Cloud Point and Microemulsion Phase Behavior of Sodium Linear

Jun 16, 2009 - Figure 1. Variation of cloud point of (surfactant−water−salt) system with salt (Bu4NBr) concentration at various surfactant (NaLAS)...
0 downloads 0 Views 1MB Size
Download PDF
8842

Ind. Eng. Chem. Res. 2009, 48, 8842–8847

Cloud Point and Microemulsion Phase Behavior of Sodium Linear Alkylbenzene Sulfonate with Tetrabutyl- And Benzyltributyl-Substituted Ammonium Halides Deeleep K. Rout,*,† Siddharth Chauhan,‡ and Ashwani Agarwal‡ UnileVer Research India and Hindustan UnileVer Research Centre, 64 Main Road, Whitefield, Bangalore 560066, India, and Indian Institute of Technology, Kanpur, India

We report cloud point phenomenon in binary water-anionic surfactant (sodium linear alkylbenzene sulfonate, NaLAS) after incorporation of organic salts, such as tetrabutyl- and benzyltributyl- ammonium halides. The effect of alkyl/aromatic hydrophobic substituents of such reagents on the cloud point phenomenon was also investigated. It was observed that the aqueous solution clouding behavior is dependent upon surfactant concentration, type of hydrophobic substituent of the salt, and operating temperature. At a given temperature, with increasing surfactant concentration, a higher amount of salt is required for the onset of clouding. Besides, at a given surfactant concentration, the amount of salt required for the cloud point is inversely proportional to the temperature. With addition of an oil phase to the binary water(salt)-surfactant system, a three-phase microemulsion could be generated. Phase diagrams for these ternary systems were determined, and the formation of the microemulsion phase behavior was established from phase visualization and oil-water interfacial tension measurements. Introduction Nonionic surfactants are believed to owe their solubility in water to hydrogen bonding. On heating, these weak hydrogen bonds break and thereby lower the surfactant solubility in water. At a certain temperature, the solution separates into two phases, one micellarrich and the other micellar-lean, and appears cloudy. The temperature at which it happens is termed the cloud point.1 Because temperature is the key parameter in this case, the cloud point is often termed as cloud temperature or lower critical consolute temperature.1 Recently, research activities have been focused on studying the relatively less common clouding phenomenon (CP) with anionic surfactants in combination with hydrophobic cations, e.g.,sodiumdodecylsulfateandquaternaryammoniumbromides2,3,8,9,16 and mixtures of sodium perfluorooctanoate and tetrapropyl ammonium bromide.10 Similarly cloud point behavior was also seen with cationic surfactants in the presence of hydrophobic anions, e.g., erucyl-bis(hydroxyethyl)methyl ammonium chloride in the presence of tosylate or salicylates,11 and solutions of cationic surfactants such as alkyltributyl ammonium bromides that have large hydrophobic head groups.12-15 It was shown by these authors2,8 that CP could be observed even at a very low surfactant concentration and that ∼2 molecules of tetrabutyl ammonium bromide [Bu4NBr] (TBAB) was required for 1 sodium dodecyl sulfate (SDS) molecule. Later on, it was shown that a mixture of surfactant/salt at 1:1 molar ratio also exhibited cloud point behavior for a wide concentration range.9 Besides, CP also changed by addition of organic ingredients such as aromatic hydrocarbons,2 aliphatic alcohols,2 amines,2 and hydrocarbons.2 In addition, CP was also seen in systems comprising cationic polymer and anionic surfactants.7 These authors reported an unusual increase in the CP of such a mixed system (oppositely charged polymer and surfactant) with addition of salts, such as Na2SO4, NaCl, NaBr, and NaSCN. Bales and Zana found that the cloud point of aqueous solutions of tetrabutyl ammonium dodecyl sulfate is a function of the concentration of counterions in the aqueous phase.17 In * Corresponding author e-mail: [email protected], [email protected]. † Unilever Research India and Hindustan Unilever Research Centre. ‡ Indian Institute of Technology.

