Solubilization-Site-Dependent Micellar Morphology: Effect of Organic

The effect of tetra-n-butylammonium bromide (R = n-C4H9) concentration was seen to substantiate the change in site of solubilization phenomenon of org...
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Langmuir 2001, 17, 4787-4792

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Solubilization-Site-Dependent Micellar Morphology: Effect of Organic Additives and Quaternary Ammonium Bromides Sanjeev Kumar, Andleeb Z. Naqvi, and Kabir-ud-Din* Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India Received January 30, 2001. In Final Form: May 3, 2001 Here we report systematic viscosity measurements under Newtonian flow conditions for sodium dodecyl sulfate (SDS) micellar solutions in the presence of aliphatic hydrocarbons (n-hexane, n-heptane, n-octane), toluene, and 1-alkanols (1-heptanol, 1-octanol) at 30 °C. The influence of 0.1 M quaternary ammonium bromides (R4NBr; R ) H, CH3, C2H5, n-C3H7, n-C4H9, n-C5H11) was seen on relative viscosity (ηr) vs additive concentration profiles. Lower members of R4NBr (R ) H, CH3, C2H5) have marginal influence on viscosity profiles and hence on micellar morphology. This may be due to the less pronounced effect on association structures as well as on solubilization sites (and on solubilizate content at a particular site) of added organics. As the alkyl part of R4NBr was increased (R g n-C3H7), the effect, which depended upon the nature of the additive (i.e., its hydrophobic/polarity character), was significant. This is possibly due to salts affecting the partitioning content of organic additives at different micellar solubilization sites with concomitant changes in micellar morphology, as well as viscosity of the micellar solutions. The effect of tetra-n-butylammonium bromide (R ) n-C4H9) concentration was seen to substantiate the change in site of solubilization phenomenon of organic additives. These studies show that, with an appropriate R4NBr concentration present in combination with a surfactant, change of solubilization site even for nonpolar compounds (like hydrocarbons) is possible.

Introduction Micellar solutions have a general tendency to solubilize a certain amount of organic additives (hydrophobic or partly hydrophobic).1,2 The emerging picture is that molecules with polar groups are mainly solubilized near the surface of the micelle3 with the polar group at the surface and that aliphatic hydrocarbons4,5 are preferentially solubilized in the interior of micelles. Many experimental efforts have been made to determine the location of a solubilizate within a micelle or related assemblies.6-9 The location, distribution, and orientation of solubilized species in micelles are of fundamental importance in understanding the nature of solubilization and its consequences on the chemical and physical behavior of solutions.10,11 The locale of different additives in or around micelles can be correlated with micellar morphology. There are many factors, including the nature and the concentration of the additive(s), that determine the shape of the micelle. These additives can be used to tune different intra- and intermicellar forces and the effective packing parameter.12,13 * Author for correspondence. (1) Lindman, B.; Wennerstrom, H. Top. Curr. Chem. 1980, 87, 1. (2) Elworthy, P. H.; Florence, A. T.; Macfarlane, C. B. Solubilization by Surface-Active Agents; Chapman and Hall Ltd: London, 1968. (3) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1. (4) Reekmans, S.; Luo, H.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1990, 6, 628. (5) Lindemuth, P. M.; Bertrand, G. L. J. Phys. Chem. 1993, 97, 7769. (6) Cerichelli, G.; Mancini, G. Langmuir 2000, 16, 182. (7) Kabir-ud-Din; Kumar, S.; Aswal, V. K.; Goyal, P. S. J. Chem. Soc., Faraday Trans. 1996, 92, 2413. (8) Teixeira, C. V.; Itri, R.; Amaral, L. Q. Langmuir 2000, 16, 6102. (9) Yue, Y.; Wang, J.; Dai, M. Langmuir 2000, 16, 6114. (10) (a) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 1620, (b) Mukerjee, P.; Cardinal, J. R.; Desai, N. R. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 1, p 241. (11) Nagarajan, R.; Chaiko, M. A.; Ruckenstein, E. J. Phys. Chem. 1984, 88, 2916.

