Asymmetric excess free energies and variable interaction parameters in mixed micellization

The electrostatic contribution to the excess free energy of mixed micelliza tion, considered to be the most important contribution for ionic/nonionic m ixtures of hydrocarbon-based surfactants with similar tail lengths, was cal culated for mixtures of sodium dodecyl sulfate and three different nonionic surfactants at two salt concentrations. A simplified analytical molecular thermodynamic approach was used where the electrostatic free energy contrib utions of both the pure ionic and mixed micelle were obtained by applying a n approximate solution of the Poisson-Boltzmann equation. The results show that in general the excess free energies are not symmetrical with respect t o the micellar composition. However, it is shown that even in this case the equations for the micellar properties given by the molecular thermodynamic approach at the optimal micellar composition remain identical to the ones produced by the phase separation approximation, including the expressions f or the activity coefficients as a function of the excess free energy. Impor tant differences in the activity coefficients, which determine the system b ehavior, are illustrated when either symmetric or asymmetric excess free en ergies are considered.