contrast, clouding behavior was not observed in a related system of tetrabutyl ammonium salts of long-chain fatty acids.18 The difference in the behavior of these two classes of surfactants with large hydrophobic cations is attributed to the surface charge and degree of ionization near the micelle surface. Mitra et al.9 reported a very detailed physicochemical study of the clouding process in tetra alkyl ammonium bromide salts of dodecyl sulfate solutions as well as aqueous solution of 1:1 molar mixtures of tetra alkyl ammonium bromide salt and sodium dodecyl sulfate. The self-assembly characteristics of the isolated surfactant as well as the 1:1 molar mixture was reported to be similar by these authors. A mixed micelle formation consisting of TBAB and dodecyl sulfate ion (DS-) with TBA+ counterion and an attractive interaction between these mixed micelles are proposed by the authors as the principal mechanism for the clouding process. With ternary (water + oil + surfactant) systems, there exists an important difference between nonionic and ionic amphiphiles regarding the temperature dependency of distribution of the amphiphile between oil and water. With ionic surfactants, the solubility in water increases with increase in temperature, and so to overcome the counteracting effect of thermal energy, the phase inversion of ionic amphiphile must, therefore, be enforced by “salting out” of the amphiphile out of the water into the oil.4 However, a lot of salt is required to achieve the phase inversion, and very often the salt requirement exceeds its aqueous solubility. In either case, a gradual poor solubility of surfactants in the aqueous solution (either by increasing the temperature or by increasing the electrolyte concentration) is often a precondition for the phase inversion and microemulsion phase formation in (oil-water-surfactant-salt) systems. In the present paper, we investigated the effect of hydrophobic substituents of R-N-X salts (R ) tetrabutyl (TBAX); benzyl,tributyl- (BTBAX)) as well as the effect of X (X ) Cl, Br) on the clouding process of an aqueous micellar NaLAS solution. We employed light scattering (DLS) technique to track the equilibrium aggregation processes near the clouding temperature. Subsequently, microemulsion phase behavior with hydrocarbons (hexane and decane) was investigated to understand the correlation between cloud point and (2φ-3φ) microemulsion phase

10.1021/ie801873f CCC: $40.75  2009 American Chemical Society Published on Web 06/16/2009

Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009

8843

transitions, as well as the effect of molecular weight of oils on the (2φ-3φ) transition boundary. Experimental Section Sodium linear alkylbenzene sulfonate (NaLAS) was prepared using LAS acid (96% active content) and NaOH flakes (obtained from S.d. fine chem ltd., Bangalore). LAS acid was obtained from Reliance Petrochem, Mumbai, India. The LAS acid is a mixture of isomers, and the distribution of these is provided as a supporting data. Tetrabutyl ammonium bromide (Bu4NBr, (TBAB)) was obtained from Aldrich Chemical Co., Mumbai, India, while all other quaternary salts (Bu4NCl (TBAC), C6H5Bu3NCl (BTBAC), Oct4NBr, and MeOct3NBr) were obtained from Fluka, India. Hexane was procured from Merck Ltd., Mumbai, India, as well as S.d. fine chem ltd., Bangalore, India. Decane was also obtained from S.d. fine chem ltd., Bangalore, India. Deionized water was used throughout the work. Cloud point measurements were carried out in a stirredjacketed vessel. Temperature was maintained by using water baths (Julabo F 25 and Grant LTD 6G). Stirring was facilitated using a Spinot magnetic stirrer. First visual appearance of turbidity was taken as cloud point. For phase-behavior studies, two centrifuges, i.e., a HERMLE Z 400 K (temperature-controlled) and Remi Centrifuging machine, were used. Samples were centrifuged at 4000-5000 rpm (×900 g) for about one-half hour at 25 °C. The occurrence of multiphasic mixtures was ascertained by visual inspection. The oil/water ratios in all phase-behavior experiments were kept at 1:1 by wt. A known volume (50 mL) of the mixture was taken in a centrifugation tube for mixing and centrifuge cycles. Extremely low oil-water interfacial tension values (0.4 µ). In ionic surfactant systems, the situation is much more complex when compared with nonionic surfactants. van der Waals attractive forces between the micelles support the aggregation. On the other hand, the solvation forces (general phenomenon for all ionic micellar systems) as well as the electrostatic repulsion between the micelles act against the occurrence of the clouding phenomenon. Alkyl chains of quaternary salts are likely to get embedded between monomers of micelle due to hydrophobic effect. Because of geometric limitations, some of the counterion alkyl chains penetrate into the micellar core while others position themselves in the water phase. Those in the water phase interact with the alkyl chains of other counterions, attached to other micelles. Such intermicellar interactions mediated via the hydrophobic alkyl tails of counterions may lead to expulsion of water between micelles, thus favoring the conditions for closer contact between the micelles. As the temperature is raised, progressive dehydration of ionic heads on the micellar surface occurs, which further increases the interaction between the anionic head and quaternary ammonium ion, leading to phase separation. Since the number of alkyl chains of counterions on the micellar surface is proportional to the salt content in the system, more alkyl chains near the headgroup region will replace more water and the CP is expected to occur at a lower temperature for a given surfactant concentration. As the surfactant concentration is increased, the effective salt content per micelle will decrease. As discussed above, the salt content (number of alkyl chains) is also responsible for the CP appearance; the CP appears at a higher concentration of quaternary salts. It seems that there exists some sort of compensation between temperature and salt concentration. However, the exact relationship between temperature and [salt] could not be derived. The asymptotic behavior (see Figure 1) can be attributed to the fact that, at high temperatures, the micellar interfacial region is nearly dehydrated and, therefore, [salt] requirement remains roughly the same for a wider range of high temperatures. It might be useful here to comment on the one-to-one correspondence between quaternary counterions and surfactant for cloud point observation. As depicted in Figure 2, the correspondence is not unique, i.e., the amount of counterions

per surfactant molecule required for the clouding phenomenon depends upon the temperature. It is well-known that the clouding phenomenon is seen for SDS/TBAB ) 1:1 molar ratio9 and at other compositions, e.g., at the lowest temperature studied in this paper (20 °C), only 1 salt molecule per 5 surfactant molecules is required for the onset of clouding. On the other hand, to obtain clouding at 50 °C, ∼1 salt molecule per 2 surfactant molecules is required. Kumar et al. reported slightly more than one Bu4N+ moiety for two SDS- molecules to produce clouding at 95 °C. In spite of the differences in the surfactant type (SDS vs NaLAS) used in these two studies, the trend is similar, i.e., [salt]/[surfactant] ratio for the onset of clouding phenomenon increases with operating temperature. However, it must be mentioned that these systems including the present one are complicated by the presence of mixed surfactants (e.g., tetrabutyl ammonium salt of alkyl benzene sulfonate in addition to sodium alkyl benzene sulfonate) and electrolytes (e.g., NaBr, RNX, R ) tetrabutyl, benzyltributyl; X ) Cl, Br). Hence, a new interpretation of the experimental results described above could only be speculative. Effect of Hydrophobic Moieties (Of Salt) and Halide Ions on the Cloud Point. We investigated the CP occurrence with a variety of salts with different anions (tetrabutyl ammonium bromide (Bu4NBr) vs tetrabutyl ammonium chloride (Bu4NCl) as well as with increasing hydrophobic volume, namely, benzyltributyl ammonium chloride (C6H5Bu3NCl), tetraoctyl ammonium bromide (Oct4NBr), and methyltrioctyl ammonium bromide (MeOct3NBr). Because of the insolubility of Oct4NBr and MeOct3NBr in water, the study was limited to the remaining three salts. CP data for micellar NaLAS (3.5 mM) in the presence of salts such as Bu4NBr, Bu4NCl, and C6H5Bu3NCl is shown in Figure 3. From Figure 3, it is clear that less salt is required for CP to occur for C6H5Bu3NCl, compared with either Bu4NBr or Bu4NCl. When comparison is made for CP-[salt] profile between Bu4NBr and Bu4NCl, there is not much difference between the two and they behave in an almost similar manner. Similar CP-[salt] profiles for Bu4NBr and Bu4NCl suggest that counterion type has a negligible or the same (if any) contribution on clouding behavior of aqueous NaLAS. On the other hand, the presence of a bulky benzene ring in C6H5Bu3NCl causes an increase in the hydrophobic volume (as compared to those for Bu4NBr and Bu4NCl). This structural difference is responsible for greater hydrophobic interactions between C6H5Bu3NCl and surfactant monomer7 as well as greater attractive interaction between micelles. It may be said that C6H5Bu3N+ ion is more effective in removing water of hydration from NaLAS micellar

Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009

8845

Figure 4. Size variation of aggregates during the clouding process by increasing the temperature in 3.5 mM NaLAS + 0.05 M TBAB solution.

Figure 5. Clouding process based on light scattering data for 3.5 mM NaLAS solution at various TBAB concentrations.

interface with a consequent steeper fall observed in CP[C6H5Bu3NCl] profile (see Figure 3.). The structural changes, accompanying the clouding process, were investigated by determining the particle (e.g., micellar) size changes as well as aggregate sizes (much larger than micelles) employing dynamic light scattering. A concentration of (3.5 mM NaLAS + 0.05 M TBAB) was chosen for the study (see Figure 1). The reason for the choice is (i) for salt concentrations > 0.04 M, the CP variations are less with increasing salt concentration and (ii) the CP is almost independent of surfactant concentration. The size changes will be representative of the micellar NaLAS system. Figure 4 shows the size changes as a function of temperature for the (3.5 mM NaLAS + 0.05 TBAB) system. At 22.5 °C, the solution is a visually clear solution and the average size of the micelles is 25 nm. It may be mentioned that the NaLAS micelle size without any salt was found to be 3.1 ( 1 nm at 25 °C. The micellar size increased considerably in the presence of salt. With subsequent increase in temperature to 33.5 °C, a bimodal distribution in the droplet sizes was noticed with one size distribution averaging at 40 nm and the other at 200 nm. The solution, at this stage, has a bluish appearance. With a further increase in temperature to 35 °C, the bimodal distribution gave rise to a more monomodal particle size distribution with an average value of 400 nm. The solution at this temperature is visually turbid and easily detectable. With a further increase in temperature, a monodispersed solution with an average particle size of 600 nm was observed. Similar experiments were repeated at other salt concentrations as well. It was interesting to note that highly monodispersed aggregate formed post the clouding phenomenon. The detailed clouding curve thus generated for 3.5 mM NaLAS at various TBAB concentrations is presented in Figure 5. As evident from Figure 5, there are two stages of the clouding process. In the first stage, a strong intermicellar interaction leads to a significant reorganization of micellar size distributions. The solution during the initial micelle aggregation process may appear bluish and, hence, is termed as the onset of clouding process. The onset of the clouding process is thus determined at various salt concentrations and appears at a higher temperature for lower salt concentrations. With a further increase in temperature, micelles aggregate because of the stronger attractive interaction between micelles due to various possible reasons, discussed in the preceding sections. The phase-separation process is accompanied by a stronger micellar aggregation process and formation of highly monodispersed aggregates of larger sizes. It is interesting to note that, at higher temperatures, the solution with a visually turbid appearance does not have

