Influence of surfactant structure on the double inversion of emulsions in the presence of nanoparticles

The double inversion of emulsions from oil-in-water (o/w) to water-in-oil (w/o) and back to o/w is a novel approach developed using oppositely charged mixtures of solid particles and surfactant molecules [B.P. Binks, J.A. Rodrigues, Angew. Chem. Int. Ed. 46 (2007) 5389]. This work extends these preliminary results reported for the cationic surfactant di-decyldimethylammonium bromide (di-C10DMAB), by studying the influence of surfactant chain length for symmetrical cationic surfactants. Two double-chain surfactants with hydrocarbon chain lengths of 8 or 12 are used here to stabilise emulsions with dodecane in the presence of silica nanoparticles (Ludox HS-30). Double inversion of emulsions is also possible with these surfactants. However, multiple emulsions of both types are observed when the most hydrophobic surfactant di-C12DMAB is used, occurring near the boundaries of the first and second inversion, respectively. A phase diagram is introduced, in which the double inversion of emulsions occurs at various particle-to-surfactant ratios, in particular at low concentrations of silica particles between 0.1 and 4 wt.%.