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Salt-Induced Cloud Point in Anionic Surfactant Solutions: Role of the Headgroup and Additives Sanjeev Kumar, Damyanti Sharma, Ziya Ahmad Khan, and Kabir-ud-Din* Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India Received August 23, 2001. In Final Form: February 12, 2002 Anionic surfactants are not known to show a clouding phenomenon in aqueous solutions. On the other hand, this is a general feature for nonionic surfactants. Here the effect of addition of tetra-n-butylammonium bromide (Bu4NBr) on the clouding phenomenon in sodium dodecylbenzenesulfonate (SDBS) has been studied by measuring cloud points (CP) for each combination. Similar type of studies were also performed with poly(ethylene glycol) t-octylphenyl ether (TX-100). The CP varies in an opposite manner for the two classes of surfactants, which is explained in terms of charge variation in each type of micelles by the addition of Bu4NBr. A relationship between [SDBS] vs [Bu4NBr] has been worked out for getting the CP - phenomenon in SDBS solutions: nearly one Bu4NBr molecule is needed for each two SDBS monomers for getting the CP in the system. With 10-mM SDBS, the addition of Bu4NBr shows an interesting phase behavior, where a stable colloidal phase with bluish - white appearance (preclouded) appears preceded by conventional clouding. Effects of the addition of ureas (urea and tetramethylurea), thioureas (thiourea and tetramethylthiourea), amino acids (glycine, alanine, leucine and phenylalanine), and sugars (xylose, arabinose and dextrose) have also been seen on the 50-mM SDBS + 35-mM Bu4NBr system (this system was chosen because its CP has a wider window available for variations below and above the CP). Ureas and thioureas affect the CP in different manners, which are explained in the light of indirect and direct interactions with micelles. CP variation in the presence of amino acids depends on their polar and hydrophobic nature. On the other hand, sugars behave in a manner similar to their effect on solubility of hydrophobic compounds in aqueous solutions.
Introduction Nonionic micellar solutions are well-known for their propensity to undergo clouding on heating followed by formation of two coexisting isotropic phases.1-4 Concentrated aqueous salt solutions of certain zwitterionic and ionic surfactants also exhibit clouding behavior.4-9 Recently, acid-induced clouding in anionic surfactants was reported.10 The threshold temperature for the clouding is known as the cloud point (CP). Clouding is attributed to the efficient dehydration of hydrophilic portion of the micelle at higher temperature. The value of the CP depends on the structure and concentration of surfactant with or without additive(s).9,11 However, the mechanism via which the phenomenon occurs remains obscure. The role of oscillations in the critical concentration and of micellar growth as mechanisms for the clouding phenomenon is still a controversial issue.12-14 Large amounts * To whom correspondence should be addressed. E-mail: kabir7@ rediffmail.com. (1) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023. (2) Karlstrom, G. J. Phys. Chem. 1985, 89, 4962. (3) Komaromy-Hiller, G.; Calkins, N.; von Wandruszka, R. Langmuir 1996, 12, 916. (4) Lang, J. C.; Morgan, R. C. J. Chem. Phys. 1980, 73, 5849. (5) Appell, J.; Porte, G. J. Phys. Lett. 1983, 44, L-689. (6) Warr, G. G.; Zemb, T. N.; Drifford, M. J. Phys. Chem. 1990, 94, 3086. (7) Yu, Z.-J.; Xu, G. J. Phys. Chem. 1989, 93, 7441. (8) Smith, A. M.; Holmes, M. C.; Pitt, A.; Harrison, W.; Tiddy, G. J. T. Langmuir 1995, 11, 4202. (9) Kumar, S.; Sharma, D.; Kabir-ud-Din Langmuir 2000, 16, 6821. (10) Casero, I.; Sicilia, D.; Rubio, S.; Perez-Bendito, D. Anal. Chem. 1999, 71, 4519. (11) Hinze, W. L.; Pramauro, E. Crit. Rev. Anal. Chem. 1993, 24, 133. (12) Lum Wam, J. A.; Warr, G. G.; White, L. R.; Grieser, F. Colloid Polym. Sci. 1987, 265, 528. (13) Strunk, H.; Lang, P.; Findenegg, G. H. J. Phys. Chem. 1994, 98, 11557. (14) Lang, P.; Glatter, O. Langmuir 1996, 12, 1193.
