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Micellar Growth in Presence of Alcohols and Amines: A Viscometric Study Kabir-ud-Din,*,† Sanjeev Kumar,† Kirti,† and P. S. Goyal‡ Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India, and Solid State Physics Division, Bhabha Atomic Research Centre, Bombay 400085, India Received August 10, 1995. In Final Form: November 21, 1995X Viscosity measurements have been made to study the effect of alcohols (C3-C8OH) and amines (C4C8NH2) on the micellar growth of 0.2 M cetylpyridinium chloride in the presence and absence of 0.1 M potassium chloride (KCl). The studies have been performed in aqueous solutions between 25 and 40 °C at intervals of 5 deg. The presence of 0.1 M KCl or organic additive of lower chain length (C3OH, C4OH, or C4NH2) singly or jointly has little effect on the viscosity of micellar solutions. As the chain length of the additive increases, the viscosity increases with the increase of additive concentration, the magnitude being substantial in presence of 0.1 M KCl. However, for equal chain lengths, the effect was greater for n-alcohols. In few cases, viscosity shows a peaked behavior when the concentration of the additive was increased beyond the maximum viscosity value. Increased effectiveness of additives in the presence of added salt was discussed in light of electrostatic and hydrophobic forces operating in the solution which are always responsible for growth processes. Changes of the effective Mitchell-Ninham parameter of the surfactant in the presence of various additives were related to viscosity behavior of the solution. Temperature dependence of the viscosity was used to compute the free energy of activation, ∆G*, for the viscous flow.
Introduction The roughly spherical ionic micelle formed at the cmc can grow on reduction of inter-headgroup repulsion.1 Generally, this is achieved by the appropriate conditions of concentration, salinity, temperature, presence of counterions, etc.2,3 Recently, we have shown that not only inorganic salts but a few organic compounds such as n-alcohols,4 n-amines,5,6 and aromatic hydrocarbons7 are also potential candidates for such structural changes. This additive-induced growth was discussed in light of additive solubilization/intercalation. Due to solubilization properties, micellar systems have several applications; thus a careful investigation of such properties is of paramount interest. The site of solubilization of different compounds within micellar systems can be correlated with the structural organization of aggregates. Solution viscosity responds to morphological changes of aggregates and their mutual interactions. Micellar growth is accompanied by a distinct rise in viscosity6 which can be connected to anisotropic susceptibilities.8 The motivation for the study came from the fact that long chain quaternary ammonium salts with sodium salicylate (NaSal) show the phenomenon of viscosity rise * To whom correspondence should be addressed. Fax: 91-0571400076. Phone: 91-0571-400515. † Department of Chemistry, AMU. ‡ Solid State Physics Division, BARC. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Missel, P. J.; Mazer, N. A.; Carey, M. C.; Benedek, G. B. In Solution Behaviour of Surfactants; Mittal, K. L., Fendler, E. J.; Eds.; Plenum Press: New York, 1982; Vol. 2, p 373. (2) Candau, S. J.; Hirsch, E.; Zana, R.; Delsanti, M. Langmuir 1989, 5, 1225 and references therein. (3) Porte, G.; Appell, J. Europhys. Lett. 1990, 12, 190. (4) Kumar, S.; Kirti; Kumari, K.; Kabir-ud-Din J. Am. Oil Chem. Soc. 1995, 72, 817. (5) Kumar, S.; Kirti; Kabir-ud-Din J. Am. Oil Chem. Soc. 1994, 71, 763. (6) Kumar, S.; Aswal, V. K.; Singh, H. N.; Goyal, P. S.; Kabir-ud-Din Langmuir 1994, 10, 4069. (7) Kabir-ud-Din; David, S. L.; Kumar, S. Langmuir, submitted for publication, 1995. (8) Khatory, A.; Lequeux, F.; Kern, F.; Candau, S. J. Langmuir 1993, 9, 1456.
