Effects of oil type and aqueous phase composition on oil–water mixtures containing particles of intermediate hydrophobicity

The behaviour of emulsions stabilised by partially hydrophobic spherical silica particles is described for systems in which the oil type is varied in mixtures with water, and for toluene-containing systems in which water is replaced by other polar solvents. Using contact angle measurements on flat glass substrates, we deduce that particles are more hydrophobic at polar oil–water interfaces (esters and alcohols), preferring water-in-oil (w/o) emulsions, and more hydrophilic at non-polar oil–water interfaces (alkanes), with the preferred emulsion type being oil-in-water (o/w). Formamide systems are similar to those of water, whereas emulsions could not be prepared with glycerol or ethylene glycol. In contrast to a surfactant-stabilised emulsion, we find that the preferred emulsion type depends on the initial location of particles in that the continuous phase is the phase in which particles are first dispersed. Using undecanol as oil, we observe an interesting phenomenon during the complete breakdown of viscous w/o emulsions involving contraction to retain the shape of the vessel. Finally, we determine the partitioning of particles between toluene and water and show that particles can be rendered more hydrophilic by simply increasing the pH of the aqueous phase. An increase in the wettability by water in this way is used to shed new light on the behaviour of different particle-containing systems involved in demulsification, flotation and antifoaming.