of added electrolytes in micellar solutions screen the repulsive electrostatic effects that usually stabilize the solutions. It is well established that the addition of ionic surfactants increases the cloud points of their nonionic counterparts11,15 and the increase depends on composition of the mixed micelles. Recently, preclouding in mixed surfactants has been reported by von Wandruszka and coworkers.16-18 Valaulikar and Manohar19 have demonstrated that the increase in cloud point can be described in terms of the surface charge per micelle which is responsible for electrostatic repulsion between the micelles. This supports the viewpoint that micelle coalescence, rather than micellar growth, is responsible for the clouding process. Hence, if introduction of charge to a nonionic micelle delays the phase separation, depletion of the charge on an ionic micelle could cause resumption of the phase separation. Thus, charge could be one of the factors to tune CP, especially in ionic micellar solutions. In most of their applications, surfactant mixtures (combinations include surfactant-surfactant, surfactantelectrolyte/nonelectrolyte, surfactant-polymer, etc.) rather than single ones are preferred as the mixed systems often exhibit enhanced properties through synergism. But systematic investigations of surfactant mixtures with oppositely charged hydrophobic counterions are sparse. The surface activity of oppositely charged systems is usually higher than that of other mixed systems. However, (15) Myers, D. Surfactant Science and Technology, 2nd ed.; VCH Publishers: New York, 1992. (16) McCarroll, M.; Toerne, K.; von Wandruszka, R. Langmuir 1998, 14, 2965. (17) McCarroll, M. E.; Toerne, K.; von Wandruszka, R. Langmuir 1998, 14, 6096. (18) Toerne, K.; Rogers, R.; von Wandruszka, R. Langmuir 2000, 16, 2141. (19) Valaulikar, B. S.; Manohar, C. J. Colloid Interface Sci. 1985, 108, 403.
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precipitation or coacervate formation occurs in such systems,20,21 which is regarded as a loss of surface activity. Thus, studying properties of mixed stable surfactant systems would be of great value to search for higher surface active systems. Keeping this view in mind we have started systematic studies on the occurrence of CP phenomenon in sodium dodecyl sulfate (SDS) solutions containing quaternary bromides with or without additives.9,22,23 Contrary to their inorganic counterparts, symmetrical quaternary ammonium ions (R4N+) are less hydrated and exhibit ambivalent nature in aqueous solutions. In these ions, the single positive charge is buried in a paraffin shell produced by the four alkyl chains. Therefore, R4N+ can interact with anionic micellar surfaces electrostatically as well as hydrophobically. In the previous studies,9,23 we have shown that the CP could be tuned with the proper [surfactant]/[R4N+] ratio and could conveniently be brought near to or below the ambient temperature. Therefore, such systems could be used to extract thermally labile analytes such as amino acids, proteins, or vitamins.10 The purpose of the present study was multifold: (i) to study clouding phenomenon in anionic sodium dodecylbenzenesulfonate (SDBS) surfactant, which is used for various commercial applications, (ii) to establish [SDBS]/[salt] ratio for the appearance of clouding in this system, (iii) to compare with SDS in order to see the effect of hydration states of the two kinds of micellar interfaces, (iv) to see the effect of addition of ureas, amino acids, and sugars on CP to make these systems compatible with thermally labile compounds such as proteins and vitamins for their possible extraction by CP extraction method,10 and (v) to automatize the CP for desired application mode. For comparison, CP measurements were also made in poly(ethylene glycol) t-octylphenyl ether (TX-100) solutions in the presence of tetra-n-butylammonium bromide (Bu4NBr). Conductivity measurements were performed to obtain cmc and degree of counterion dissociation (β). Experimental Section SDBS (∼99%) obtained from Tokyo Kasei, Japan, was successively recrystallized from methanol and water, then rinsed with cold methanol and finally dried under vacuum. Purity was further ascertained by the absence of minimum in surface tension vs [SDBS] plot. TX-100 was obtained from Fluka, Switzerland (Prod. No. 93420), and used as received. Bu4NBr and SDS were the same as used earlier.9 All other chemicals viz., urea, tetramethylurea, thiourea, tetramethylthiourea, glycine, L-alanine, L-leucine, L(-)phenylalanine, D(-)-arabinose, D(+)-xylose, and D(+)glucose(dextrose) were of best analytical grade available. Demineralized double-distilled water was used throughout. Freshly prepared stock solutions of the surfactants were used to obtain samples for CP measurements (containing Bu4NBr with or without different additives). CPs were obtained by placing Pyrex glass tubes (containing the sample solutions) into a temperature controlled bath, the temperature of which was ramped at the rate of 0.1 °C/ min near the CP and onset clouding was noted by visual inspection. However, the temperature was oscillated slowly through the CP until reproducible (( 0.1 °C).9,23 (20) Barry, B. W.; Gray, G. M. T. J. Colloid Interface Sci. 1975, 52, 327. (21) Mukhayer, G. I.; Davis, S. S. J. Colloid Interface Sci. 1976, 56, 350. (22) Kumar, S.; Aswal, V. K.; Naqvi, A. Z.; Goyal, P. S.; Kabir-udDin. Langmuir 2001, 17, 2549. (23) Kumar, S.; Sharma, D.; Khan, Z. A.; Kabir-ud-Din. Langmuir 2001, 17, 5813.
