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The aggregation behavior of sodium dioctylsulfosuccinate (AOT) in aqueous media containing tetraalkylammonium bromide (TAAB, where alkyl ¼ ethyl (TEAB), propyl (TPAB) and butyl (TBAB)) was studied by surface tension, fluorescence (with pyrene as the probe), small-angle neutron scattering (SANS) and dynamic light scattering (DLS) measurements. A comparison of the critical micelle concentration (cmc) values of AOT in the presence of the salts showed the order TBAB < TPAB < TEAB < NH 4 Cl < NaCl. Synergism in the cmc occurs when the solution contains a mixture of sodium and tetraalkylammonium counterions. The counterion binding behavior was examined by applying the modified Corrin-Harkins (CH) equation which revealed that a special counterion binding behavior of AOT exists in aqueous solutions with tetraalkylammonium salts. The modified CH equation and DLS data indicate a change in the shape of the surfactant aggregates, which was confirmed by the SANS data.Dehydration of the head group and the counterion during their interaction appears to induce a micelleto-vesicle transition in the aggregates.

The dependence of critical micelle concentration (cmc) of sodium dioctylsulfosuccinate (AOT) on the amount of ethylene glycol (EG) in water + EG medium was reported to be unusual and different from that of other surfactants to the extent that the cmc of AOT in EG is lower than in water. It is yet to be understood why AOT behaves so in water + EG medium, although AOT is known to have some special properties. Hence in the present study cmc of AOT in water + EG medium in the range from 0 to 100% (by weight) EG is measured by using surface tension and fluorescence emission methods. In contrast to what was reported, this study revealed that with respect to EG amount the cmc of AOT follows the general trend and AOT has higher cmc in EG than in water. On the other hand, it was surprisingly found that a break in the surface tension isotherm occurs in the premicellar region when the amount of EG exceeds 50% rendering a bisigmoidal shape to the surface tension isotherm. UV spectral study showed that AOT and EG undergo hydrogen bonding in the premicellar region when the EG amount is ≥50% and this hydrogen bonding becomes less on adding NaCl. The density functional theory calculations also showed formation of hydrogen bonds between EG and AOT through the sulfonate group of AOT providing thereby support to the experimental findings. The calculations predicted a highly stable AOT-EG-H(2)O trimer complex with a binding energy of -37.93 kcal mol(-1). The present system is an example, which is first of its kind, of a case where hydrogen bonding with surfactant and solvent molecules results in a surface tension break.

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