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 Fe2 O3 NPs. Finally, we demonstrate that our magnetic-patchy JMPs can assemble at the oil-water interface, enabling magnetic-responsive reversible recovery of Pickering emulsions.