Surfactant solutions containing spherical micelles are isotropic and of low viscosity.14 The presence of reasonably long wormlike micelles in the solution causes increased viscosity due to mutual interactions (entanglement).15,16 It has been proposed that interfacial partitioning of organic additives causes micellar growth while interior solubilization produces swollen micelles.17-19 These two types of micelles impart different viscosity behavior to solutions. It was reported earlier that the viscosity increased with the increase of additive concentration and magnitude of viscosity was substantial when organic additives and inorganic salts were added simultaneously.17,18,20 It is well-known that addition of inorganic salts decreases both the electrostatic interactions between micelles and the partitioning of organic additives between micelles and bulk solvent.21 Recently, we have studied the solution behavior of ionic micelles in the simultaneous presence of organic additives and quaternary ammonium bromides (R4NBr).22-24 The R4N+ ions interact both electrostatically and hydrophobically with the micellar surface.25 R4NBr salts exhibit an ambivalent (12) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1991. (13) Mileva, E. J. Colloid Interface Sci. 1996, 178, 10. (14) Kohler, H.-H.; Strnad, J. J. Phys. Chem. 1990, 94, 7628. (15) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, 566. (16) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988, 92, 4712. (17) Kabir-ud-Din; Kumar, S.; Kirti; Goyal, P. S. Langmuir 1996, 12, 1490. (18) Kabir-ud-Din; Bansal, D.; Kumar, S. Langmuir 1997, 13, 5071. (19) Hoffmann, H.; Ebert, G. Angew. Chem., Int. Ed. Eng. 1988, 27, 902. (20) David, S. L.; Kumar, S.; Kabir-ud-Din J. Chem. Eng. Data 1997, 42, 198. (21) Hoiland, H.; Ljosland, E.; Backlund, S. J. Colloid Interface Sci. 1984, 101, 467. (22) Kumar, S.; Bansal, D.; Kabir-ud-Din Langmuir 1999, 15, 4960. (23) Kumar, S.; Naqvi, A. Z.; Kabir-ud-Din Langmuir 2000, 16, 5252. (24) Kumar, S.; Sharma, D.; Kabir-ud-Din Langmuir 2000, 16, 6821. (25) Jansson, M.; Eriksson, L.; Skagerlind, P. Colloids Surf. 1991, 53, 157.

10.1021/la0101550 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/03/2001

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nature in aqueous solutions. In these ions the single positive charge is buried in a paraffin shell. The saltingin effect of these salts is in contrast to the salting-out effect of the inorganic salts.26 In the earlier studies17,18,22,23 we were able to show that the presence of salts in ionic micellar solutions changed the expected solubilization sites of organic additives. In this regard it could be mentioned that the presence of inorganic counterions caused the change of partitioning site of alcohols/amines (surface active additives in Mukerjee’s terminology27) from the interfacial region to the interior of the micelle.17,18 On the other hand, the presence of R4NBr salts has changed the partitioning of aromatic hydrocarbons from the interfacial region to the bulk aqueous phase.22 In all the above studies it was further shown that it is the interfacial partitioning of the organic additive that has the major contribution toward micellar growth and to the resultant viscosity of the solution. Assuming that the variation in interfacial partitioning content (IPC) of the organic additive would influence micellar growth (or viscosity of the micellar solution), we can expect a change in viscosity behavior (and also solubilization site) on addition of hydrocarbons/alcohols to anionic sodium dodecyl sulfate (SDS) micellar solutions in the presence of R4NBr. In the present work we wish to demonstrate that indeed one can bring a change in the solubilization site even of aliphatic hydrocarbons from micellar interior to the interfacial region. Further, viscosity behavior is compared with representatives of alcohol and aromatic hydrocarbon families. This is due to the fact that alcohols/hydrocarbons are the organics most frequently added to the surfactant solutions and any generalization should be verified with the presence of such additives.4,5 Experimental Section Sodium dodecyl sulfate (SDS) and quaternary ammonium bromides (R4NBr) were the same as used earlier.7,23 All the alcohols and hydrocarbons were the highest purity chemicals available and used as received. The R4NBr salts were dried before use in a vacuum-drying oven. Demineralized, double-distilled water was used to prepare sample solutions. The sample solutions were prepared by taking precalculated amounts of alcohols/hydrocarbons by disposable micropipets in standard flasks and making up volumes with the stock solution (0.3 M SDS containing either a fixed concentration of R4NBr or no salt). The samples were left for equilibration (∼24 h). Viscosity measurements were carried out by an Ubbelohde viscometer thermostated at 30 ( 0.1 °C for at least 1 h with the sample solution. To avoid evaporation, the flasks/viscometer were kept properly stoppered and sealed during equilibration. At higher salt/organic concentrations, the viscosities were found to be dependent on the rate of flow. Such viscosity measurements were performed as reported elsewhere.7 No density corrections were made, since these were negligible.28