at very low ionic strengths9,10 and then viscosity drops off drastically. Another viscosity maximum was observed at further high concentrations of the NaSal. Many contradictory interpretations were invoked to explain the results.11 The most plausible one seems to be due to the dual role of NaSal as an inorganic salt and an organic molecule. The former depresses the electrostatic repulsions between the aggregates while the latter increases the effective hydrophobic volume of the surfactant monomer. Both of these effects are favorable to increase the Mitchell-Ninham parameter (Rp)12 which could be responsible for the formation of long cylindrical micelles as evident from a large increase in viscosity at low concentrations of NaSal. In an attempt to show that the presence of salt and certain organic molecules jointly can behave like a surfactant (cationic)-NaSal (at low concentration) system, we have selected cetylpyridinium chloride (CPC) as the surfactant, KCl as the salt, and n-alcohols/n-amines as the organic molecule. The effect of chain length of organic molecules and temperature on the magnitude of relative viscosity (ηr) was also observed. KCl (as salt) was chosen because it was reported that the micellar size remains constant over a wide range of concentration and its addition only screens out Coulombic forces between micelles.13 The study aims to show that growth efficiency of an organic molecule could be enhanced by the presence of a salt which itself has no effect on the micellar growth. In such a way, the dependence of micellar growth on high ionic strength could be replaced by organic molecules if their presence is tolerable. In this manner a wide spectrum of such systems would be available for reaction media or other application modes14 where high salt presence is undesirable. (9) Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. J. Chem. Soc., Chem. Commun. 1986, 379. (10) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (11) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081; 1988, 4, 354; 1989, 5, 398. (12) Mitchell, D.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (13) Goyal, P. S.; Menon, S. V. G.; Dasannacharya, B. A.; Rajagopalan, V. Chem. Phys. Lett. 1993, 211, 559. (14) Sein, A.; Engberts, J. B. F. N.; Linden, E. van der; Pas, J. C. van de. Langmuir 1993, 9, 1714.
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Figure 1. Relative viscosities (ηr) of 0.2 M CPC micellar solutions as a function of added n-alcohols at 25.0 °C. The chain lengths of added n-propyl to n-octyl alcohols are indicated as C3 to C8. Lines joining the experimental points are guides for the eyes.
Langmuir, Vol. 12, No. 6, 1996 1491
Figure 2. Logarithms of relative viscosities of 0.2 M CPC + 0.1 M KCl solutions as a function of added n-alcohols at 25.0 °C.
Experimental Section CPC (Sigma, St. Louis, MO) was recrystallized twice from an ethanol-ethyl acetate mixture and dried at 60 °C under moderate vacuum. The purity of CPC was ensured by the absence of a minimum in a plot of surface tension vs logarithm of concentration. KCl (purity >99%) was from E. Merck (India) while all alcohols (1-propanol, C3OH; 1-butanol, C4OH; 1-pentanol, C5OH; 1-hexanol, C6OH; 1-heptanol, C7OH; 1-octanol, C8OH) were BDH high-purity chemicals and were used as supplied. The amines (n-butylamine, C4NH2; n-hexylamine, C6NH2; n-heptylamine, C7NH2; n-octylamine, C8NH2, all “purum grade”) were obtained from Fluka (Buchs, Switzerland). Water was distilled twice over alkaline KMnO4 in an all glass still. The viscosity measurements were carried out by using a Ubbelohde viscometer thermostated at fixed temperatures (25, 30, 35, or 40 °C; accuracy, (0.1 °C). The method of measurements of viscosities under Newtonian flow conditions was the same as that described in the literature.15 The solvent flow time in the viscometer was always longer than 200 s, and no kinematic corrections were introduced.16
Results and Discussion Typical plots of the variation of relative viscosities, ηr ) η/η0 (η and η0 are viscosities of solution and solvent water, respectively), of 0.2 M CPC micellar solutions with added alcohols and amines with or without 0.1 M KCl at 25 °C are shown in Figures 1-4. It is clear from these figures that lower chain length additives (e.g., C3OH, C4OH, or C4NH2) show marginal effect on viscosity changes which remain nearly the same even in the presence of 0.1 M KCl. These additives are mainly hydrophilic molecules with excellent solubilities in water and very little in micelles. They will not affect micellar structure appreciably, and hence, no substantial change occurs in the viscosity of 0.2 M CPC with or without added 0.1 M KCl. The shape of the CPC micelle remains almost unchanged, i.e., spherical. There are at least two opposing factors responsible for the micellar growth process. One is the electrostatic repulsion term originating from intermicellar and intramicellar Coulombic interactions which favors micelles with (15) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, 566. (16) Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1980, 77, 219.
