Magnetic-Patchy Janus Colloid Surfactants for Reversible Recovery of Pickering Emulsions

We present a straightforward and robust method for the synthesis of Janus colloid surfactants with distinct amphiphilicity and magnetic responsiveness. To this end, hydroxyl-functionalized amphiphilic Janus microparticles (JMPs) are synthesized by seeded monomer swelling and subsequent photopolymerization. By incorporating controlled amounts of hydroxyl groups on poly(styrene-co-vinyl alcohol) seed particles, we adjust the interfacial tension between the seed polymer and the poly(tetradecyl acrylate) secondary polymer (γ₁₃). From theoretical and experimental observations, we verify that when γ₁₃ is tuned to ∼8.5 mN/m in a medium with controlled solvency, which corresponds to a 0.6 volume fraction of ethanol in water, the particles bicompartmentalize to form oval or ellipsoidal JMPs with controllable bulb dimensions. We also show that bulb site-specific patching of magnetic nanoparticles (NPs) can be achieved using the electrostatic interaction between the polyethylenimine-coated bulb surface and the polyvinylpyrrolidone-stabilized Fe₂O₃ NPs. Finally, we demonstrate that our magnetic-patchy JMPs can assemble at the oil-water interface, enabling magnetic-responsive reversible recovery of Pickering emulsions.