Shape transitions in the aqueous phase of the system hexadecyltrimethylammonium bromide—hexanol—water

Abstract

Viscosity, conductivity, density, and ultrasonic velocity measurements have been used to characterize aqueous solutions of hexadecyltrimethylammonium bromide and hexanol, i.e., the L 1 phase of the system. At surfactant molalities below 0.055 m, conductivity and ultrasonic velocity measurements show that a change in the solubilization pattern takes place as hexanol is added. It seems that above a certain hexanol content, the additive is solubilized not only in the palisade layer but also in the interior of the micelles, thus producing swollen micelles. The viscosity remains practically constant as hexanol is added. At surfactant molalities above 0.055 m the relative viscosity increases drastically above a certain hexanol content and the conductivities show a maximum at this point. This suggests a shape transition from spherical to larger rod- or disklike micelles. At surfactant molalities between 0.055 and 0.09 m hexadecyltrimethylammonium bromide, the viscosities start to decrease at even higher hexanol contents. The conductivity and ultrasonic velocity measurements also show that a change in solubilization pattern takes place at this hexanol content. It seems likely that hexanol becomes solubilized in the micellar interior and that a breakdown from large aggregates to spherical swollen micelles takes place. It seems that the L 1 phase of the system hexadecyltrimethylammonium bromide—hexanol—water can be divided into four structurally different regions; one of monomeric species, one of ordinary spherical micelles, one of swollen spherical micelles, and one of rod- or disklike micelles.