Stability and Stabilization of Water-in-Oil-in-Water Multiple Emulsions

Water-in-oil-in-water (w/o/w) and oil-in-water-in-oil (o/w/o) multiple emulsions are inherently unstable dispersions because of the several possible routes to physical breakdown, some of which are peculiar to these 'double' emulsions. In w/o/w systems coalescence of the oil droplets carrying the dispersion of smaller water droplets occurs with the water droplets influencing only marginally the process; coalescence of the internal water droplets may also occur, diffusion of surfactant from the original w/o interface to the interface of the oil droplets with the bulk aqueous phase complicating formulation and stabilization. The most difficult feature to control is the osmotic flux of water molecules across the oil lamellae in w/o/w systems. Electrolyte and other additives including drugs can affect the stability of the systems by both osmotic and interfacial effects. Strategems to overcome some of these basic problems include (i) the use of a high viscosity oil phase in w/o/w emulsions to prevent or decrease diffusion of individual surfactant molecules and water molecules, (ii) the polymerisation of interfacially adsorbed surfactant molecules, and (iii) the gelation of the oily or aqueous phases of the emulsions.