Concentration and temperature dependence of the counterion self-diffusion coefficient in aqueous solutions of hexadecyltrimethylammonium bromide

Abstract

The concentration and temperature dependence of the counterion self-diffusion coefficient, D Br − , have been measured for aqueous solutions of hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide, CTAB), from below the critical micelle concentration (CMC = 0.036 wt%) to 17.3 wt% CTAB. The radiometric-porous frit method was used. The concentration dependence of D Br − and of the activation energy for self-diffusion both apparently exhibit extrema near the CMC. However, above about 1% CTAB D Br − is a linear function of the concentration, and this concentration dependence continues even at high surfactant concentrations where the viscosity of the solution becomes very large. The data are analyzed in terms of a “two-site” model which has been applied to the NMR linewidths of Br − in CTAB solutions, and it is shown that within the model the accepted interpretation of the linewidths is inconsistent with the bromide self-diffusion coefficient.