ABSTRACT
The goal of this chapter is to demonstrate that the thermodynamics of microemulsions presented by Ruckenstein in 1982 at the Lund meeting on surfactants in solution and published in Refs. 1 and 2, can provide insight with respect to some of the relevant experimental results gathered regarding microemulsions. The following experimental results are particularly meaningful in the present context:
Schulman and coworkers [3–7] observed that isotropic and optically transparent dispersions of oil in water or water in oil are formed spontaneously in the presence of a surfactant and a cosurfactant such as an alkyl alcohol. Using low-angle X-rays, light scattering, ultracentrifugation, electron microscopy, and viscosity measurements, they concluded that the dispersed phase consists of almost uniform spherical droplets with diameters ranging between 8 and 80 nm.
Winsor [8,9] found that in ternary nonionic surfactant-oil-water phase diagrams there are three types of systems. In Winsor’s type I systems, an oil-in-water (O/W) microemulsion coexists with excess oil, while in Winsor II systems a water-in-oil (W/O) microemulsion coexists with excess water. In both types of systems, almost spherical globules are dispersed in the continuous phase. In Winsor III systems, a microemulsion coexists with both excess phases, and one can no longer identify dispersed and continuous phases. For given amounts of the components, the type I system is present at relatively low temperatures; at intermediate temperatures, the type III system is generated; and, finally, at relatively high temperatures, the type II system is formed. Similar observations have been made with more complex systems that contained oil, water, surfactant, cosurfactant, and salt [10,11]. Indeed, if the temperature, the cosurfactant content, or the ionic strength increases, the system passes from an O/W microemulsion in equilibrium with excess oil to a middle-phase microemulsion in equilibrium with both excess phases and finally to a W/O microemulsion in equilibrium with excess water.
