Micellar shape driven counterion binding. Small-angle neutron scattering study of AOT micelle.

Sodium dioctylsulfosuccinate (AOT) micelle has a special counterion binding behavior in aqueous electrolyte medium, viz., the counterion binding constant (β) abruptly increases by 2-fold at about 0.015 mol dm(-3) NaCl concentration (c*), but not in sodium salicylate (NaSa) solution. Since counterions affect the structure and performance of ionic surfactants, ascertaining the cause for the sudden shift in the β value of AOT micelle is of fundamental importance. In this study the special counterion binding behavior of AOT micelle has been ascertained at 40 °C by carrying out surface tension, zeta potential, and fluorescence emission (pyrene probe) measurements. The results of the small-angle neutron scattering experiment carried out at 40 °C showed that at c* the shape of AOT micelle changes from prolate spheroid to rodlike in NaCl solution, but not in NaSa solution, thus establishing micellar shape change as responsible for the abrupt change in β value. The absence of sudden shift in β of AOT micelle in NaSa solution is attributed to the binding of salicylate coanion to AOT micelle through hydrophobic interaction.