Modification Of Nano-Fe₃O₄ By Conjugated Linoleic Acid And The Application

Conjugated linoleic acid（CLA）,a natural polyunsaturated fatty acid,has the advantages of green non-toxic and good biocompatibility.The carboxyl group of CLA makes it has p H response and weak reducibility.Besides,its sodium salt,sodium conjugated linoleate（SCL）,is a polymerizable anionic surfactant,and thus,both CLA and SCL possess simple self-assembly and self-crosslinking activities.Nowadays,CLA has received much attention.Meanwhile,nano-Fe₃O₄ has been broadly applied due to its good ferromagnetism and biocompatibility,but naked nano-Fe₃O₄ tends to agglomerate due to high surface energy and strong magnetism,and is easily oxidized in air resulting in demagnetization.At present,oleic acid is often used as a modifier of nano-Fe₃O₄.However,the outer layer of oleic acid is only modified by hydrophobic force on the surface of nano-Fe₃O₄ during bimolecular modification,which is easy to fall off the surface of nanoparticles with environmental changes,thus limiting its application in various aspects.In this paper,a series of multifunctional interfacial materials with different morphologies and structures are constructed using nano-Fe₃O₄ as base particles and SCL admicelle（SCA）as modification layers.The applications of these materials in Pickering emulsifiers,oil-water separation materials and interfacial catalysts are explored.The structure of nanoparticles mainly includes uniform nanoparticles and Janus nanoparticles,and the asymmetric structure and versatility of Janus particles have attractive application prospects in solid particle emulsifiers,interfacial catalysis,biomedicine and other fields.However,most Janus particles reported are obtained by prepolymer graft or in situ polymerization of polymer monomer and magnetic particle surface,and the thickness of modification layer is difficult to control.The size of Janus particles changed obviously before and after modification.In addition,Janus particles have good interfacial activity due to their special structure,which is mostly used as the carrier of precious metals and then used for Pickering interfacial catalysis.However,currently reported Au supported Janus catalysts are mostly prepared by reduction method,and reducing agents generally need to be added,which makes the preparation process of catalysts more complicated.In view of the current research status and existing problems,the main research contents and results of this paper are as follows:（1）Using the self-assembly and self-crosslinking activities of SCL,the SCL was self-assembled into an admicelle on the surface of nano-Fe₃O₄,which was then transformed into a self-crosslinked admicelle（SCA）by thermal polymerization,and finally the stable structure of SCA@Fe₃O₄ with p H/CO₂-N₂/Magnetism multiple stimulus response was synthesized.The Pickering emulsion stabilized with SCA@Fe₃O₄can change between emulsification and demulsification by adjusting p H value or bubbling CO₂/N₂ into the emulsion.Under an external magnetic field,the particles can be recovered and reused.The particle size of the stable emulsion has almost no change after 4 times of circulation,and the recovery of the particle is up to 98%.When SCA@Fe₃O₄ as the emulsifier is used to treat the oily wastewater,a formulated dilute oil-water dispersion（5 wt%）is concentrated into a high internal phase emulsion(V_oil=75%)by simply stirring with,and the magnetic Pickering emulsion is magnetically separated following broken by CO₂-bubbling to turn SCA@Fe₃O₄into the demulsifier,finally the CO₂-hypnotized SCA@Fe₃O₄ is regenerated by N₂-bubbling and used as the aroused emulsifier repeatedly.Moreover,liquid paraffin is recovered with excellent recyclability more than 95.5%in all four cycles.（2）By skilfully manipulating self-assembly and self-crosslinking activities of SCL,the true nanoscale Janus self-crosslinked admicelle-decorated nano-Fe₃O₄（SCA-Fe₃O₄）was synthesized under protection by Pickering emulsion.The interface activity/amphipathicity of SCA-Fe₃O₄ results in the unique low-energy emulsification（LEE）effect on Pickering emulsion preparation;its high detachment energy（E_d）ensures the emulsion ultra-stable against extreme environmental stresses of temperature（25-90℃）,salinity（0.1-1.8 M）and p H（1.8-11.7）.SCA-Fe₃O₄ indeed reduces the requirement for E_e and achieves an new energy-saving effect in Pickering emulsion preparation similar to that of LEE in nanoemulsion technology.The ultra-stable Pickering emulsion can be switched on–off by simultaneously applying an external magnetic field,adjusting acidic p H,along with the hydraulic effect provided by slow agitation,and reversibly tunable at least five times by the dual triggers.（3）Janus nano-Au catalyst（Au-SCA-Fe₃O₄）was synthesized by SCA surface bonding method,taking advantage of the p H sensitivity and weak reducibility of carboxyl array of SCA.This method solves the defect of additional reducing agents,and provides an economical and simple method for the preparation of nano-Au catalyst.Au-SCA-Fe₃O₄ was investigated to act as both emulsifier and catalyst in the Pickering interfacial catalytic oxidation of benzyl alcohol to benzaldehyde.As a microreactor,the contact area between the insoluble reactants and the catalyst was significantly improved by emulsion droplets.The catalytic activity of benzyl alcohol oxidation at 90℃is 2 times that of uniform nanoparticles,3 times that of non-emulsion,and 19 times higher than that of air oxidation.In addition,due to the non-rotatability of Au-SCA-Fe₃O₄ at the interface,the benzaldehyde catalyzed on the aqueous side of the interface was quickly extracted to the oil phase,avoiding its excessive oxidation to benzoic acid,making the selectivity of benzaldehyde in the reaction model higher than 99.9%.（4）A new type of Janus Au nanocatalyst with emulsifying,catalytic and magnetic responsiveness Au-SCA@Fe₃O₄ was prepared by SCA surface bonding method using SCA@Fe₃O₄ as a precursor.The structure-activity relationship of Au/Fe₃O₄ Janus nanocatalyst was investigated by using aqueous catalytic reduction of 4-nitrophenol and Pickering interfacial catalysis of 4-nitroanisol as reaction models.In the aqueous catalytic reduction reaction of 4-nitrophenol,compared with Au-SCA@Fe₃O₄,Au-SCA-Fe₃O₄ is more hydrophobic,which is conducive to the enrichment of organic phase,and increases the reaction rate.The TOF value is twice that of Au-SCA@Fe₃O₄.In the interfacial catalytic reduction reaction of Pickering emulsion with 4-nitroanisol,both of the two Au-Fe₃O₄ Janus nano-catalysts have good emulsification performance for toluene/water system,and stable Pickering emulsion can be formed when the dosage is only 0.04 wt%.In addition,the amphiphilicity and non-rotatability of Au-SCA-Fe₃O₄ at the interface avoid the rotation of Au-SCA@Fe₃O₄ catalyst in the process of reaction,increase the contact area between Au surface and reactant of Au-SCA-Fe₃O₄,so that the reaction rate is accelerated,and the TOF value is twice that of Au-SCA@Fe₃O₄.In terms of recycling,Janus catalyst can be used for water phase catalysis of 4-nitrophenol and emulsion interface catalysis of 4-nitroanisole,and can be recycled at least 4 times under the condition that the catalytic rate remains unchanged.