Interfacial phase transitions of microemulsions

Abstract

In this paper we study the interfaces between phases in a phenomenological model of a microemulsion that is in equilibrium simultaneously with an oil-rich and a water-rich phase. The tensions and chemical-composition profiles of the interfaces are calculated. We ask whether the oil–water, oil–microemulsion and microemulsion–water tensions σ_ow, σ_om and σ_mw are related by σ_ow < σ_om+σ_mw or by σ_ow=σ_om+σ_mw. In the former case the microemulsion phase does not wet the oil–water interface, whereas in the latter it does. We find separate ranges of values of the model's parameters in which each possibility is realized, while the microemulsion is a middle phase related symmetrically to the oil and water phases. When a parameter that breaks that symmetry is varied and a critical endpoint of the three-phase equilibrium is approached, an originally non-wet oil–water interface becomes wet (while an originally wet interface remains wet). The transition is of first order, accompanied by a change in interfacial structure. A microscopic lattice model of such three-phase equilibria is also described. In its context we raise (but do not fully answer) the same questions that we treated in the earlier phenomenological model.