any micelles and the turbidity is due to a strongly monodispersed aggregates. The dispersions were later checked under a polarizing microscope for any other ordered phase formation, but they were found to contain none. It may be pointed out that the above cloud point curves were determined from light scattering experiments only, and a comparison with the curve in Figure 1 (determined from visual observation) indicates a small difference in the cloud point boundary at higher salt concentrations, which may be accounted as error in visually estimating the cloud point. Microemulsion Phase Behavior with Oil Addition: NaLAS-TBAB-Water-Alkane System. After introducing hexane as an oil to the (NaLAS-TBAB-water) system, microemulsion phase behavior was studied following the protocol of Kahlweit et al. (Fish diagram). The oil/water ratio by wt was chosen to be 1:1. Because the salt was required to induce the clouding behavior in a (salt concentration-temparature) phase space at a fixed NaLAS concentration, microemulsion phase behavior was studied in the (salt concentration-temperature) space, keeping NaLAS concentration fixed at 30 mM. Usually, critical microemulsion concentration is ∼5-100 × CMC, depending upon the alkyl carbon number (ACN) or equivalent carbon number (EACN) of a given oil. In the present case, however, surfactant concentration was fixed at 15 × CMC of NaLAS (in deionized water). A minimum of 0.035 M TBAB is needed for the appearance of 3-φ microemulsion, which is roughly equivalent (by molar concentration) to the NaLAS concentration in the system. It was well-known that even 1:1 molar ratio of SDS/TBAB could exhibit clouding phenomenon,9 similar to that of the isolated tetrabutylammonium salt of dodecyl sulfate (TBADS). In our study, we found that the CMC of the mixture NaLAS/TBAB at 1:1 molar ratio is 0.4 mM, which is 5 times lower than NaLAS alone. The critical microemulsion concentration for (NaLASTBAB-water-hexane) system is ∼75 times that of the corresponding CMC. The CP for 0.03 M NaLAS and 0.035 M TBAB mixture was observed at 30 °C (Figure 1). The head of the 3-φ region is also seen at 30 °C (Figure 6). As seen from the CP behavior of the present (surfactantsalt-water) system, a compensation mechanism between salt and temperature in the formation of the three-phase microemulsion also exists. These two field variables provide the required thrust by decreasing the hydrophilicity of surfactant sufficient enough for them to phase separate and form a microemulsion phase. A right shift in the three-phase boundary with decane against hexane (see Figure 7) shows that, at fixed temperature and amphiphile concentration, the amount of salt required for three-phase formation increases with increasing carbon number of the oil.

8846

Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009

Figure 6. Partial 3-φ region depicting the 2φ-3φ boundary of (NaLASTBAB-water-hexane) system. The upper regions of the phase boundary were not determined on account of loq BP of hexane. NaLAS concentration in the aqueous part is 30 mM. Oil/aqueous phase ratio ) 1:1 by wt.

Figure 7. Partial microemulsion phase region for (NaLAS-TBABwater-decane) system. NaLAS concentration in the aqueous part is 30 mM. Oil/aqueous phase ratio ) 1:1 by wt. The cloud point boundary is also shown as a separate line.

Figure 8. Partial microemulsion phase region for (NaLAS-BTBACwater-decane) system. NaLAS concentration in the aqueous part is 30 mM. Oil/aqueous phase ratio ) 1:1 by wt.

It is well-known4 that the three-phase body in the microemulsion system shifts to a higher temperature with increasing molecular weight of the oil. In the present case, by replacing hexane with decane, the three-phase body (lower temperature boundary) moved up from 5° to 20 °C, in addition to the requirement of more salt (0.05 M Bu4NBr compared to 0.035 M for the system containing hexane). These observations are in line with well-known microemulsion phase behavior.4,5