Results and Discussion The viscosity data for the 0.3 M SDS system in the presence of different R4NBr and organic additives can be used to explain the changes in micellar structure that occur at varying concentrations of additive and R4NBr. In the present context, the alkyl chains of R4NBr (higher members) may get embedded between SDS monomers of the anionic micelle due to the hydrophobic effect. But the geometric constraints make it difficult, the result being (26) Burns, J. A. In Thermodynamic Behaviour of Electrolytes in Mixed Solvents; Furter, W. F., Ed.; Adv. Chem. Series 155; American Chemical Society: Washington, DC, 1976. (27) Mukerjee, P. In Solution Chemistry of Surfactants; Mittal, K. L. Ed.; Plenum Press: New York, 1979; Vol. 1, p 153. (28) Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1980, 77, 219.

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Figure 1. Relative viscosities (ηr) of 0.3 M SDS + 0.1 M R4NBr micellar solutions as a function of added n-heptane (upto the solubility limit indicated by dotted lines) at 30 °C: R ) H (O), CH3 (4), C2H5 (0), n-C3H7 (y), n-C4H9 (b), n-C5H11 (Q); K stands for no added salt.

that two directions may be chosen for bending: one is toward the water phase and the other penetrating toward the micellar interior.29 The alkyl chains pointing toward bulk water may produce a temporary hydrophobic region around micellar surface.23 There may exist a number of solubilization sites for various additives as one moves toward the micellar interior from the surface region. The purpose of the present work was to see whether one can change the established solubilization site (vide supra) of an organic additive by the addition of R4NBr and, if so, can viscometry be used to study such site changes and the resultant micellar morphologies. Figure 1 shows the relative viscosity (ηr) variation of 0.3 M SDS solutions (with or without 0.1 M R4NBr) upon addition of n-heptane at 30 °C. The viscosity remains almost constant when n-heptane was added gradually with or without 0.1 M R4NBr (upto R ) n-C3H7). This result indicates that very little (or no change) occurs in the micellar association structure irrespective of 0.1 M salt (upto R ) n-C3H7)/n-heptane combinations either added individually or simultaneously. This is in agreement with the earlier findings obtained for similar systems.5,30,31 The initial viscosity of 0.3 M SDS + 0.1 M R4NBr (where R ) n-C4H9 or n-C5H11) becomes comparatively very high. The reason being the micellar growth that takes place with (29) Almgren, M.; Swarup, S. J. Phys. Chem. 1983, 87, 876. (30) Smith, M. B.; Alexander, A. E. Proceedings of the 2nd International Congress of Surface Activity; Butterworth: London, 1957; Vol. 1, p 311. (31) Eriksson, J. C.; Gillberg, G. Acta Chem. Scand. 1966, 20, 2019.

Solubilization-Site-Dependent Micellar Morphology

Figure 2. Relative viscosities (ηr) of 0.3 M SDS + 0.1 M R4NBr micellar solutions as a function of added toluene (upto the solubility limit indicated by dotted lines) at 30 °C: R ) H (O), CH3 (4), C2H5 (0), n-C3H7 (y), n-C4H9 (b), n-C5H11 (Q); K stands for no added salt.

