Illuminating the Impact of Submicron Particle Size and Surface Chemistry on Interfacial Position and Pickering Emulsion Type

Pickering emulsions are increasingly applied in the production of medicines, cosmetics, and in food technology. To apply Pickering emulsions in a rational manner it is insufficient to examine properties solely on a macroscopic scale, as this does not elucidate heterogeneities in contact angles (θ) of individual particles, which may have a profound impact on stability and microstructure. Here, we apply the super-resolution technique iPAINT to elucidate for the first time the microscopic origins of macroscopically observed emulsion phase inversions induced by a variation in particle size and aqueous phase pH. We find θ of single carboxyl polystyrene submicron particles (CPS) significantly decreases due to increasing aqueous phase pH and particle size, respectively. Our findings confirm that θ of submicron particles are both size- and pH-dependent. Interestingly, for CPS stabilized water-octanol emulsions, this enables tuning of emulsion type from water-in-oil to oil-in-water by adjustments in either particle size or pH.