Figure 3. Relative viscosities (ηr) of 0.2 M CPC micellar solutions as a function of added n-amines at 25.0 °C. The chain lengths of added n-butyl- to n-octylamines are indicated as C4 to C8.
a high surface area per headgroup, i.e., spherical micelles. The other is due to the hydrophobic interactions between the hydrocarbon part of the micelles/monomers which tries to achieve aggregates with tightly packed chains, i.e., rods or disks. Mukerjee17 had proposed that an additive which is surface active to a hydrocarbon-water interface will be mainly solubilized at the micellar surface and will promote micellar growth. The higher chain length additives (e.g., C8OH, C7OH, or C8NH2) have a strong chance to get embedded between monomers comprising a micelle. This penetration of a surfactant-rich film by these additives helps to overcome headgroup repulsion by holding these molecules in between headgroups of similar charge and is, therefore, responsible for the decrease in surface area occupied per surfactant headgroup (A0). Consequently, the MitchellNinham parameter Rp ) Vc/lcA0 (Vc being the volume of hydrophobic portion of the surfactant monomer, and lc its length)12 increases. An increase in this parameter could (17) Mukerjee, D. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; p 153.
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Kabir-ud-Din et al. Table 1. Relative Viscosities, ηr, of 0.2 M CPC + n-Alcohols with (Sp) or without (Sa) Added 0.1 M KCl at Different Temperatures alcohol concn (M) 0.00 C3OH 0.50 1.00 1.50 2.00 3.05 4.00 C4OH 0.42 0.50 0.83 1.00 1.50 1.66 2.50
Figure 4. Logarithms of relative viscosities of 0.2 M CPC + 0.1 M KCl solutions as a function of added n-amines at 25.0 °C.
be understood by considering the CPC-higher chain length additive couple as a single surfactant. The penetration of additive will result in increasing the volume of the micellar core, which is equivalent to increasing the Vc.18 This seems to result in an increase of the Rp value. Thus, CPC-higher chain length additives should have a tendency to form large micelles, and it seems to do so as reflected by the viscosity rise on addition of higher alcohols and amines to 0.2 M CPC micellar solutions (Tables 1 and 2). Further, the increase in Rp would be greater with C8OH or C8NH2 than with C5OH or C6NH2. This is due to the larger hydrophobic volume of C8 which would increase Rp more with a concomitant formation of larger micelles and have a higher viscosity of the solution. The viscosity plots (Figures 1-4) are consonant with the explanation. Hartel and Hoffmann19 used such arguments to design lyotropic nematics. It is worth recalling that both the anionic SDS and cationic CTAB micellar solutions with or without added electrolyte show similar behavior with C6OH concentration in the whole range investigated.4,20-22 It is interesting to note that the presence of both 0.1 M KCl and additives (above a certain chain length: alcohols, C5-C8OH; amines, C6-C8NH2) imparts very high viscosity to 0.2 M CPC micellar solutions in comparison to the situation where 0.1 M KCl or various additives were present singly. This is not the end of the story; another point of interest is that, in the presence of 0.1 M KCl, the viscosity of the 0.2 M CPC micellar solution increases with the increase in additive concentration (similar to the case without KCl), reaches a maximum, and then decreases (this behavior was obtained with C5OH, C6OH, and C7NH2 only). In all probability, the other alcohols and amines, viz. C7OH, C8OH, and C8NH2, would show the same phenomenon at higher additive concentrations, but studies were hampered as turbidity appeared at concentrations beyond that shown in Tables 1 and 2. These two effects show that a special phenomenon exists when (18) Lin, Z.; Cai, J. J.; Scriven, L. D.; Davis, H. T. J. Phys. Chem. 1994, 98, 5984. (19) Hartel, G.; Hoffmann, H. Liq. Cryst. 1989, 5, 1983. (20) Prasad, Ch. D.; Singh, H. N. Colloids Surf. 1990, 50, 37. (21) Backlund, S.; Bakken, J.; Blokhus, A. M.; Hoeiland, H.; Vikholm, I. Acta Chem. Scand. 1986, A40, 241. (22) Tominaga, T.; Stem, T. B.; Evans, D. F. Bull. Chem. Soc. Jpn. 1980, 53, 795.
