Preparation And Properties Of CO₂-responsive Microemulsion

Microemulsion has the characteristics of nanometer particle size,ultra-low interfacial tension,stable thermodynamics,high water-oil solubilization and emulsifying capability,etc.It is widely used in the fields of chemical reaction medium,drug delivery,fine chemical industry,daily chemical industry,oil exploitation,etc.However,it is very difficult to achieve demulsification after being used due to the thermodynamics and stability of microemulsion,which is not conducive to resource recovery.It is easy to cause secondary environmental pollution and waste of resources.In this study,a microemulsion system based on reversible solvent was prepared by using N,N-Dimethylcyclohexylamine（CO₂-switching solvent）,Sodium dodecyl sulfate（SDS）,n-butanol and water as raw materials.By alternately bubbling through CO₂/N₂,the reversible switching of microemulsion wasformated,thus demulsification was achieved.The microstructure of the microemulsion was studied by the conductivity method and the dyeing method.The effects of the main factors such as salt concentration,temperature,the mass ratio of the surfactant/cosurfactant on the phase behavior were investigated.The reversible switching characteristics of the prepared microemulsion by the dynamic and static light scattering and electrical conductivity were verified.The reversible switching mechanism of the microemulsion was analyzed by X ray diffraction（XRD）,and the HLB value of the microemulsion was calculated according to the experimental results and the theoretical calculation formula of HLB.The main conclusions are as follows:（1）It is feasible to the switchable microemulsion was prepared with N,N-dimethylcyclohexylamine（CO₂ switching solvent）,sodium dodecyl sulfate（SDS）,n-butanol and water as raw materials.（2）The conductivity and dyeing methods showed that the microstructure of the microemulsion changed from W/O type to double continuous type to O/W type microemulsion with the increase of water content.（3）The smaller the area of microemulsion formed along with the increase concentration of NaCl solution.The area of ionic surfactant microemulsion was not affected by temperature.When the mass ratio of surfactant to cosurfactant was 1:1,the area of microemulsion formed was the largest.（4）The dynamic and static light scattering method and conductivity method were used to verify the reversible switching characteristics of the microemulsion system.（5）The reversible switching mechanism of the designed microemulsion system was characterized by X ray diffraction（XRD）:After bubbling CO₂ at 25°C,the reaction of N,N-dimethylcyclohexylamine was carried out under the condition of water participation to form N,N-dimethylcyclohexylamine bicarbonate.The tertiary amine bicarbonate and anionic surfactant SDS in the system reacted to produce neutral oil-like substance,which has surface activity and sodium bicarbonate.After bubbling N₂ at 65°C,the tertiary amine bicarbonate was easily decomposed into tertiary amine,the decrease of the tertiary amine bicarbonate maked the reaction balance toward the direction of the decomposition of the anion and cation complex,and the surfactant SDS was released slowly,and then the microemulsion was restored.（6）The suitable HLB value of the microemulsion prepared by SDS/n-butanol/N,N-dimethylcyclohexylamine/H₂O is between 22.80²8.55.In this study,a CO₂/N₂-swicthable microemulsion system has been designed to achieve demulsification under mild conditions,and the composition is simple.It can be used to recycle the components and solve the problems of demulsification and recycling.