Interfacial behaviour in water-oil-amphiphile mixtures

Using a lattice model for water-oil-amphiphile mixtures, we examine the interfaces between the water-rich, oil-rich and disordered phases in the mean-field and Bethe approximations. By calculating the surface tensions between these phases along their coexistence triple line, we find that the disordered phase wets the water-oil interface when it is an ordinary disordered fluid but not when it is a microemulsion, where the associated wetting transition is weakly first order. Here, we use the disorder line as the dividing line between the « ordinary » and microemulsion regions, rather than the Lifshitz line. In the region of two-phase coexistence between water-rich and oil-rich phases, we find three distinct water/oil interfacial phases and the transitions between them. One of these interracial phases, which contains a strongly oriented amphiphile monolayer, typically exhibits ultralow surface tension.