Illuminating the Impact of Nanoparticle Size and Surface Chemistry on Interfacial Position

In this manuscript we begin by preparing bulk Pickering emulsions of water:octanol at varying aqueous pH values, using ionizable carboxyl polystyrene nanoparticles of 320 nm and 810 nm diameter. Remarkably we observe two emulsion phase inversions, one resulting from an increase in pH and a second due to an increase in particle size. To illuminate the mechanism of the macroscopic phase inversion, we turn to the super resolution microscopy technique interface Point Accumulation for Imaging and Nanoscale Topography (iPAINT) to measure in-situ the contact angles of single carboxyl polystyrene particles at the water:octanol interface. Importantly, we identify a significant decrease in contact angle over the increase of pH and particle diameter, respectively. Additionally, the non-negligible dependence of contact angle on particle size was shown to hold regardless of particle surface chemistry.