Mixed micellization of dodecylethyldimethylammonium bromide and dodecyltrimethylammonium bromide in aqueous solution

Conductivity and steady-state fluorescence measurements have been carried out at 25 degreesC to study the monomeric and micellar phases of the aqueous solutions of the mixed system constituted by two cationic surfactants, dodecylethyldimethylammonium bromide and dodecyltrimethylammonium bromide. From conductivity data, the total critical mixed micellar concentration, cmc*, and the dissociation degree of the mixed micelle, beta, have been obtained, while fluorescence experiments allow for the determination of the total aggregation number, N*, and the micropolarity of the micellar inside. A new experimental method has been designed to determine the partial contribution of each surfactant to the mixed micellization process, through their critical micellar concentrations, cmc(1)* and cmc(2)*, and their aggregation numbers, N-1* and N-2*. A phenomenological limit mixed micellization line is proposed to define the limit conditions to form mixed micelles. The mixing of two cationic surfactants of identical tail but polar heads which differ only by the substitution of a methyl group by an ethyl group results in a small but detectable and negative deviation from ideality. Several descriptive and predictive theoretical models have been applied to interpret this experimental behavior.