CO₂-responsive Pickering Emulsion Based On Tertiary Amine And Amine Oxide

Pickering emulsions stablized by colloidal particles are typical soft materials,which play key roles in numerous fields,such as food processing,cosmetics formula,oil recovery,paints,and so forth.Compared to common emulsions stabilized by low-weight surfactants,Pickering emulsions only require a smaller amount of traditional emulsifier?even to zero?,and show higher stability because of the nearly irreversible adsorption of colloidal particles at the oil-water interface.Whereas,the high stability is not desirable sometimes after a particular stage.Therefore,responsive emulsions,whose stability can be smart controlled through a tiny external stimulus,emerge in response to the requirement.Recently,switching Pickering emulsion“on”?stable?and“off”unstable states using CO₂ as trigger has aroused considerable interesting.Here,we describe three CO₂-responsible Pickering emulsion systems based SiO₂nanoparticles,which were modified by N,N-dimethyl-N-dodecyl amine(C₁₂A),11-?benzylselanyl?-N,N-dimethylundecan-1-amine?SeTA?and myristylamidopropyl amine oxide(C₁₄PAO),respectively.Then,the stability of the emulsion at macro and micro levels was observed to investigate the CO₂-responsive performance of the emulsion.In addition,Zeta potential,contact angle,adsorption isotherm and the adsorption amount of SiO₂ on the surface of emulsion droplets were measured to explore the mechanism of emulsion's formation and its responsive behaviors.The concrete results are as following:1?C₁₂A was used to modify silica nanoparticles by controlled hydrophobic effect,and then CO₂-responsive Pickering emulsion was constructed.Upon alternately bubbling CO₂ and N₂ at a moderate conditions(35 ^oC,40 mL·min^-1),C₁₂A could be reversibly switched between cationic and nonionic forms,and then adsorbed on or desorbed from the surface of silica nanoparticles.As a consequent,when CO₂ was bubbled,interfacial active particles were formed,stabilizing the emulsion,or when N₂ was bubbled,interfacial active particles were disrupted,breaking the emulsion.Above a critical concentration,the Pickering emulsion can be formed over the whole concentration scope without interruption,even as zeta potential equals to zero,because of the unusual adsorption of the protonated C₁₂A.Such a method can be used to producing high internal phase Pickering emulsion for different oil,showing significant application value.2?We described for the first time a CO₂ and redox dual responsive paraffin oil-in-water Pickering emulsion stabilized by the modified silica nanoparticles with SeTA,in which the tertiary amine serves as a CO₂-sensitive group,and the Se atom serves as a redox-sensitive center.When CO₂ was bubbled,SeTA protonated into the positively charged cation?SeTA-CO₂?,and then SeTA-CO₂ was adsorbed on the surface of silica by electrostatic action,which endowed silica particles with amphiphilic property,stabilizing the emulsion.Next,when N₂ was bubbled,SeTA-CO₂ is reduced to uncharged SeTA,and then SeTA was desorbed by the surface of silica,which made silica particles lost amphiphilic property,breaking the emulsion.Moreover,after the addition of H₂O₂,the hydrophobic selenide is transformed into the hydrophilic selenoxide,which results in that SeTA-Ox can be adsorbed on the surface of silica nanoparticles through the hydrogen bond,forming interfacial active particles and stable emulsion.With the addition of reducing agent Na₂SO₃,the hydrogen bond disappeared with the transformation from selenoxide to selenide,damaging the interfacial activity of the particles and breaking the emulsion.However,in the presence of CO₂,redox can only induce a change in the droplet size,not demulsification.These interesting and unique multiresponsive behaviors endow the Pickering emulsion with a capacity for intelligent control of emulsification and demulsification,as well as the droplet size.3?A novel CO₂-switchable oil-in-water Pickering emulsion stabilized by functionalized silica nanoparticles with myristylamidopropyl amine oxide(C₁₄PAO)was developed. Upon alternately bubbling CO₂ and N₂ under mild conditions(30 ^oC,40 mL·min^-1), C₁₄PAO is reversibly switched between cationic and nonionic forms,and is thereby adsorbed on?forming interfacial active particles?or desorbed from?breaking interfacial active particles?the surface of the particles.In this way,interfacial active particles are formed and adsorbed on the surface of oil droplets,stabilizing the emulsion?CO₂?,or disrupted and desorbed from the surface of oil droplets,breaking the emulsion?N₂?.Switching the current system with CO₂/N₂ multiple times does not lead to any evident changes in either macroscopic appearance or microscopic size. Moreover,this CO₂-responsive Pickering emulsifier can be recycled when fresh oil was added after removing the original oil,and theoretically the cycling can be maintained.It offers a green,efficient,and recyclable container for oil product transportation.Such a strategy is also suitable for other amine oxide-based surfactants.