피커링 에멀젼 마이크로반응기 제조를 위한 야누스 콜로이드 계면활성제 합성 연구

Abstract

Janus particles are biphasic colloids that have two parts, which are on the opposite sides of the particles, with different compositions or properties. The advantage of using Janus particles is that their shape and chemical anisotropies are independently controllable, which allows us to give asymmetric functionality to the particles. The well-designed Janus particles have explored various applicabilities as the building blocks for supra-nanostructures, optical biosensors, functional surfactants, and carriers of biomacromoleclues. The Janus particles with chemical anisotropy have been usually synthesized by employing advanced technologies, including the macrophase separation during polymerization, controlled drop polymerization in microfluidic channels, and toposelective surface modification from particle monolayers. Moreover, shape anisotropy can be achieved by means of dewetting a bulb from the seed particle, controlling packing of hard spheres in confined drops, and by generating elastic force in cross-linked particles. To date, the successful development of uniform Janus particles still remains a challenge mainly owing to their limitation in large-scale productivity, complicated and time-consuming process. This study introduces a facile method for synthesis of catalytically activated Janus microparticles with controllable compartmented bulb dimensions as well as surface amphiphilicity. In chapter 2, we show that using the seeded monomer swelling and polymerzation technique allows us to obtain novel bicompartmentalized Janus micrparticles that are generated depending on the phase miscibility of the poly (alkyl acrylate) chains against the polystyrene seed, thus minimizing the interfacial free energy. When tetradecyl acrylate is used, complete compartmentalization into two distinct bulbs can be achieved, while tuning the relative dimension ratio of compartmented bulb against the whole particle. Finally, we have domonstrated that selectively patching the silica nanoparticles onto one of the bulb surfaces gives amphiphilicity to the particles that can assemble at the oil-water interface with a designated level of adhesion, thus leading to development of a highly stable Pickering emulsion system. In chapter 3, 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 are synthesized by seeded monomer swelling and subsequent photo-polymerization. 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 (g13). From theoretical and experimental observations, we verify that when g13 is tuned to ~8.5 mN/m in a medium with controlled solvency, which corresponds to the 0.6 volume fraction of ethanol in water, the particles bicompartmentalize to form oval or ellipsoidal Janus microparticles with controllable bulb dimensions. We also show that bulb site-specific patching of magnetic nanoparticles can be achieved using the electrostatic interaction between the polyethylenimine-coated bulb surface and the polyvinylpyrrolidone-stabilized Fe2O3 nanoparticles. Finally, we demonstrate that our magnetic-patchy Janus microparticles can assemble at the oil-water interface, enabling magnetic-responsive reversible recovery of Pickering emulsions. In chapter 4, we introduce a new colloid surfactant catalyst platform exhibiting remarkable catalytic activities in Pickering emulsion microreactors that can be reversibly recovered in response to the applied magnetic field. The colloid surfactant catalyst is fabricated by site-specific patching of catalytic nanoparticles (NPs), including Ag NPs and Pd NPs, respectively, on the surface of amphiphilic Janus microparticles (JMPs). The catalytic performance of JMP catalysts is confirmed through carrying out three model organic reactions; oxidation, amination, and reduction. As the JMP catalysts have amphiphilicity, they readily adhere to the reactant-water interfaces, thus producing structurally stable Pickering emulsions. We figure out that the reaction kinetics are controllable by tuning the concentration of catalytic NPs on the JMPs and the reaction temperature. When Fe2O3 NPs were co-patched, the JMP catalysts show magnetic responsiveness, thus enabling the repeated recovery of the products and catalysts by simple application of an external magnetic field. The catalytic activity of the recovered JMP catalysts remains unchanged even to five cycles. The JMP catalysts developed in our study are expected to advance the environment-friendly catalyst system for green chemistry.