Evolution of water-in-oil emulsions stabilized with solid particles
Abstract Water-in-oil emulsions made of water droplets dispersed in a continuous paraffin oil phase were prepared and stabilized with hydrophobic silica particles alone. Differential scanning calorimetry (DSC) experiments were carried out to characterize the water droplets freezing transitions and their evolution with time. Water droplet size distribution, oil–water interfacial tension, and rheological stress–strain properties were determined alongside to better understand the role of particles in the formation and stabilization of emulsions. The results obtained were compared to the properties of emulsions prepared with a non-ionic emulsifier, sorbitan monooleate. No interfacial tension reduction was observed at the oil/water interface in presence of particles. As a consequence, the fragmentation of water into droplets required more energy in absence of surface-active emulsifiers. The resulting emulsion droplet size distribution is polydisperse and contains large droplets. Rheology measurements showed that the stability of the emulsion prepared with particles originates from the formation of a 3D network of particles in the continuous oil phase. In emulsion samples containing sorbitan monooleate, calorimetry experiments revealed a progressive displacement of the water droplet freezing transition due to a change in ice nucleation mechanism. The interpretation points out a possible reorganization of the emulsifier film at the water/oil interface, which could modify the conditions of crystallization of dispersed water droplets and affect the emulsion long-term stability. Calorimetry, when used together with other techniques, such as laser light scattering, provides complementary information on the evolution with time of the structure of the emulsion.