Study On Surfactant-free Microemulsions And Their Temperature-responsiveness

Microemulsion are defined as optically isotropic,thermodynamically stable and transparent soft colloidal dispersion of water?polar phase?,oil?non-polar phase?and emulsifier.It's generally considered that surfactants and co-surfactants?such as medium-chain alcohols?are used as emulsifier for stabilizing microemulsions,such microemulsions are called“surfactant-based microemulsions”?SBME?.In recent years,it has been found that a ternary mixture of two immiscible liquids?polar phase and non-polar phase?and amphiphilic solvent can also form a dispersed small droplets of optically isotropic,thermodynamically stable and transparent in specific area of phase diagram,this ternary system is called“surfactant-free microemulsion”.Stimuli-responsive surfactants can be reversibly switched between active and inactive with certain stimuli.Therefore,emulsions stabilized by stimuli-responsive surfactants can be intelligentlyswitched between“emulsification?open?”and“demulifiaction?close?”through smaller externalstimuli.Compared with emulsions,thermodynamically stable SBME are more difficult for demulifiaction.To the best of our knowledge,switchable SBME have been to date triggered by light,magnet,pH?CO₂/N₂?and redox reaction.Owing to limited understand of stimuli-responsive SFME,stimulus-responsive SFME is confronted with more challenges.Only CO₂/N₂ stimulus-responsive SFME have been reported.It was reported that the main driving force for the formation of SFME is the weak interaction between molecules.However,the magnitude of this weak interaction between molecules is related to the temperature,so we guess that temperature has a certain effect on the phase behavior of SFME.In this paper,we studyed surfactant-free microemulsions and their temperature responseness.Firstly n-octanol-water-ethanol SFME was considered as a research object.Whether the phase separation of microemulsions can be achieved by changing the temperature,and further study whether the phase separation is complete and whether this regulation is reversible.It was found that a minor fluctuation in temperature induced the microemulsion could be reversibly switched between"demulifiaction"and"emulsification",but there was a drawback of incomplete demulsification.Furthermore,the influence of changing the non-polar phase and amphiphilic solvent in the SFME system on the temperature response of SFME was investigated,and finally the SFME that could achieve complete phase separation by changing the temperature,namely the n-butanol-DMSO-water SFME system.Therefore,n-butanol-DMSO-water SFME and its temperature responseness was systematically studied.Within the scope of this paper,the main research contents and conclusions are summarized as follows:?1?Study on n-octanol-ethanol-water SFME and its temperature-responsivenessIn the single-phase region of n-octanol-ethanol-water SFME phase diagram,the pre-ouzo zone was confirmed by dynamic light scattering?DLS?,and the type of microemulsion was judged via the conductivity and polarity probe methods.The effects of temperature on the phase behavior and droplet size of the n-octanol-water-ethanol microemulsion system were systemically evaluated by the ternary phase diagram and DLS techniques.The results showed upon increasing temperature from 10?to 40?,the area percentage of single-phase calculated on the basis of the phase diagram to increase linearly from 47.5%to 53.5%,and the temperature sensitive coefficient k=1.78×10^-3.The area of pre-ouzo zone decreases accompanied by a decrease in the droplet size.Moreover,the critical point gradually draws close to the n-octanol corner with increasing temperature.When one formulation is far away from the demixing border,the droplet size can be reversibly and precisely regulated by changing temperature.When one formulation is located on the vicinity of the boundary,a minor variation in temperature can induce a prominent phase transition between Winsor IV?high temperature?and Winsor II?low emperature?.Such a temperature-responsive SFME can be used as a microreactor for Knoevenagel condensation.The reaction was carried out at 35?.After the temperature was cooled to 25?,the microemulsion was demulsified.The products were precipitated in the lower phase and were collected from the water phase by simple filtration at 25?.When temperature was increased to 35?again,microemulsion reformed and could be recycled,.Such a reaction cycle can be performed at least nine times,and the product yield after the second reaction cycle can be as high as about 95%.?2?Study on n-butanol-DMSO-water SFME and its temperature-responsivenessThe ternary phases of n-butanol-amphiphilic solvent?ethanol,DMF,DMSO?-water SFME and non-polar phase?n-butanol,n-pentanol,n-octanol?-ethanol-water SFME at different temperatures were plotted.The temperature sensitivity coefficient of n-butanol-DMSO-water SFME is the largest,indicating that its temperature response is better.Therefore,the n-butanol-DMSO-water SFME and its temperature response were studied.In the single-phase region of n-butanol-DMSO-water SFME phase diagram,the pre-ouzo region was confirmed by dynamic light scattering?DLS?,and the type of microemulsion was determined by ultraviolet spectroscopy.The effect of temperature on the phase behavior and droplet size of the n-butanol-DMSO-water SFME system was systematically evaluated by ternary phase diagram and DLS.The results showed with increasing temperature from 20?to 40?,the area percentage of single-phase calculated on the basis of the phase diagram to increase linearly from72.9%to 79.5%,and the temperature sensitive coefficient k=1.78×10^-3.The critical point gradually draws close to the n-butanol corner and the droplet size and the pre-ouzo zone decreases with increasing temperature.However,when the microemulsion formula is near the boundary,the system can be reversibly switched between a monophasic microemulsion and nearly complete phase separation by regulating temperature.When the temperature rises from 20?to 40?,the interfacial tension of the system decreases from 1.19 mN·m^-1 to 0.52 mN·m^-1,which explains that when the temperature decreases,the microemulsion will undergo phase separation.Such temperature-responsive SFME can be used as recovering silver nanoparticles.The size of the recovered silver nanoparticle can be controlled.What's more,it has excellent catalytic performance without any extra process comparing with silver nanoparticle recycled from cationic SBME.