C5OH 0.10 0.14 0.30 0.42 0.50 0.56 0.70 0.76 1.00 1.52 C6OH 0.06 0.10 0.12 0.15 0.20 0.25 0.27 0.30 C7OH 0.03 0.05 0.09 0.10 0.12 0.15 0.17 C8OH 0.05 0.07 0.08 0.09 0.11 0.12 0.13
ηr in H2O Sa
25 °C Sp
Sa
30 °C Sp
Sa
35 °C Sp
Sa
40 °C Sp
1.65
1.40
1.63
1.34 1.60
1.33 1.59
1.27
1.83
1.58 1.88
1.81
1.56 1.80 1.85 1.81 2.23 2.25 2.79 2.96
1.56 1.78 1.85 1.79 2.20 2.19 2.77 2.61
1.56 1.81
1.66
1.65
1.64
1.89 2.48
2.34 2.82
3.05
1.83 2.31 2.83
1.67 1.88
1.82 1.94
2.05 2.19 2.67
2.58 3.26
1.78 1.85 2.24 3.70 4.82 5.55
1.93
1.91 2.29 2.52
2.56
2.42 3.21 2.53
2.40 3.15 2.45
1.71 1.58 3.16 1.80 4.91 2.13 5.82 5.60 5.60 3.38 4.86 4.28 5.03
1.68 1.55 2.55 1.77 4.35 2.07 5.29 5.22 5.19 3.15 4.72 4.05 4.82
1.44
1.44
1.82
3.50 4.65 5.38
5.80
1.84 2.08 4.92 12.04
1.75 4.85 14.54 48.43 99.12 85.47 68.30 1.43 1.63 4.31 20.14 65.06 695.17 turbid
1.89 1.97 2.14
1.45 1.76
1.91 2.01 2.16
2.16 6.66 6.34 6.12 5.23
1.79
2.03 2.18
1.72 1.63 3.90 5.89
1.81
1.85 2.16 2.25 4.69
1.73 3.59 8.63 25.30 64.56 60.26 51.80
1.42 1.55 3.27 1.96 10.68 28.08 3.74 314.98 7.63 419.11 1.78
1.76 1.93 2.03 3.72
2.31 3.07
1.36 2.30 3.63 4.82 5.08 5.00 4.43
1.43 1.71
2.88 5.73 14.53 41.28 41.75 42.66
1.41 1.54 2.67 1.90 6.93 14.37 3.08 133.79 5.47 282.34 1.76
2.17 2.74
1.75 1.91 2.02 3.25
2.41 3.89 8.57 23.78 26.86 28.87
1.40 1.53 2.19 1.83 4.86 8.12 2.62 56.21 3.90 129.90 1.74
1.85 1.91
1.72 1.81 1.63 1.79 1.61 1.76 1.53 5.51 1.86 3.99 1.84 3.25 1.81 2.74 17.46 10.18 6.75 4.81 2.60 78.21 2.29 35.32 2.12 18.53 1.91 10.65 turbid 356.58 139.50 84.21 29.72 787.82 360.35 138.98 54.19 1401.00 840.00 369.66 155.08
0.1 M KCl and additives are present in the same system. The manifold increase in viscosity is the result of variation of different forces favorable for micellar growth. Addition of KCl to the CPC solution weakens the Coulombic repulsion between the micelles, and intercalation of higher chain length additives decreases intramicellar Coulombic repulsive forces and increases hydrophobic forces among the monomers of the CPC micelle. As mentioned earlier, the decrease of Coulombic repulsion and/or increase in hydrophobic interactions are favorable conditions for micellar growth (either one of which exists if 0.1 M KCl or organic additive is present singly). The evidence of the growth process due to the combined effect is provided in Figure 5 where ln ηr vs concentration of the additives
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Langmuir, Vol. 12, No. 6, 1996 1493 Table 3. Values of Activation Free Energies, ∆G*, for the Viscous Flow of 0.2 M CPC Solutions with (Sp) and without (Sa) 0.1 M KCl in Presence of Added n-Alcohols/n-Amines alcohol concn (M) 0.00 C3OH 0.50 1.00 1.50 2.00 3.05 4.00 C4OH 0.42 0.50 0.83 1.00 1.50 1.66 2.50
Figure 5. Viscosity variation of 0.2 M CPC micellar solutions in the presence (b, 9) and absence (O, 0) of 0.1 M KCl with equal chain length additives (C7OH and C7NH2) at 25.0 °C. Table 2. Relative Viscosities, ηr, of 0.2 M CPC + n-Amines with (Sp) or without (Sa) Added 0.1 M KCl at Different Temperatures ηr in H2O
amine concn (M)
25 °C Sp Sa