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Figure 1. Variation of cloud point (CP) with [Bu4NBr] for 50-mM surfactant solutions: O, SDS; b, SDBS; k,TX-100.
The precise conductances of SDBS in water containing different amounts of Bu4NBr were measured at 25 ( 0.1 °C with ELICO conductivity Bridge (type CM 82T) using platinized electrodes with cell constant 1.02 cm-1. The conductivity can be linearly correlated to the [surfactant] in both the pre-micellar and post-micellar regions, having a slope in the former region greater than that in the latter. The intersection point of the two straight lines gives the cmc and the ratio of the slopes gives the degree of counterion dissociation (β). Results and Discussion Figure 1 shows the variation of CP with the addition of Bu4NBr to solutions of different surfactants (SDBS, SDS, and TX-100). Bu4NBr has a different effect on the two classes of surfactants (anionic and nonionic). A detailed discussion on the mechanism of CP appearance in the SDS with Bu4NBr has been published elsewhere.9,23 However, for the sake of brevity, a few main characteristics of micellar surface region need be considered again. The micellar surface is characterized by the presence of water of hydration. The Bu4N+ cation, in addition to a positive charge, carries four butyl chains. Therefore, as pointed out earlier, it can interact with the micellar surface hydrophobically as well as electrostatically. The two modes of interaction influence CP in opposite manners for the two classes to which the surfactants under study belong (SDS, SDBS - anionic, pure ionic surfactants have no CP; TX-100 - nonionic). In the following paragraphs, the influence of Bu4NBr addition on each class is discussed separately. The first appearance of CP in a 50-mM solution of both SDS and SDBS occurs at roughly the same [Bu4NBr] (Figure 1), which indicates that the hydration states of interfaces with both the anionic surfactants are nearly similar. It is quite obvious that at such a high temperature (∼90 °C) the micellar interfacial region is nearly completely dehydrated and, therefore, needs roughly the same [Bu4NBr]. However, as the [Bu4NBr] increases, CP [Bu4NBr] profiles (Figure 1) show different variation, which demonstrates that Bu4NBr is more effective in decreasing CP in the SDBS systems (SDBS + Bu4NBr). This behavior could be understood in view of basicity and phase data which led to the following rank ordering of headgroups with respect to their relative hydrophilicity24 (24) Laughlin, R. G. Adv. Liq. Crystals 1978, 3, 99.
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Table 1. Critical Micelle Concentration (cmc) and Apparent Degree of Counterion Dissociation (β) for Different Surfactant Systems at 30 °C surfactant system
103 cmc (mM)
β
SDSa SDBSa TX - 100b SDBS + 2 mM Bu4NBra SDBS + 4 mM Bu4NBra SDBS + 6 mM Bu4NBra
8.5 2.5 0.24 1.28 0.71 0.47
0.52 0.86 0.85 0.77 0.78
a Present work, obtained conductometrically. b Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant System; NSRDS - NBS 36; Washington, D.C., 1971.
