Barrier to Coalescence in Stabilized Emulsions

Abstract

ALTHOUGH emulsions of immiscible liquids are thermo-dynamically unstable, it is well known that the rate of coalescence may be reduced by adding small quantities of a third component (stabilizer) which is tensioactive. Many theories have tried to explain emulsion stability but none has been perfectly general. The most general theory is based on the stabilization of thin liquid films during approach of droplets as a result of the Gibbs–Marangoni effect1. This stabilizing effect may operate during the early stages of emulsion formation. Electrical repulsion must be a barrier to coalescence in emulsions stabilized by ionized detergents2, but many emulsions of high stability may be prepared in which no electrical potential barrier is believed to be present. The purely hypothetical “solvation barriers”3 cannot explain the stability of water/oil emulsions where the oil is a paraffin, for example, in which no orientation at the interfaces would be expected. A barrier resulting from the high viscosity of the stabilizing film4 cannot account for the high stability of emulsions stabilized by monolayers which do not increase the interfacial viscosity. The difficulty of flow of the continuous medium from thin films between approaching droplets results in a hydrodynamic barrier which would not be expected to be influenced very greatly by the presence of adsorbed monolayers. These latter two mechanisms do not offer an explanation of emulsion type.