Nanoparticle silica-stabilised oil-in-water emulsions: improving emulsion stability

The rapid phase separation of oil-in-water (o/w) emulsions prepared from non-polar oil and alkaline dispersions of hydrophilic silica nanoparticles alone is described. A study of the roles of the surface chemistry of the particles and the type and composition of the oil and aqueous phases in improving the stability of these emulsions is then reported. Alteration of the particle charge and flocculation by control of pH and addition of divalent electrolyte causes temporary improvements in emulsion stability, while the addition of cationic surfactant results in the preparation of stable emulsions. Increasing the oil phase polarity is found to improve emulsion stability and this is attributed to adsorption of polar solvent molecules to the silica surface changing the particle wettability. Attempts to reduce the affinity of the particles for the aqueous phase by adding non-aqueous solvents to that phase are unsuccessful, perhaps due to the extreme hydrophilicity of the particles. For the stable emulsions, the effect of silica particle size on the emulsion drop size is also investigated.