Whereas hydrophobic colloidal particles are known to destabilize foams and emulsions stabilized with surfactants, their use for destabilizing Pickering emulsions is unexplored. Pickering emulsions differ from surfactant-stabilized emulsions because they are stabilized with colloidal particles that are adsorbed to the oil/water interface, which provide a steric barrier to droplet coalescence. This can make Pickering emulsions very stable, but it can also make their subsequent destabilization difficult. The hypothesis of this work is that destabilizing a Pickering emulsion should be possible with colloidal particles provided they are sufficiently hydrophobic, which will enable the particles to dewet an emulsion film and induce coalescence.
A model oil-in-water Pickering emulsion was stabilized with polyethylene glycol-modified silica nanoparticles and its stability was assessed by centrifugation and by stirring on a stir plate. Three different fumed silica particles, A200 (bare, hydrophilic), R816 (hexadecylsilane modified, intermediate hydrophobicity), and R805 (octadecylsilane modified, hydrophobic), were added to the emulsion and stirred to evaluate their ability to macroscopically induce coalescence. Optical microscopy was used to visualize the interaction between the model Pickering emulsion and the three different fumed silica particles.
The results from this work show there is a strong correlation between the wettability of a fumed silica particle and its ability to destabilize a model Pickering emulsion, with more hydrophobic particles showing a greater tendency to coalesce the Pickering emulsion. The hydrophilic and partially hydrophobic particles, at all concentrations tested, were unable coalesce the model Pickering emulsion. This was because the particles were almost immediately wetted by the continuous phase of the emulsion, which prevented any interactions between the emulsified oil drops and the silica particle surface. The hydrophobic fumed silica particles coalesced 60% of the emulsified oil with just 0.01 wt% added fumed silica, which further increased to 85% with 0.05 wt% added silica.