30 °C Sa Sp
35 °C Sa Sp
40 °C Sa Sp
0.00
1.65 1.40
1.63 1.34
1.60 1.33
1.59 1.27
C4NH2 0.50 1.00 1.50 2.00 C6NH2 0.20 0.28 0.50 1.00 1.50 1.70 2.75
1.99 2.04 2.08 2.10
1.89 1.99 2.00 2.08
1.96 2.00 2.05 2.09
1.86 1.91 1.92 2.00
1.92 1.98 1.99 2.06
1.86 1.89 1.90 1.98
1.90 1.99 1.93 2.03
1.83 1.88 1.89 1.96
1.73
1.66
1.64
1.61
1.78 2.06 3.30 4.59 5.10 5.09 5.27 5.87 5.87
1.74 2.00 3.27 4.37 4.94 4.72 5.20 4.88 5.67
1.73 1.95 3.25 4.26 4.78 4.63 4.99 4.78 5.46
1.67 1.89 3.20 4.01 4.70 4.31 4.78 4.42 5.32
C7NH2 0.20 2.01 0.30 2.56 0.40 7.27 0.45 0.48 0.50 13.98
3.20 11.77 20.17 20.67 18.79 17.43
1.87 2.86 2.37 9.92 6.55 17.34 17.83 16.89 9.35 16.66
1.81 2.54 2.21 8.25 5.74 15.18 16.04 15.65 8.18 15.07
C8NH2 0.10 2.03 3.32 1.92 2.71 1.85 0.15 10.64 7.75 0.18 44.64 26.97 0.20 2.37 70.60 2.26 42.50 2.11 0.25 6.86 158.16 5.02 105.23 4.32 0.30 17.28 632.70 12.36 265.07 8.67 0.33 21.92 turbid 15.44 turbid 10.96
2.37 6.07 18.42 28.90 72.66 125.21 turbid
1.79 2.40 2.14 7.03 5.00 13.39 14.67 14.58 7.35 14.76 1.76 2.15 4.65 13.85 2.01 18.84 3.39 49.59 6.97 67.10 8.55 turbid
with the heptyl chain is shown for 0.2 M CPC with and without 0.1 M KCl. The observed viscosity decrease at higher concentrations with the aforementioned additives is possibly due to the fact that they may be salted-out by the added 0.1 M KCl and start dissolving in the micellar core rather than remaining in the vicinity of interfacial region; therefore the requirement of the surfactant chains to reach the
C5OH 0.10 0.14 0.30 0.42 0.50 0.56 0.70 0.76 1.00 1.52 C6OH 0.06 0.10 0.12 0.15 0.20 0.25 0.27 0.30 C7OH 0.03 0.05 0.09 0.10 0.12 0.15 0.17 C8OH 0.05 0.07 0.08 0.09 0.11 0.13
∆G* (kJ mol-1) Sa Sp 2.05
4.94
1.25
0.90 1.80
2.65 6.16
amine concn (M)
∆G* (kJ mol-1) Sa Sp
0.00
2.05
4.94
C4NH2 0.50 1.00 1.50 2.00
2.66 2.42 3.94 2.33
1.66 3.45 2.96 2.56
4.98 6.94
8.00 13.84 2.07 1.24 1.93 1.24 4.03
5.44 2.99
2.68 1.39
3.21 31.00 25.31
3.87 7.84 9.33 7.59
13.54 11.07 12.72 7.73
3.53 1.49 4.81 9.95 9.93 30.39
2.67 6.39 32.28 59.07 2.50 2.44 15.58
36.11 66.44 143.96 65.00 69.77 44.29 5.19 10.38 32.41 72.93 109.16 129.65 106.03 6.64 33.90 68.74 106.56 127.78 110.74
C6NH2 0.20 0.28 0.50 1.00 1.50 1.70 2.75 C7NH2 0.20 0.30 0.40 0.45 0.48 0.50 C8NH2 0.10 0.15 0.18 0.20 0.25 0.30 0.33
3.603 4.347 6.685 7.987 7.775
5.890 9.38 19.38 31.978
7.25 7.66 34.91 47.63 34.66
2.59 1.47 4.03 13.59 33.22
15.25 26.58 21.31 17.99 13.29 9.49
21.78 43.44 61.19 68.39 59.80 119.17
center of the micelle becomes relaxed.23 It is, thus, possible that at high additive contents the larger micelles disintegrate to smaller ones and form the basis of the viscosity decrease (Figures 2 and 4). The solubility and packing constraints do not allow C7OH, C8OH, or C8NH2 to do so as there is not a sufficient amount of these additives available to form the core of the micelle (solubilized system). Figure 5 also shows that for equal chain lengths (C7OH and C7NH2) the viscosity rise is more with alcohols than with amines. It was reported earleir that C4-C10 n(23) Lindemuth, P. M.; Bertrand, G. L. J. Phys. Chem. 1993, 97, 7769.