such salts as they possess large hydrophobic volume and can increase the surfactant packing parameter.12 A detailed discussion on this topic has appeared elsewhere.32 Here also the addition of n-heptane shows a sharp decrease in ηr followed by a rather constancy in plots of ηr vs n-heptane concentration. This behavior demonstrates that initially present cylindrical micelles change their shape to spherical ones, which is similar to behavior observed by Hoffmann and Ebert.19 For micelles to maintain spherical (or globular shape), some of the surfactant monomeric tails should be able to reach the micellar center. Aliphatic hydrocarbons, generally thought to solubilize in the micellar interior, relax the above precondition for a spherical micelle. Now the micelle can remain spherical with the radius, which was earlier prohibitive. In this manner one can understand the viscosity trends observed in the presence of the above salts. It is noticeable, however, that even the presence of higher R4NBr salts does not, in principle, change the ηr behavior with n-heptane addition. Maybe the concentration of R4NBr is not sufficient to bring about a change in the solubilization site. To check this point, we increased the R4NBr (R ) n-C4H9) concentration and studied further the effect of n-heptane addition. This point will be discussed a little later (vide infra). Figure 2 shows the plots of ηr vs toluene concentration with different R4NBr salts. Perusal of the data shown in Figures 1 and 2 makes it clear that lower R4NBr salts behave similarly with toluene as with n-heptane. However, the ηr patterns change with R g n-C3H7, which fairly indicate that the sites of solubilization are changing as the chain length (R) of the salt increases. The toluene could be considered slightly more polar than n-heptane and would have slightly less objection to go toward the interface. Also, the presence of the propyl chains of R4NBr salt would reduce the polarity near the interfacial region. These two factors would increase the IPC of toluene and would produce micellar growth, as indeed observed (32) Kumar, S.; Aswal, V. K.; Goyal, P. S.; Kabir-ud-Din J. Chem. Soc., Faraday Trans. 1998, 94, 761.

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Figure 3. Relative viscosities (ηr) of 0.3 M SDS + 0.1 M R4NBr micellar solutions as a function of added 1-heptanol (upto the solubility limit indicated by dotted lines) at 30 °C: R ) H (O), CH3 (4), C2H5 (0), n-C3H7 (y), n-C4H9 (b), n-C5H11 (Q); K stands for no added salt.

in Figure 2, which supports earlier propositions.23,33 This effect is more pronounced with R ) n-C4H9 salt. In light of the above discussion, one can expect that with this salt the polarity of the interfacial region would further reduce, due to the larger alkyl part with the concomitant increase in its IPC. This IPC increase gives a steep rise in the ηr-[toluene] plot, which is due to the distinct micellar growth with this salt (R ) n-C4H9)/toluene combination. The low solubility of (n-C5H11)4NBr precludes collection of ηr data at the experimental temperature. However, quite high ηr values indicate that micelles are large, which is obvious due to the higher hydrophobic volume of the salt. Figure 3 illustrates the variation of ηr with 1-heptanol concentration with or without 0.1 M R4NBr. Contrary to n-heptane or toluene, 1-heptanol shows a viscosity rise right from beginning. This is because of the different solubilization region (i.e. interfacial site) in the micelle for 1-heptanol. It was reported by SANS34 studies that the volume of SDS micelle increases with the increase of longer chain alcohols, which is due to the increase in aggregation number as well as the increase in alcohol molecules in the micelle. Such a volume change in the SDS micelle is expected here also and explains the viscosity rise in the systems shown in Figure 3. A detailed discussion on the variation of the packing parameter of the surfactant on addition of longer alcohols can be found elsewhere.17,18 It is interesting to see that with tetra-n-butylammonium bromide (R ) n-C4H9) the figure shows a peculiar behavior of viscosity increase, followed by a level off, and thereafter a slow decrease. As discussed earlier, few alkyl chains of this salt and the alkyl chain of 1-heptanol would partition at a similar solubilization site and would be quickly saturated as 1-heptanol content is increased. At higher 1-heptanol content, the solubilization of additional 1-heptanol will either be in the micellar interior or in the exterior (33) Kandori, K.; McGreevy, R. J.; Schechter, R. S. J. Phys. Chem. 1989, 93, 1506. (34) Caponetti, E.; Martino, D. C.; Floriano, M. A.; Triolo, R. Langmuir 1997, 13, 3277.

