Influence of a mixed particle/surfactant emulsifier system on water-in-oil emulsion stability

Abstract Water-in-oil (W/O) emulsions consisting of water and paraffin oil were prepared and stabilized by using a dual emulsifier system including hydrophobic silica particles and non-ionic surfactant. The surfactant/particles synergistic interaction and more specifically the effect of surfactant concentration (from 0 to 1.8%, w/w) at fixed amount of particles (1.8%, w/w) on the emulsions properties were investigated. Interfacial tension, emulsion morphology, water droplet size distribution, stability with time, rheological stress–strain and oscillatory properties were determined to better understand the interaction and mechanism of emulsion stabilization by solid particles in presence of surfactant. The results showed the existence of strong interaction between surfactant molecules and silica particles. The critical micellar concentration for the mixed-emulsifier system was significantly higher than that of surfactant alone. In presence of a low amount of surfactant (0.1%, w/w), the emulsion was characterized by maximal stability (up to 21 days without any phase separation). This dependence was confirmed by rheological measurements which revealed a highest storage modulus (G′) value for this emulsion. Optical microscopy revealed the formation of closely packed aggregates of small droplets forming bridges between neighboring large drops, at low surfactant concentration. The possible explanation for this phenomenon could be the flocculation of small emulsion droplets in the presence of low amount of surfactant. The formation of flocs caused the thickening of the continuous oil phase, the increase in emulsion rigidity and consequently enhanced emulsion stability.