Thermodynamics of microemulsions: relation between their structure and phase behaviour

Abstract

The treatment previously developed by the author to explain the thermodynamic stability and phase behaviour of microemulsions is presented and some of its physical consequences are emphasized. General thermodynamic equations are written concerning the singel phase microemulsions, the transition to a microemulsion in equilibrium with an excess dispersed phase, and for the transition from the latter two phase system to one in which a microemulsion coexists with both excess phases (three phase system). On this basis, it is shown that the spherical shape of the globules of the dispersed phase becomes unstable to thermal perturbations, both in some single phase microemulsions as well as near the transition from the two to the three phase systems. While in the former case the radius of the globules at which the instability occurs is finite, in the latter case, it tends to infinity and the inter-facial tension at the surface of the globules as well as their bending stress tend to zero proportionally to the square of the curvature. This explaines the ill defined, fluctuating interfaces observed in the former case as well as the behaviour similar to that near a critical point observed in the latter case.