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Figure 4. Relative viscosities (ηr) of 0.3 M SDS micellar solutions as a function of added n-hexane (upto the solubility limit indicated by dotted lines) at various fixed concentrations (x) of (n-C4H9)4NBr at 30 °C: x ) 0.0 (K), 0.10 (b), 0.15 (O), 0.20 (y), and 0.30 M (X).

of the micelle. Figure 3 shows a constancy in viscosity with this salt (R ) n-C4H9) at fairly higher 1-heptanol content. Due to the presence of the -OH group, 1-heptanol would prefer a surface region35 and its core solubilization can be safely ruled out. As the interfacial region is saturated with the alkyl chains of R4NBr (R ) n-C4H9) and a part of 1-heptanol content, additional 1-heptanol could be solubilized in the exterior of the micelles, where the remaining chains of the tetraalkylammonium salt (vide supra) exist, producing another temporary hydrophobic region around the micelle. Now, this 1-heptanol partitioning in the micellar exterior will not affect the micellar growth (due to elongation) and the viscosity, therefore, will remain constant. Similar reasoning was put forth earlier to explain the effect of similar additives on the viscosities of ionic micellar solutions.23,33,36 Figures 4-6 show the interplay of the concentrations of R4NBr and aliphatic hydrocarbons (n-hexane, nheptane, n-octane) on the variation of ηr of 0.3 M SDS micellar solutions (R ) n-C4H9). From the previous discussions of Figures 1 and 2 it is clear that hydrocarbons solubilized in the micellar interior provide less viscosity to the micellar solution, while their interfacial solubilization increases the viscosity due to micellar growth. However, in the real situation (our case) for an aliphatic hydrocarbon, intramicellar partitioning at different sites (headgroup or interior regions) seems more important and the content of the hydrocarbon at each site would decide whether micellar growth (due to elongation) or micellar swellenity (due to interior solubilization) will dominate toward the overall viscosity of solutions. In view of the above discussion, the data shown in Figures 4-6 are selfexplanatory. Upto 0.1 M salt concentration, all the aliphatic hydrocarbons behave similarly, which can be understood in the context of the predominance of micellar interior solubilization (change of grown to swollen spherical micelles). At g0.15 M R4NBr salt (R ) n-C4H9), viscosity (35) De Gennes, P. G., Taupin, C. J. Phys. Chem. 1982, 86, 2294. (36) Bunton, C. A.; Cowell, C. P. J. Colloid Interface Sci. 1988, 122, 154.

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Figure 5. Relative viscosities (ηr) of 0.3 M SDS micellar solutions as a function of added n-heptane (upto the solubility limit indicated by dotted lines) at various fixed concentrations (x) of (n-C4H9)4NBr at 30 °C: x ) 0.0 (K), 0.10 (b), 0.15 (O), 0.20 (y), and 0.30 M (X).

Figure 6. Relative viscosities (ηr) of 0.3 M SDS micellar solutions as a function of added n-octane (upto the solubility limit indicated by dotted lines) at various fixed concentrations (x) of (n-C4H9)4NBr at 30 °C: x ) 0.0 (K), 0.10 (b), 0.20 (y), and 0.30 M (X).

patterns start changing, which also show a dependence on the alkyl chain length of the aliphatic hydrocarbon. With n-hexane, viscosity starts increasing at lower overall n-hexane content addition. However, at higher n-hexane concentration, usual viscosity decrease was observed. This effect was even more pronounced at higher salt concentration, and the viscosity-decreasing region diminished, which indicates that n-hexane is now predominantly solubilized at the interfacial region and the viscosity increase gives a signature of further micellar growth. This effect is comparatively less pronounced in case of nheptane (Figure 5) and is nearly absent with n-octane (Figure 6). This shows that with increase in chain length of the aliphatic hydrocarbon, the preference for interfacial solubilization is depleted at constant salt concentration.

Solubilization-Site-Dependent Micellar Morphology

Figure 7. Relative viscosities (ηr) of 0.3 M SDS micellar solutions as a function of added toluene (upto the solubility limit indicated by dotted lines) at various fixed concentrations (x) of (n-C4H9)4NBr at 30 °C: x ) 0.0 (K), 0.10 (b), 0.20 (y), and 0.30 M (X).

