Reversible Self-assembly Of Stimulus-responsive Surfactants And Nanoparticles At Oil-water Interface

In the past decades,the self-assembly of stimuli-responsive surfactants or particles at the fluid interface has been attracted considerable attention with the rapid development of nanoscience and technology.Switchable surfactants can be transformed between on and off states under appropriate triggers,including pH,light irradiation,magnetic fields,temperature,redox,CO₂/N₂.To date,the interaction between opposite charged particles and surfactants has been widely studied.However,the reports about the interaction between same charged particles and surfactants are quite few.Recently,our group first proposed the interaction of surfactants with the same charged inorganic nanoparticles to construct a novel emulsion?non-Pickering emulsion?.In this emulsion system,an ionic surfactant is adsorbed on the oil/water interface,and the surface of the droplets is charged,while the nanoparticles dispersed in the continuous phase increase the thickness of the continuous phase liquid film.The electrostatic repulsion between particles prevents the droplets and the oil droplets from agglomerating and stabilizes the novel emulsion.Based on these academic conditions,this article attempts to study the self-assembly behavior of inorganic nanoparticles with different charged stimuli-responsive surfactants to construct stimuli-responsive Pickering emulsion or novel emulsions.The main results are summarized as follows:?1?Redox and pH dual stimuli-responsive Pickering emulsionA sodium carboxylate-derived selenium surfactant(C₁₀-Se-C₁₀·?COONa?₂)in combination with positively charged alumina nanoparticles were applied to formulate Pickering emulsion.Multistimuli responsive Pickering emulsions can be stabilized by C₁₀-Se-C₁₀·?COONa?₂ and alumina nanoparticles at concentrations as low as 0.001 mmol/L and 0.1 wt.%,respectively.Rapid and efficient demulsification can be successfully achieved by both pH and redox triggers.The Pickering emulsion can be reversibly switched between “on?stable?”and“off?unstable?”states by both pH and redox stimuli for several times.?2?Redox and pH dual stimuli-responsive novel emulsionC₁₀-Se-C₁₀·?COONa?₂ in combination with negatively charged silica nanoparticles were applied to formulate novel emulsion.Multistimuli responsive novel emulsions can be stabilized by C₁₀-Se-C₁₀·?COONa?₂ and silica nanoparticles at concentrations as low as0.0003 mmol/L and 0.0001 wt.%,respectively.Rapid and efficient demulsification and emulsificationcan be successfully reversibly switched by both pH and redox triggers for several times.In the novel emulsion,the surfactants were adsorbed at the oil-water interface,and the nanoparticles were distributed in the continuous phase?aqueous phase?.On the one hand,the nanoparticles increase the thickness of the water film and reduce the van der Waals'gravity between oil droplets.On the other hand,the electrostatic repulsion between particles-particles,droplets-droplets,and particles-droplets prevents aggregation and coalescence between oil droplets,so ionic surfactants in combination with the same charged particles can stabilize the novel emulsion,which also has dual redox and pH stimulus response properties.?3?Light stimuli-responsive novel emulsionPositively charged alumina nanoparticles in combination with a photo-responsive cationic surfactant 4-butyl-4'-?4-N,N,N-trimethylhexyloxyammonium bromide?azobenzene?C₄AzoC₆N?were applied to formulate a photo stimuli-responsive novel emulsion.Rapid and efficient demulsification and emulsificationcan be successfully reversibly switched by both UV/blue light triggers at least 10 times.The regulation will not introduce new chemicals into this system,which offers a green,efficient,and recyclable strategy for oil transportation and microfluidics.?4?Novel emulsion and Pickering emulsion reversible conversionA novel pH stimuli-responsive surfactant 11-tertiary amine sodium undecanoate(?CH₃?₂N-C₁₀-COONa)was successfully synthesized.Since the charge of the head group in CH₃)₂N-C₁₀-COONa reversible transformed between negative,neutral,and positive through pH variation,the type of the surfactant can be switched between anionic,zwitterionic,and cationic forms.Not only rapid demulsification can be achieved,but also the emulsion can be reversible switched between Pickering emulsion and“non-Pickering”emulsion,in which the nanoparticles at the oil-water interface are transformed between interfacial adsorption and non-interfacial adsorption.This strategy endows the emulsion with a capacity for intelligent and precise control of stability/demulsification as well as the viscosity and droplet size.It may offer a generic model which enables one emulsion to satisfy the requirements in different applications,such as emulsion polymerization,biphasic catalysis,cutting fluids,and oil pipeline transportation.