Stable, metastable and unstable oil-in-water droplets

Abstract

In this paper we bring together some recent results concerning the stability and properties of O/W microemulsion droplets in a ternary system composed of water, decane and the nonionic surfactant pentaethylene glycol dodecylether (C₁₂E₅). Stable microemulsion droplets can be prepared when the spontaneous curvature has a finite but not too low value. Near the limit of maximum oil solubilisation the droplets adopt a spherical shape with low polydispersity. Experimental results obtained from low shear viscosity, collective and long time self-diffusion and static light scattering show that the spherical droplets interact to a very good approximation as hard spheres over a large range of volume fractions. A supersaturated microemulsion can be prepared by a rapid temperature quench (drop) into the two-phase area where a smaller droplet size coexists with excess oil. In the two-phase area, we can distinguish a region near the microemulsion phase boundary where the droplets are metastable, from a region further away from the boundary where the droplets are unstable and the oil-phase nucleates instantaneously. Treating the initial phase separation as a homogeneous nucleation it is possible to calculate an activation energy within the curvature energy approach.