Synergistic effect of silica nanoparticle and cetyltrimethyl ammonium bromide on the stabilization of O/W emulsions

The preparation and stability of emulsions stabilized by A200 (silica nanoparticles, Degussa) and cationic surfactant CTAB (Cetyltrimethyl Ammonium Bromide) are investigated. The effects of NaCl concentration, aqueous dispersion pH, A200 particles concentration and CTAB concentration on the stability of emulsions are also investigated. Clear synergistic stabilization concentration ranges for A200 particles and CTAB are found. The results show that A200 particles can strongly adsorb on the oil-water interface with appropriate addition of CTAB concentration, but desorbed from the interface into bulk solution in the form of larger floes in an excessive increase of CTAB concentration. The synergistic effect comes from three sources: CTAB can change the surface of A200 particles from being full hydrophilic to being appropriately hydrophobic; promote A200 particles into aggregates with appropriate size; reduce the oil-water interfacial tension to facilitate the formation of emulsions. The interfacial adsorption of A200 particles is promoted with appropriate addition of NaCl, but not affected by aqueous dispersion pH and the interfacial structure contains monolayer and multilayers of A200 particle aggregates. In synergistic area, the super-high coalescence stability of emulsions is mostly attributed to the higher elasticity of dense A200 particles layers around the surface of emulsion droplets, while the high creaming stability comes from the increase of viscosity induced by the networks formed by the particles around emulsion droplets and the particle aggregates in continuous phase. (C) 2007 Elsevier B.V. All rights reserved.