Probing the Interactions between Pickering Emulsion Droplets Stabilized with pH-Responsive Nanoparticles.

The presence and adsorption of particles at the oil/water interface play a critical role in stabilizing Pickering emulsions and affecting their bulk behavior. For water-in-oil (W/O) and oil-in-water (O/W) Pickering emulsions with pH-responsive nanoparticles, their interaction forces and stabilization mechanisms at the nanoscale have not been reported. Herein, the Pickering emulsions formed by oil/water mixtures under different pH values with bilayer oleic acid-coated Fe3O4 nanoparticles (Fe3O4@2OA NPs) were characterized using microscopy imaging and zeta potential and interfacial tension (IFT) measurements. The interaction forces between formed emulsion droplets were quantified using an atomic force microscope (AFM) drop probe technique. A W/O emulsion formed at pH 2 and 4 is mainly stabilized by the steric barrier formation of confined particle layers (with Fe3O4@2OA NPs and aggregates). At pH 9 and 11, an O/W emulsion is formed, and its stabilization mechanism is mainly due to relatively low IFT, strong electrostatic repulsion due to carboxyl groups, and steric repulsion from confined nanoparticles and aggregates, leading to a stable confined thin water film. Increasing the maximum loading force and dwelling time enhances the confinement of Fe3O4@2OA particles and aggregates at the oil/water interface. This work provides useful insights into the interaction and stabilization mechanisms of Pickering emulsions with stimuli-responsive interface-active particles.