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

[Image: see text] 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.