Solution thermodynamics of mixed nonionic/ionic and ionic/ionic surfactant micelles

By considering the main contributions to the composition-dependent free energy of mixed micelles and relying on the pseudophase approximation, we have devised new schemes for analyzing the synergistic effects often observed for the CMCs of binary surfactant mixtures. It turns out that merely free energy contributions (other than the one due to ideal mixing) which are nonlinear with respect to the surfactant composition in the micelles can give rise to synergism. Contrary to what is commonly presumed, the synergistic effects are to a large extent caused by entropic rather than energetic contributions to the head group free energy. Specifically, the following cases have been treated: (i) a mixture of a nonionic and an ionic surfactant with and without an excess of added salt and (ii) a mixture of two ionic surfactants, in all instances whilst making use of the standard Poisson-Boltzmann theory to account for the electrostatic free energies. The agreement with experimental CMC data for systems of this kind is satisfactory.