Encyclopedia of Colloid and Interface Science

Abstract

Synonyms Coalescence of emulsions Definition Coalescence of emulsions is a process whereby two or more emulsion droplets become joined together forming larger droplets with the ultimate separation of the emulsion into two phases (oil and water). The driving force for emulsion coalescence is the thinning and disruption of the liquid film between the droplets. This can happen if the emulsion droplets closely approach each other in a cream layer, in a floc, or during Brownian diffusion. When the droplets approach each other very closely, the liquid film between them undergoes some fluctuation, and when the film thickness h approaches a critical value h cr , film collapse occurs resulting in its rupture. Alter-natively, the surface of the droplets may undergo some fluctuation resulting in the formation of waves. The van der Waals attraction at the apex of these waves becomes sufficiently strong, and eventually, the waves join. Coalescence can be prevented by having a sufficiently high positive disjoining pressure p. The latter is the sum of three contributions, electrostatic p el (i.e., repulsive and positive), steric p st (i.e., repulsive and positive), and van der Waals p A (i.e., attrac-tive and negative). For the film to remain stable, p el + p st >> p A . This can be achieved by using mixed surfactant films of ionic and nonionic sur-factants; use of polymeric surfactants of the A-B, A-B-A block, or BA n types; and formation of lamellar liquid crystalline phases formed of sev-eral surfactant bilayers that wrap around the droplets.