Nano-emulsion formation by emulsion phase inversion

The droplet size distribution of an emulsion governs emulsion properties such as long-term stability, texture and optical appearance. Consequently, means to control the droplet size during emulsification are of interest when well-defined emulsion properties are needed. In this work, we study emulsions consisting of water, paraffin oil and a mixture of non-ionic surfactants and fatty alcohols by means of laser light scattering. We investigate the influence of the route of preparation as well as the surfactant concentration on the droplet size distribution. Above a critical surfactant-to-oil ratio and following the standard way of emulsion phase inversion, a significant amount of oil droplets with diameters less than 1 mum were obtained. When changing the way of emulsification and thereby avoiding a phase inversion to occur, such fine droplets are absent and the droplet size distribution is solely governed by the input of mechanical energy. We demonstrate that emulsification by the phase inversion method makes use of two effects for the achievement of finely dispersed oil-in-water emulsions. The lamellar or bicontinuous structure formed by the surfactant at the inversion point determines the size of the resulting droplets while the corresponding minimal interfacial tension facilitates the droplet formation, explaining why the droplet size distribution only depends on the weight ratio between surfactant and oil rather than on the water concentration. (C) 2004 Elsevier B.V. All rights reserved.