- COO- . - SO3- > - OSO3The structural differences in (or near) the surfactant headgroup regions of SDBS and SDS seem to affect their cmc and apparent degree of counterion dissociation (β) (Table 1). The presence of a benzene ring in the SDBS monomer causes lengthening of the hydrophobic chain (as compared to SDS), which is responsible for lower cmc,25 while the higher hydrophilicity of -SO3- in SDBS monomer24 seems to be responsible for a higher β value. The only difference in the two surfactant monomers is that SDBS has Figure 2. Variation of cloud point (CP) with [Bu4NBr] for 10-mM SDBS: the shaded area shows preclouding region.
while SDS has -O-. This structural difference is responsible for greater hydrophobic interactions between the benzene ring of the SDBS monomer and the alkyl chains of Bu4NBr. Also, a higher β value observed in case of SDBS shows a greater attraction between the micelles and Bu4N+. These two types of interactions (increased hydrophobic and stronger electrostatic) in the case of SDBS would cause Bu4N+ to be more effective to remove water of hydration from SDBS micellar interface with a consequent steeper fall observed in the CP - [Bu4NBr] profile for SDBS (Figure 1). Addition of Bu4NBr to the nonionic TX-100 shows an opposite effect which indicates that a different mechanism is operating toward the CP - variation. CP increase has been observed in nonionic micellar solutions on the addition of different organic acids/salts and also a few ionic surfactants.16-19,26-30 As shown by Valaulikar and Manohar,19 addition of the latter to solutions of nonionic surfactants increased the CP by introducing electrostatic repulsion between the resultant mixed ionic micelles. They further demonstrated that the increase in CP could be described in terms of the surface charge per micelle. The increase in charge also favors an increase in solubility. The rise in CP of TX-100 in the presence of Bu4NBr (Figure 1) could thus be anticipated, and is in tune with the accepted norms. Figure 2 shows the variation of CP in 10-mM SDBS solutions by the addition of Bu4NBr. CP decreases with [Bu4NBr] in the similar fashion as observed with 50-mM (25) Lin, I. J.; Somasundaran, P. J. Colloid Interface Sci. 1971, 37, 731. (26) Manohar, C.; Kelkar, V. K.; Mishra, B. K.; Rao, K. S.; Goyal, P. S.; Dasannacharya, B. A. Chem. Phys. Lett. 1990, 171, 451. (27) Kelker, V. K.; Mishra, B. K.; Rao, K. S.; Goyal, P. S.; Manohar, C. Phys. Rev. A 1991, 44, 8421. (28) Pandya, K.; Lad, K.; Bahadur, P. J. Macromol. Sci., - Pure Appl. Chem. 1993, A30, 1. (29) Cardoso da Silva, R.; Loh, W. J. Colloid Interface Sci. 1998, 202, 385. (30) Ghosh, S.; Moulik, S. P. Indian J. Chem. 1999, 38A, 10.
SDBS (Figure 1) with the difference that a range of sample compositions exists (shaded part of Figure 2) in which the clouding process proceeds differently. This is manifested during the rise of temperature in the process of achieving CP. A bluish stable dispersion (preclouding) appeared before the solutions clouded in a true conventional sense. Similar types of observations were made with nonionic surfactants during the addition of ionic surfactant, without much discussion.16,31,32 Only recently, a tentative mechanism has been presented for the preclouding in a nonionic - ionic surfactant system.17 The size range of the particles in such a system is dictated by the relative proportions of nonionic and ionic surfactants. In a separate study,23 the variation of CP was correlated with the charge per micelle. In the above referred systems as well as in the present one, the charge of the micelle, which is on the change progressively, seems responsible for such an interesting phenomenon (preclouding).16 The present results, however, do not allow us to discuss more about preclouding and one has to wait for further results. Earlier,23 we worked out a correlation between [SDS] and [Bu4NBr] and pointed out that slightly more than one salt counterion per two surfactant monomers was required to produce clouding (∼ 95 °C). To get more insight and to find out similar relations at different temperatures, we have performed similar kind of experiments (like Figure 1) at several fixed [SDBS] (all are not shown). In fact, this exercise was done to gather information regarding compensation behavior (if any) between [salt] and temperature as increasing the [salt] decreases the CP (one should keep in mind that an initial [Bu4NBr] is necessary to observe the phenomenon that also depends on the initial [surfactant]). Figure 3 shows the effect of the variation of [Bu4NBr] with [SDBS] at different temperatures (straight line plots). It can be seen that the positive intercepts reduce near to zero as the temperature for determining [Bu4(31) Maclay, W. N. J. Colloid Sci. 1956, 11, 272. (32) Nishikido, N.; Akisada, H.; Matura, R. Mem. Fac. Sci., Kyushu University Ser. C 1977, 10, 92.
