Monte Carlo study of a microscopic lattice model for microemulsions

Abstract

A microscopic lattice model of microemulsion-forming ternary solutions has been studied by Monte Carlo simulation. Compelling evidence for three-phase equilibrium among oil-rich, water-rich, and bicontinuous microemulsion phases is reported. The simple two-surfactant bending energy term used in our earlier mean-field study of this model was not found to produce three-phase equilibrium in our simulations. Additional three-surfactant terms were found to be necessary, indicating that the bending energy term in a lattice model must be chosen with care. These new bending energies have a simple physical motivation. The results reported here are obtained for a particularly simple set of interaction energies, involving only a single nonzero bending energy. A method is presented for calculating the surface excess densities and interfacial tension from the simulation data. It yields a plausible excess surface density of surfactant and surface tensions among the coexisting phases in three-phase equilibrium that fall into the ‘‘ultralow’’ regime, about 1000 times smaller than typical oil–water surface tensions.