Effect of Hydrophobic Moiety of Salt on the Microemulsion Phase Behavior. By replacing Bu4NBr with a more hydrophobic salt (C6H5Bu3NCl), it was seen earlier that the clouding phenomenon of aqueous surfactant solution occurred at a lower salt concentration. A similar trend is also expected in the microemulsion phase behavior. Figure 8 shows the microemulsion phase behavior for (NaLAS-water-hexanebenzyltributyl salt) system. The CP data trends, obtained for different salts (as shown in Figure 3), indicated that more hydrophobic C6H5Bu3NCl is far more effective to salt out the surfactant from its aqueous solution when compared with Bu4NBr or Bu4NCl. With oil, the threephase microemulsion body occurred at lower salt concentration (0.022 M, in contrast to 0.035 M with Bu4NBr). In addition, the three-phase body with C6H5Bu3NCl widens in the temperature space significantly. Oil-Water Interfacial Tension in the Microemulsion Phase. Microemulsion systems are characterized by a low oil-water (O-W) interfacial tension (IFT). The phase behavior of quaternary systems indicated the formation of a three-phase microemulsion. Additional evidence, e.g., ultra low oil-water interfacial tension during the three-phase region, was required to confirm the formation of microemulsion systems. In view of this, oil-water IFT was measured as a function of temperature for the (NaLAS-C6H5Bu3NCl-water-hexane) system. The phase diagram in Figure 8 indicates that, at 0.025 M C6H5Bu3NCl concentration, the transition from 2φ-3φ and 3φ2φ j occurred at 15 and 35 °C, respectively. The representations 2 and 2j, respectively, indicate 2-φ with most surfactant residing in the bottom water phase (O/W) and 2-φ with most surfactant residing in the top oil phase (W/O). It is, therefore, expected for hexane-water IFT to exhibit a low value within this temperature range. Figure 9 shows the oil(hexane)-water IFT (γ) as a function of temperature. Water contains 0.03 M NaLAS and 0.025 M C6H5Bu3NCl. The interfacial tension of hexane in aqueous solution of (surfactant + salt) decreased from 0.16 dynes/cm (at 10 °C) to 0.06 dynes/cm (at 25 °C) and then increased with increasing temperature. Such γmin within the 3-φ region is indicative of microemulsion phase behavior. Conclusion Salts (RN+X-) with hydrophobic substituents, such as tetrabutyl and benzyltributyl, X ) Cl, Br, which are also phasetransfer catalysts, are capable of inducing a phase separation of surfactant-rich phases from its aqueous surfactant solution. This

Figure 9. Plot between IFT of (30 mM NaLAS + 0.025 M BTBAC) and hexane as a function of temperature.

Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009

phase-separation process, often called the clouding phenomenon, is marked by a two-stage micellar aggregation processes. First, in the presence of salt, the micelles grow in size, leading to a bluish appearance of the solution (onset of clouding). With further increase in the temperature, micelles first form loose aggregates, manifested by large polydispersity in particle size distribution (phase separation). With further increase in temperature, a highly monodispersed aggregate structure formation gives rise to a highly turbid solution. C6H5Bu3NCl was found to be the most effective salting-out reagent among the quaternary salts tried in the present study. The presence of bulky hydrophobic moiety in the salt is the reason for such efficiency, when compared to salts with alkyl substituents, such as butyl. The anion type (of salt) seems not to affect the cloud point. Such clouding phenomenon is a prerequisite for microemulsion formation when oil is added to the (water-surfactant-salt) system. The 3-φ boundary succeeds the cloud point boundary, and it seems to correlate well with the cloud point boundary. The position of the three-phase body in the phase space agree well with the known microemulsion phase behavior The microemulsion forming systems with quaternary salts display low oil-water IFT (0.06 dynes/cm) in the 3-φ region. Acknowledgment We are thankful to the journal referees for their constructive comments. S.C. and A.A. acknowledge Hindustan Unilever Limited (HUL) for working at the Research Centre at Bangalore as Summer Trainees. The authors acknowledge several useful inputs obtained from Gautam Kini, P. Rejitha. The authors also acknowledge the permission given by the Research Management of HUL to publish the work. Supporting Information Available: Chemical composition of the raw material and titration curve of the LAS acid and sodium hydroxide reaction. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Evans, F. D.; Wennerstorm, H. Colloidal Domain; VCH: NewYork, 1994. (2) Kumar, S.; Sharma, D.; Kabir-ud-Din. Cloud point phenomenon in anionic surfactant plus quaternary bromide systems and its variation with additives. Langmuir 2000, 16, 6821.

