Destabilisation of Pickering emulsions using pH

Pickering oil-in-water (o/w) emulsions, with equal volumes, stabilised solely by partially hydrophobic silica nanoparticles (80% SiOH) were destabilised by changing the pH of either the silica dispersions before emulsification or that of the freshly prepared emulsions. It was found that the aqueous silica dispersions were unstable at pH < 4.2 in accordance with classical DLVO theory. When the pHs of the emulsions were adjusted after preparation, the resulting emulsions showed creaming and coalescence to different extents and the maximum oil loss (75%) occurred at pH 2.94, which is around the iso-electric point (i.e.p.) of silica particles, and at pH 7.1 whereas ∼45% of the oil was lost at intermediate pHs. Complete emulsion destabilisations was achieved when the pHs of the silica dispersions were adjusted before the emulsification process particularly between pH 7 and 8 where most of the oil and aqueous phases were released. The results suggested that the wettability of the silica particles, hence their contact angles, is greatly affected by changing the pH.