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Figure 3. Plot between [SDBS] and the minimum [Bu4NBr] needed for the appearance of CP at different temperatures: O, 30; b, 40; k, 90 °C.
NBr], needed to get clouding, increases. For the system SDBS - Bu4NBr, the minimum [Bu4NBr] needed to get clouding (∼90 °C, Figure 3, lower curve) follows a relation in which it is computed (by the slope of the curve) that ca. one Bu4N+ is required per two SDBS monomers in the system. A less [Bu4N+] required than for SDS23 confirms that Bu4NBr is more effective with SDBS (vide supra). At first sight, it seems that there is some sort of compensation between temperature and [Bu4NBr] (see Figure 3). However, the exact relationship between temperature and [salt] could not be derived. As mentioned earlier, these studies could prove useful in cloud point extraction methods for the separation/ preconcentration of different analytes.10 As the analytes can also affect CP of the test samples, we have seen the effect of the addition of different nonelectrolytes (Figures 4 - 6) on the CP behavior of the 50-mM SDBS + 35-mM Bu4NBr system. This system was chosen as a reference because it has a CP of 44.5 °C, which has a wider temperature window for making variations below and above the CP. The CP variations of 50-mM SDBS + 35-mM Bu4NBr with some ureas are shown in Figure 4. All the ureas initially increase the CP followed by a significant decrease (with thioureas) or near constancy (with ureas). Two different mechanisms for urea action on the properties of micellar solutions have been proposed: (i) an indirect mechanism whereby urea causes the loss of water “structure” that facilitates the hydration of a nonpolar solute,33,34 and (ii) a direct mechanism whereby urea replaces some of the water molecules in the hydration shell of the solute.35,36 In view of the above, the actions of ureas and thioureas are seemingly different, which is obvious in the light of different crystal structures,37,38 as well as different atoms involved in chemical bonds in their molecules.9 Jencks and co-workers39,40 have proposed that the increased solubility of hydrocarbons in aqueous urea (33) Franks, H. S.; Franks, F. J. Chem. Phys. 1968, 48, 4746. (34) Franks, F. In Water - A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1975; Vol. 4, Chapter 1. (35) Enea, O.; Jolicoeur, C. J. Phys. Chem. 1982, 86, 3370. (36) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218. (37) Masunov, A.; Dannenberg, J. J. J. Phys. Chem. A 1999, 103, 178. (38) Masunov, A.; Dannenberg, J. J. J. Phys. Chem. B 2000, 104, 806. (39) Robinson, D.; Jencks, W. P. J. Am. Chem. Soc. 1965, 87, 2462. (40) Roseman, M.; Jencks, W. P. J. Am. Chem. Soc. 1975, 97, 631.
Figure 4. Cloud point variation in the system 50-mM SDBS + 35-mM Bu4NBr with the concentration of ureas and thioureas: x, urea; b, tetramethylurea; Y, thiourea; O, tetramethylthiourea.
results primarily from a smaller free energy of cavity formation in the mixed solvent, resulting from the replacement of water by the larger urea molecule in the solvation region. It has also been reported that the counterion dissociation degree (β) of micelles increases with urea addition.41 As a result of increase of β, the micelle hydration would increase, which would contribute toward the CP increase. Another factor is the solubility increase of surfactant (with urea addition) in the bulk solvent. The two factors taken together would need extra heating in order to observe clouding, and hence the temperature where the surfactant solubility would decrease drastically in the bulk solvent, viz. CP, would be higher. The above observations can be rationalized in terms of competing attractive van der Waals and repulsive steric interactions between the hydrated micelles.42,43 In the present case, the latter effect seems responsible for the increase of CP in case of urea. At higher [urea], both the effects seem equally important and thus give a rather constancy in CP vs [urea] plots. Because of the presence of additional methylene groups in tetramethylurea, the initial steep rise in CP may be due to enhanced solubility of nonpolar parts of the SDBS monomer and Bu4N+. The resultant stabilized system (due to enhanced solubility) will obviously show a higher CP. These results corroborate the influence of the presence of additional methylene group in ureas on cmc variation which shows an increasing trend as the number of methylene groups increase.44 The interesting behavior of drastic CP decrease in the presence of thioureas could be understood in the light of direct interaction of thioureas and SDBS + Bu4NBr mixed micelles. Since the >CdS bond has a different nature (41) Asakawa, T.; Hashikawa, M.; Amada, K.; Miyagishi, S. Langmuir 1995, 11, 2376. (42) Mitchell, D. J.; Tiddy, G. J. T.; Warring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. I 1983, 79, 975. (43) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: New York, 1985. (44) Costantino, L.; D'Errico, G.; Roscigno, P.; Vitagliano, V. J. Phys. Chem. B 2000, 104, 7326.
