Adsorption of dodecyltrimethylammonium bromide on aqueous surfaces of sodium bromide solutions

Abstract

Surface tension of aqueous NaBr solutions of dodecyltrimethylammonium bromide has been measured by the drop weight method at different NaBr concentrations from 0 to 6.00 M at 25°C. Surface tension decreases sharply with increasing surfactant concentration below the critical micelle concentration (CMC) at a given NaBr concentration, but it has a constant value above the CMC. By applying the Gibbs adsorption isotherm for the surface tension at surfactant concentrations lower than the CMC, surface excess densities of surfactant cation, Na + and Br − have been derived as functions of surfactant concentration at different NaBr concentrations. Generally, surface excess densities of surfactant cation and Br − increase with increasing surfactant concentration, and they approach saturated values at the CMC. The adsorption of Na + is very low in magnitude and negative. The Corrin—Harkins plot of the CMC consists of two straight lines intersecting at 1.30 M NaBr, which can be assigned to the threshold NaBr concentration for the sphere—rod transition of the micelle.