Highly Concentrated Emulsions: Physicochemical Principles of Formulation

This review deals with the preparation, stability, rheology and different applications of highly concentrated emulsions. These emulsions, which are known as high internal phase ratio emulsions (HIPRE), gel-emulsions, biliquid foams, etc., containing over 90% internal phase by volume, have a swollen micellar (L₁ or L₂) solution of nonionic or ionic surfactants as a continuous phase. These emulsions have the structure of biliquid foams and behave as gels since they present viscoelastic and plastic properties. The functional macroscopic properties of gel-emulsions are dependent on the structural parameters of the microemulsion continuous phase as well as of the interfacial properties (interfacial tension, bending modules, spontaneous curvature) of surfactant monolayers. The depletion interaction between emulsion droplets due to the non-compensated osmotic pressure of micelles is revealed as a main factor, along with surface forces, which determine the aggregative stability and the rheological properties of these emulsions. The effect of electrolyte and surfactant concentration, temperature, as well as other physicochemical parameters on the fiocculation threshold, stability, and yielding properties of highly concentrated emulsions is explained by the effect of these parameters on the critical micelle concentration (CMC) and the aggregation number of surfactants, and, consequently, on the depletion interaction. The thermodynamic theory of adhesion of fluid droplets in micellar solution and the suggested model of elasticity of gel-emulsions permit to explain the effect of mentioned physicochemical parameters on the elasticity modulus, the plastic strength and the linear deformation of these emulsions. A novel mechanism for the spontaneous formation of gel-emulsions by the phase inversion temperature (PIT) route is suggested, allows the selection of ternary systems able to yield these emulsions, and explains how the droplet size can be controlled during the PIT process. An original model for liquid film rupture is also suggested, and allows the prediction of the effect of structural parameters of micellar solutions and the interfacial properties of surfactant monolayers on the stability of gel-emulsions.