8847

(3) Cross, J. Nonionic Surfactants: Chemical Analysis, Surfactant Science Series; Marcel Dekker: NewYork/Basel, 1986. (4) Kahlweit, M. How to prepare microemulsions at prescribed temperature, oil and brine. J. Phys. Chem. 1995, 99, 1281. (5) Balzer, D.; Luders, H. Nonionic Surfactants: Alkyl Polyglucosides, Surfactant Science Series; Marcel Dekker: NewYork/Basel, 2000. (6) Kumar, S.; Sharma, D.; Khan, Z. A.; Kabir-ud-Din. Salt-induced cloud point in anionic surfactant solutions: Role of the head group and additives. Langmuir 2002, 18, 4205. (7) Yan, Y.; Li, L.; Hoffmann, H. Clouding: Origin of Phase Separation in oppositely charged polyelectrolytes/surfactant mixed solutions. J. Phys. Chem. B 2006, 110, 1949. (8) Kumar, S.; Sharma, D.; Kabir-ud-Din. Temperature-[salt] compensation for clouding in ionic micellar systems containing sodium dodecyl sulfate and symmetrical quaternary bromides. Langmuir 2003, 19, 3539. (9) Mitra, D.; Chakraborty, I.; Bhattacharya, S. C.; Moulik, S. P. Interfacial and Solution Properties of Tetraalkylammonium Bromides and their Sodium Dodecyl Sulfate Interacted Products: A Detailed Physicochemical Study. Langmuir 2007, 23, 3049. (10) Yu, Z. J.; Neuman, R. Non-critical behavior near the cloud point in perfluorinated ionic micellar solutions. Langmuir 1994, 10, 377. (11) Raghavan, S. R.; Edlund, H.; Kaler, E. W. Cloud point phenomena in wormlike micellar systems containing cationic surfactant and salt. Langmuir 2002, 18, 1056. (12) Drifford, M.; Belloni, L.; Dubois, M. J. Light scattering on concentrated micellar systemssInfluence of monomers. J. Colloid Interface Sci. 1987, 118, 50. (13) Warr, G. G.; Zemb, T.; Drifford, M. Liquid-liquid phase separation in cationic micellar solution. J. Phys. Chem. 1990, 94, 3086. (14) Jansson, M.; Warr, G. G. Self-diffusion coefficients in attractive micellar solutions. J. Colloid Interface Sci. 1990, 140, 541. (15) Buckingham, S.; Garvey, C.; Warr, G. G. Effect of head group size on micellization and phase behavior in quaternary ammonium surfactant systems. J. Phys. Chem. 1993, 97, 10236. (16) Keller, J. M.; Lundeman, H. D.; Warr, G. G. Structure and transport in concentrated micellar solutions with a lower consolute boundary. J. Chem. Soc., Faraday Trans. 1994, 90, 2071. (17) Bales, B. L.; Zana, R. Cloud Point of Aqueous Solutions of Tetrabutylammonium Dodecyl Sulfate Is a Function of the Concentration of Counterions in the Aqueous Phase. Langmuir 2004, 20, 1579. (18) Zana, R.; Schmidt, J.; Talmon, Y. Tetrabutyl Ammonium Alkyl Carboxylate Surfactants in Aqueous Solution: Self Association Behavior, Solution Nanostructure, and Comparison with Tetrabutyl Ammonium Alkyl sulfate Surfactants. Langmuir 2005, 21, 11628. (19) Rout, D. CMC determination for NaLAS, NaLAS/TBAB ) 1:1 by molar ratio, and NaLAS/ BTBAC ) 1:1 molar ratio. Personal communications.

ReceiVed for reView December 5, 2008 ReVised manuscript receiVed May 21, 2009 Accepted May 21, 2009 IE801873F