CO₂-Responsive Pickering Emulsion

Pickering emulsions have been widely studied and applied in food,cosmetics,medicine,material synthesis,petroleum industry and catalysis due to their excellent properties.In many cases,we want the emulsion to be as stable as possible,but in many industrial processes,such as emulsion polymerization,oil transportation,nano-material preparation and fuel production,it is usually only necessary to form a temporarily stable emulsion,which is then demulsified on demand.Thus,how to resolve the conflict between the long-term stability and the rapid demulsification of the emulsions has been the focus of Pickering emulsion researches.The emergence of stimuli-responsive Pickering emulsions provides a new strategy and has become a research hotspot in recent years.So far,stimulus-responsive triggers that have been reported include pH,temperature,light,magnetic field,redox and CO₂.Among these triggers,CO₂ has the advantages of non-toxic and harmless,low cost,good biocompatibility and easy to remove from the system.It is a environmentally friendly stimulus-responsive trigger.However,this mild trigger has not received resonable attention in the field of Pickering emulsion studies,and various problems exist such as difficulty in surfactant synthesis,singleness in microstructure and phase behavior of the emulsion,and harsh regulation conditions.Therefore,it is nacessary to develop new Pickering emulsifiers that respond to CO₂.Based on the above analysis,this work intends to design and synthesize new ionic liquids and functionalized nanomaterials as emulsifiers for preparing CO₂-responsive Pickering emulsions,to study regulation characteristics and mechanism of CO₂ for microstructure and phase behavior of Pickering emulsions at ambient temperature and pressure,and to explore their possible applications.The main contents are as follows.1.In order to develop a cheap and readily available CO₂-responsive Pickering emulsion system,a series of CO₂-responsive ionic liquid precursors N-alkylimidazole([C_nim],n=6,8,10,12,14)was designed and synthesized,where[C_nim]could be easily protonated in aqueous solution saturated with CO₂ and transformed into ionic liquid N-alkylimidazole bicarbonate([C_nim][HCO₃]).How[C_nim][HCO₃]and SiO₂ synergistically stabilize a mixture of n-decane and water to form a stable Pickering emulsion,and how alkyl chain length and concentration of the ionic liquids affect the microstructure of emulsion and particle size of emulsion droplets were systematically studied.The reversible regulation of microstructure and phase behavior of Pickering emulsion by CO₂/N₂ and its microscopic mechanism was also studied by various methods such as zeta potential,water contact angle,adsorption amount,¹³C-NMR and FT-IR.2.Monooctylsilane-functionalizedSiO₂nanosphere(SM-O),monoaminosilane-functionalized SiO₂ nanosphere(SM-BIS),and octyl-and bis(2-hydroxyethyl)-3-amino-bifunctionalized SiO₂ nanosphere(SM-O-BIS),were designed and synthesized to construct CO₂-responsive Pickering emulsions with various oil-water mixtures,by balancing hydrophobicity of the alkyl chain and the hydrophilicity of the amino group using CO₂.It was shown that SM-O-BIS was the best CO₂-responsive emulsifier for oil-water mixtures.The effects of emulsifier mass fraction and oil phase polarity on emulsion formation and emulsion droplet size were also investigated.It was found for the first time that the Pickering emulsion stabilized by SM-O-BIS could be easily and reversibly inverted from water-in-oil(w/o)to oil-in-water(o/w)by alternately bubbling of CO₂ and N₂ at room temperature and atmospheric pressure.By measuring the changes in zeta potential,contact angle,¹³C-NMR,and FT-IR data before and after the reaction of emulsifier and CO₂,the possible switching mechanism of CO₂/N₂ for the phase inversion of Pickering emulsion was revealed.Furthermore,based on the phase inversion characteristics of the CO₂-triggered Pickering emulsions,the encapsulation and release of curcumin molecules were achieved.3.By using the Williamson ether synthesis method,bromoalkylimidazole was chemically grafted on a metal-organic frameworks(MOF)material UiO-66-(OH)₂ to synthesize alkyl imidazole-functionalized MOF UiO-66-(OH)₂-C_nim(n=8,10,12).The effects of the content of these porous hybrid materials,alkyl chain length and the introduction of CO₂ on the microstructure of Pickering emulsion were studied.At room temperature and pressure,reversible emulsification and demulsification of the Pickering emulsion stabilized by MOF was realized for the first time by using CO₂.Combined with zeta potential,water contact angle,¹³C-NMR,interfacial tension and other testing methods,the possible mechanism of CO₂/N₂-switchable emulsification and demulsification of Pickering emulsion was inffered.4.Aiming at the currently reported problems of harsh synthesis conditions and difficult large-scale preparation of functional MOF emulsifiers,triethylene tetramine(TETA)-functionalized ZIF-90 was designed and synthesized with the post-modification strategy at room temperature and pressure,and then used to emulsify n-hexane-water mixture for the formation of stable Pickering emulsions.The main factors affecting the microstructure of Pickering emulsion and its responsive performance to CO₂ were investigated.It was found that ZIF-90/TETA stabilized Pickering emulsion could be reversibly switching between emulsification and demulsification by alternately bubbling and removing of CO₂ under atmospheric pressure.At the same time,a variety of test methods were used to study the possible switching mechanism.In addition,by utilizing the CO₂-switchable Pickering emulsion,highly efficient Knoevenagel reactions,product separation and emulsion components recycling were integrated to achieve a sustainable chemical process.