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Figure 6. Typical plots showing the variation of ln ηr with 1/T for 0.2 M CPC aqueous solutions in the presence of C7OH. The concentration of C7OH is shown in parentheses beside each curve.
alkylamines are solubilized in SDS and CTAB micelles by electrostatic and hydrophobic effects with the amine group left on the surface of the micelle.24 Their partial dissociation into -NH3+ and OH- (though feebly) may affect electrostatic interactions with cationic pyridinium headgroup, which will hinder the micellar growth. Therefore, for equal chain lengths and concentrations of alcohols and amines, alcohols will be more effective for cationic micellar growth. This indeed is observed in the present viscosity results (Figure 5, Tables 1 and 2). The effect of temperature on the viscosity values showed an Arrhenian behavior. The variation of ln ηr with 1/T was used to calculate activation free energy, ∆G*, for the viscous flow (Table 3), as was done earlier.4,5 Typical plots are shown for C7OH (Figure 6); similar types of plots were obtained for all other alcohols and amines. We have (24) Yamashita, T.; Yano, H.; Harada, S.; Yasunaga, T. J. Phys. Chem. 1983, 87, 5482.
Kabir-ud-Din et al.
shown6 that ∆G* could be linked to the difference in curvature elasticity of the spherical end-caps and the cylindrical part of the micelles. A higher value of ∆G* implies that very high energy is required to convert cylindrical micelles to small micelles which indicates that the lifetime of cylindrical micelles is increased with an increase of additive concentration. For equal chain length alcohols and amines, higher ∆G* has been found with the former (Table 3) which shows that long cylindrical micelles with higher lifetimes are formed with added alcohols. This confirms our earlier proposition that micellar growth is slightly hindered by the presence of amine molecules. Thus ∆G* values could be used as a measure of the lifetime in micellar structures.25,26 With the above discussion we can conclude that in micellar growth the presence of a salt and an organic additive in the system produced favorable conditions which do not exist in presence of either the salt or the additive alone. The electrostatic effect produced by additives at the micellar surface is a governing factor in addition to the hydrophobic part of the additives. In case of additives of equal chain length, its efficacy of decreasing A0 decides the potential of the additive for structural growth. Thus, we are able to address the phenomenon shown by the CTAB/NaSal system which is mainly the consequence of the dual role played by NaSal (i.e., as a salt and an organic moiety). If conditions are favorable, other systems can also show the same behavior to some extent. Here we have shown how organic molecules can be used to study such theoretically important phenomenon and also their potential for viscosity enhancement which is desirable for various industrial applications/reaction media where lower ionic strength is required. Acknowledgment. The authors thank the Chairman, Department of Chemistry, for providing research facilities. The Inter-University Consortium for Department of Atomic Energy Facilities, India, is gratefully acknowledged for a research grant (IUC:(PB-41):94-95/2590). LA950677D (25) Kern, F.; Zana, R.; Candau, S. J. Langmuir 1991, 7, 1344. (26) Makhloufi, R.; Hirsch, E.; Candau, S. J.; Binana-Limbele, W.; Zana, R. J. Phys. Chem. 1989, 93, 8095.