The effect could be understood in light of the fact that for equal interfacial volume available for each hydrocarbon, more n-hexane molecules would be solubilized at the interface, due to its lower molar volume.5 The increased IPC for n-hexane would influence the overall course of micellar association structure variation. Figure 7 shows the ηr-[toluene] curves for different R4NBr concentrations (R ) n-C4H9) at fixed 0.3 M SDS. The overall patterns are similar to aliphatic hydrocarbon additions with the difference of the complete disappearance of the viscosity decreasing region. In light of the previous discussion, one can say that toluene has different solubilization sites than the aliphatic hydrocarbons, which is not surprising in the context of its different nature. The viscosity behavior can be understood with the similar reasoning as put forth in earlier paragraphs. The predominant site of solubilization from viscosity results seems to be around the interfacial region.5,30,31 Figures 8 and 9 show the plot of ηr vs 1-alkanol concentration variation for different fixed concentrations of R4NBr (R ) n-C4H9) in 0.3 M SDS micellar solutions (for 1-heptanol or 1-octanol). In the case of 1-heptanol (Figure 8), the viscosity rise takes place even without salt, which is due to micellar growth, which occurs by interfacial partitioning of 1-heptanol.34 The presence of salt (R ) n-C4H9) increases the viscosity due to the synergistic effect of the salt and 1-heptanol. Further addition of the salt shows a flattening in ηr-[1-heptanol] plots. Additionally, at [salt] ) 0.3 M, practically no change in the overall viscosity was observed upon the addition of 1-heptanol. This shows that the site of solubilization of 1-heptanol is changed from the interfacial region. The nature of alcohol (the presence of the -OH group) and the higher salt content permit us to say that the site of alcohol solubilization has been changed to the micellar exterior (vide supra). As discussed earlier, the exterior solubilization of an additive has no significant effect on the overall viscosity as well as on micellar growth. Figure 9 shows similar viscosity trends with n-octanol with the difference in overall solubilization content, which is obviously due to the higher hydrophobic volume and less polar nature of the alcohol.

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Figure 8. Relative viscosities (ηr) of 0.3 M SDS micellar solutions as a function of added 1-heptanol (upto the solubility limit indicated by dotted lines) at various fixed concentrations (x) of (n-C4H9)4NBr at 30 °C: x ) 0.0 (K), 0.10 (b), 0.20 (y), and 0.30 M (X).

Figure 9. Relative viscosities (ηr) of 0.3 M SDS micellar solutions as a function of added 1-octanol (upto the solubility limit indicated by dotted lines) at various fixed concentrations (x) of (n-C4H9)4NBr at 30 °C: x ) 0.0 (K), 0.10 (b), 0.20 (y), and 0.30 M (x).

On the basis of the viscosity data, we can make the following conclusions concerning hydrocarbon and alkanol additions in SDS + R4NBr systems. For partitioning of an additive, this is its polarity, which should be compatible with the major part of the volume of a micelle, as it has different polarity segments constituting the overall micelle. (It would not be out of context to mention here that Davies et al.37 have recently been successful in discussing their kinetic data based on a multiple micellar pseudo(37) Davies, D. M.; Gillitt, N. D.; Paradis, P. M. J. Chem. Soc., Perkin Trans. 2 1996, 659.

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phase model in which, depending on particular amphiphilic properties, the reactants show various distributions of concentration with distance from the center of the micelle.) The polarity of the whole micelle (particularly the headgroup or interfacial region) can be tuned by the presence of a few selected additives. In this respect, the names of salts,17,18 aromatic hydrocarbons,6 and amphiphilic molecules4,38 could be mentioned. There seems to be a greater advantage with R4NBr salts due to their ionic nature as well as higher hydrophobic volumes. In favorable situations (proper R and concentration), they can get solubilization sites of organic additives changed to the interfacial region that otherwise are known to (38) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208.

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solubilize in the interior of micelles. Further, this study modifies the existing two-region model39 by providing another temporary hydrophobic region around the micelle (micelle exterior). Over and above, this study could provide a route to understand the solubilization of various polar/ organic compounds in the presence of salts that is not defined convincingly in the literature. Acknowledgment. This work was performed under the collaborative research Scheme No. IUC/CRS-M-72 of the Inter-University Consortium for Department of Atomic Energy Facilities, India. LA0101550 (39) Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1987, 91, 338.