Rheological responses of Pickering emulsions prepared using colloidal hydrophilic silica particles in the presence of NaCl

Abstract

We investigated the effects of flocculated structures of silica nanoparticles on the interfacial and rheological properties of Pickering emulsions. Pickering emulsions were prepared using diisopropyl adipate and aqueous colloidal silica nanoparticles flocculated in the presence of different NaCl concentrations. Averaged silica particle sizes were measured by dynamic laser scattering measurements; and we obtained a critical flocculation concentration (cfc) determined as a NaCl concentration at which size of silica particles was steeply increased. The resulting emulsions were classified as oil-dispersed-in-water type, and characterized by measuring droplet diameters, oil volume fraction, the amount of adsorbed silica nanoparticles on droplet surfaces, and the stress–strain (S–S) sweep curves. The Pickering emulsions increased droplet size over time due to partial coalescence, except at the cfc of NaCl for silica nanoparticles. The droplet size at two weeks after preparation decreased as NaCl concentration increased, while droplet size increased for NaCl concentrations above the cfc. The resulting yield stress and critical shear strain determined from the S–S sweep curves increased with increasing NaCl concentration at or above the cfc. This can be attributed to the formation of a three-dimensional network of silica particles between the droplets.