Creaming of emulsions: the role of depletion forces induced by surfactant

Abstract

We show that excess surfactant, or salt, in the bulk phase of an oil in water ionic emulsion has strong consequences on the thermodynamical behavior of the dispersion. Static light scattering experiments have been performed to investigate the attractive interaction induced by micelles according to a depletion mechanism. This interaction can be largely reduced by adding salt. This depletion interaction leads to a phase transition which is characterized as a fluid-solid transition. The long range ordering of the droplets dense phase is characterized by visible light diffraction. The experimental phase diagram is quantitatively analysed as a transition between a perfect gaz and a harmonic solid. An analytical model is worked out from which we analyze the general phase diagram in terms of the volume fraction of the droplets, of the depth of their interaction potential and of the ratio (δ/σ), where δ is the range of the interaction and σ the droplet diameter. Cuts of the phase diagram at constant (δ/σ) feature a liquid-gas or a liquid-solid transition.