The adsorption of cetyltrimethylammonium bromide on aqueous surfaces of sodium bromide solutions

Abstract

Surface tension of aqueous solutions of Cetyltrimethylammonium bromide has been measured by the drop weight method, at different NaBr concentrations ranging from 0 to 0.05 M at 25°C. Surface tension generally decreases with increasing concentrations of surfactant and NaBr, but it reaches a constant value at the critical micelle concentration for a given NaBr concentration. By applying the Gibbs adsorption isotherm for the surface tension at surfactant concentrations lower than the critical micelle concentration, surface excess densities of surfactant cation, Na + and Br − have been derived as functions of surfactant concentration at different NaBr concentrations. Surface excess densities of surfactant cation and Br − increase with increasing surfactant concentration, except for those of surfactant cation at 0.0004 and 0.001 M NaBr, which reach maxima at 3 and 1 × 10 −4 M, respectively. Furthermore, they do not reach constant values even at the critical micelle concentrations. The adsorption of Na + is negative, and its absolute value is large at high NaBr concentrations. The Corrin-Harkins plot of the critical micelle concentration consists of two straight lines intersecting at 0.01 M NaBr, which could correspond to the regions of spherical and rod-like micelles.