Ternary oil-water-amphiphile systems: Self-assembly and phase equilibria

The micellization properties and phase behaviour of model ternary surfactant-oil-water systems were studied by grand canonical Monte Carlo simulations assisted by histogram reweighting techniques. Larson's lattice model was used for a symmetrical H₄T₄ surfactant, where H represents hydrophilic and T hydrophobic groups. In contrast to earlier studies, oil chains of variable length, from T to T₄, were studied and the effect of added oil on the critical micelle concentration (cmc) was determined quantitatively. It was found that the cmc decreases as the volume fraction of oil in the aqueous phase is increased. Longer oil chains have a stronger effect than shorter chains for the same volume fraction. The oil, tail and head density profiles were obtained for typical micelles, Simulation results for the density profiles indicate that long chain oils are almost exclusively located within the micellar aggregates, in agreement with single-chain mean-field theory predictions. Ternary phase diagrams were obtained both by simulations and by quasichemical theory. The results suggest that the critical micelle concentration line is directed towards the oil-lean corner of the three-phase triangle, if there is one present in the system. It was also found that the amount of oil that can be dissolved in the ternary system increases sharply at the surfactant critical micelle concentration, indicating that micellization greatly enhances oil solubility.