(136c) Impact of Hydrophilic Particle Additives in Classical Emulsions: A New Method of Emulsion Stabilization?

Classical (surfactant-stabilized) and Pickering (particle-stabilized) type emulsions have been widely studied for applications to industries ranging from cosmetics, to food sciences and petroleum technology.  Pickering emulsions are formed when particles assemble at the oil-water interface between emulsion droplets and the continuous phase, analogous to surfactant stabilization.  These particles must be amphiphilic, as to prefer the oil-water interface over either of the continuous phases.  This amphiphilicity is commonly achieved through the surface modification of hydrophilic particles with a hydrophobic ligand (or vice versa) or through tuning of the ionic strength of the water phase as to decrease electrostatic repulsion between charged hydrophilic particles, thereby increasing their hydrophobicity.  Recently a synergistic effect has been reported in which interactions between nanoparticles and surfactants enabled stable emulsions.  In this work, we further explore this synergy through the study of bare silica particles as additives to classical emulsions to enhance emulsion stability.   These silica particles are fluorescently labelled by incorporation of a fluorescent dye into the core of the particle, enabling imaging of the particles by laser scanning confocal microscopy with greater confidence than by use of a passive fluorescent tag post-synthesis.  This imaging reveals that the particles do not assemble at the oil-water interface, depending on the surfactant utilized, but nevertheless impact the properties and stability of the formed emulsion.  Steric hindrance leading to limited droplet coalescence is attributed to these changes.  Moreover, confocal rheology (simultaneous confocal microscopy and rheology) experiments show significant differences in how the purely classical and particle-added classical emulsions respond to shear due to interparticle and particle-droplet interactions at high shear stress.  This discovery has important practical implications, as particles that do not require surface modification or a specific ionic strength for the stabilization of emulsions would be cheaper and simpler to use.