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Figure 5. Cloud point variation in the system 50-mM SDBS + 35-mM Bu4NBr with the concentration of R-amino acids: Y, glycine; x, alanine; O, leucine; b, phenylalanine.
than the >CdO bond, one can understand the consequences of an electron deficient S-atom toward its interaction with SDBS anionic micelle.9 It seems that thiourea interacts with the SDBS + Bu4NBr micellar system indirectly in the beginning, causing an increase of CP followed by a direct interaction with micelle (due to S-atom) causing removal of water from the micellar headgroup region, in addition to that of possible charge neutralization. However, this explanation needs an independent verification. Figure 5 shows the variation of CP of 50-mM SDBS + 35-mM Bu4NBr with the addition of amino acids. Surprisingly, amino acids divide themselves into two categories: glycine and alanine produce decreasing effects, while leucine and phenylalanine increase the CP. All R - amino acids have similar functional groups with different side chains. In one of their micellar mediated chemical reactions, Kabir-ud-Din et al.45 have shown that the reaction rate increases with an increase in the hydrophobicity of the side chain. The effect of amino acids on the CP may be explained taking cognizance of polarity and hydrophobicity. Glycine (and alanine to some extent) prefer a polar environment, and thus they would partition in the headgroup region. This partitioning would replace the amount of water near the headgroup region with a lower temperature to obtain the CP phenomenon. This indeed was observed (Figure 5). On the contrary, hydrophobic nonpolar amino acids would prefer the micelle interior and in doing so they could compete with Bu4N+ for the micellar surface. A steeper rise in the CP in the (45) Kabir-ud-Din; Salem, J. K. J.; Kumar, S.; Khan, Z. Colloids Surf. A 2000, 168, 241.
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Figure 6. Cloud point variation in the system 50-mM SDBS + 35-mM Bu4NBr with the concentration of sugars: Y, xylose; b, arabinose; O, dextrose.
presence of leucine or phenylalanine is expected and this is what we observe in Figure 5. The effect of adding sugars on the CP variation of 50mM SDBS + 35-mM Bu4NBr system is shown in Figure 6. The addition of each of the compounds of this category produces a decrease in the CP. These observations are similar in form to the decrease in the water solubility of hydrophobic derivatives caused by sugars and reinforce the belief that ‘water structure makers’ strengthen the hydrophobic interactions.46 The CP depression indicates a ‘salting out’ effect because the temperature range in which the single phase solutions prevail is reduced. In conclusion, one can say that there exists a certain surfactant - quaternary salt ratio for the appearance of CP in anionic surfactant solutions. The ratio is found to be dependent on the nature of the surfactant headgroup. One can tune the CP to a desired direction with appropriate selection of additives. Cloud point variation through additives (especially for ionic surfactants) presently lacks predictive power, so we believe that it is important to establish a good CP- database of the systems undergoing phase separation for their desired mode of applications (e.g., CP extraction methodologies). This paper could prove a step forward to break the boundaries between ionic and nonionic surfactants with regard to CP observance. One could also widen spectrum of the cloud point extraction methods by including the above interesting systems. Acknowledgment. This work was performed under collaborative research scheme, No. CRS-M-72, of the InterUniversity Consortium for the Department of Atomic Energy Facilities, India. LA011343T (46) Lakshmi, T. S.; Nandi, P. K. J. Phys. Chem. 1976, 80, 240.
