Volumetric behavior and micelle formation of aqueous surfactant mixtures

A thermodynamic treatment of the volumetric behavior of surfactant mixtures in water have been developed on the basis of the thermodynamic treatment of mixed micelle by Motomura et al. Densities of aqueous solutions of mixtures of decyltrimethylammonium bromide (DeTAB) and dodecyltrimethylammonium bromide (DTAB) have been measured as a function of total molality at constant compositions. The apparent molar volumes of the mixtures have been derived from the density data and the mean partial molar volume of monomeric surfactant mixture V_t^w, the molar volume of mixed micelle V^M/N_t^M, the voluem of formation of mixed micelle Δ_W^MV, and the composition of surfactant in the mixed micelle have been evaluated. The V_t^W, V^M/N_t^M, and Δ_W^MV have been observed to depend on the composition. The linear dependence of V_t^W and V^M/N_t^M on the composition indicates that the mixing of DeTAB and DTAB is ideal both in the monomeric and micellar states. This has been confirmed further by the shape of the critical micelle concentration vs. composition curves.