Stability of emulsions under equilibrium and dynamic conditions

In the present article we analyse the influence of various factors, both thermodynamic and hydrodynamic, on the stability of emulsion systems. The effect of the droplet size on the droplet lifetime in an emulsion cream is quantified. The comparative importance of kinetic factors such as surface and bulk diffusion fluxes, or viscous and elastic surface stresses, is investigated. The fact that the emulsion films drain much more slowly when the surfactant is dissolved in the continuous phase (rather than in the disperse phase) provides a new understanding of the Bancroft rule and the process of chemical demulsification. New thermodynamic aspects of emulsion stability are also discussed. One of them is the relatively high surface electric potential of pure oil-water interfaces and adsorption monolayers of nonionic surfactants. Another aspect is the role of non-DLVO surface forces, such as the hydration repulsion, oscillatory structural and depletion forces due to the presence of surfactant micelles. A criterion of emulsion stability is formulated synthesizing the effects of the major factors. Finally we consider the importance of the kinetic phenomena in emulsions formed from non-pre-equilibrated phases. (C) 1997 Elsevier Science B.V.