation influences the aggregation process and that both ionic liquids contribute to forming mixed micelles with CTAB. Contrary to the usual behaviour of surfactants, the higher CMC is observed with more hydrophobic C4Br. The difference is rather small, that suggests the similar micellization mechanism occurring with two ionic liquids. Therefore, the mechanism of mixed micelle aggregation is based rather on polar/hydrogen bonding than hydrophobic interactions. The influence of temperature on CMC in ionic surfactant solutions is illustrated with Figs. 7 and 8. As expected, CMC increases with rising temperature. However, this increase is significantly lower with the {CTAB + ionic liquid} as compared with the pure CTAB solution. This is probably due to the hydrogen bonding of hydroxyl groups of C3Br and C4Br. Thus, mixed micelles are probably more stable due to new interactions appearing in the polar shell of the micelle.
Table 1 Values of CMC determined with electrical conductivity data; CMC of CTAB are in good agreement with literature data [25]; CMC (298.15 K) = 0.001, CMC (328.15 K) = 0.00133 T (K)
298.15 308.15 318.15 328.15
CMC × 104 (mol dm−3 )
Fig. 8. Critical micellar concentrations (CMC) as a function of temperature: (䊉) CTAB; (×) {CTAB + C3Br}; the molar ratio CTAB/C3Br = 0.5; () {CTAB + C4Br}; the molar ratio CTAB/C4Br = 0.5.
4. Conclusions
CTAB
CTAB/ C3Br = 1
CTAB/ C3Br = 0.5
CTAB/ C4Br = 1
CTAB/ C4Br = 0.5
9.7 11.0 12.1 13.5
6.3 7.3 8.1 9.2
5.7 6.2 6.9 7.2
6.6 7.6 8.5 9.6
6.2 6.8 7.6 8.5
We have studied the aggregation behavior of CTAB with two ionic liquids. Previous studies on the mechanisms of {surfactant + ionic liquid} aggregation were concerned with anionic surfactant. In this case, the organic cation of the ionic liquid was the counterion of the surfactant. In the present study
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