Droplet dynamics in water-in-oil microemulsions and macroemulsions stabilised by non-ionic surfactants. Correlation of measured rates with monolayer bending elasticity

Abstract

We have investigated the structure and inter-droplet exchange rates of water-in-oil (w/o) microemulsions, and the stabilities of w/o emulsions stabilised by three different non-ionic surfactants of the alkylpolyoxyether (C_nE_m) type. The surfactants investigated were C₈E₃, C₁₀E₄ and C₁₂E₅. The aim of the study was to test whether there is a correlation between the measured rates and the bending elasticity constant K for the different surfactant systems. Care was taken to ensure that, as far as possible, the different systems were compared in equivalent states with regard to equilibrium-phase behaviour. For the three systems studied, it was found that the energy barrier to inter-droplet exchange between microemulsion droplets increases in the order C₈E₃ < C₁₀E₄ < C₁₂E₅. The emulsion stability also increases in this same order. For both the microemulsion and macroemulsion systems, the effectiveness of the different monolayers in stabilisng the droplets is found to scale approximately with the bending elasticity constant K. Thus, the data suggest that K is an important determinant of kinetic stability in surfactant monolayer stabilised emulsions.