Shape-Tunable Biphasic Janus Particles as pH-Responsive Switchable Surfactants

We report a simple and robust strategy to prepare pH-responsive biphasic Janus particles composed of polystyrene/poly(2-vinylpyridine) (PS/P2VP) homopolymers that are able to control the reversible formation, breakage, and switching of Pickering emulsions depending on their geometry- and pH-dependent hydrophilic–lipophilic balance. The chemical stability of these PS/P2VP Janus particles was tuned through the incorporation of cross-linkable benzophenone units along the backbone of the homopolymers. By employing these stabilized particles as emulsifiers for toluene and water, a facile transformation of emulsion types (i.e., from toluene-in-water to water-in-toluene emulsions) was achieved by adjusting the pH of the aqueous phase. More importantly, this pH-dependent switching behavior and associated stability of the emulsions could be actively controlled by adjusting the relative size ratio of PS to P2VP. When the PS volume fraction (ϕPS) was between 0.33 and 0.67, a wide range tuning of emulsion phase including rapid and reversible pH-triggered emulsion inversion was achieved by the Janus surfactants. Finally, incorporation of iron oxide nanoparticles facilitated magnetic separation of oil droplets from O/W emulsions and recovery of the Janus particles, which represents a considerable advantage for these systems.