Study On The Microstructure And Aggregation Behavior Of Several Types Of New Microemulsions

Microemulsion is a thermodynamically stable and isotropic dispersion system generated by two immiscible liquids in the presence of surfactant molecules.It has the advantages of large mass transfer area,high transfer rate,and easy preparation.Because of these advantages,it can be widely used as a microreactor in the synthesis of nanostructured materials,encapsulation and delivery of drugs,extraction and separation,etc.In recent years,some intelligent new microemulsions have been reported,such as light response,temperature response,CO₂ response,p H response,high temperature stability,surfactant-free and other types of microemulsions.The development of these new microemulsion systems is of great significance for solving problems such as difficult demulsification,small reaction application range,harsh reaction conditions,and cumbersome separation operation process.However,there are few studies on the microstructure and aggregation behavior of these microemulsions,and the interaction mechanism is not clear,which makes it difficult for molecular design.As a part of the National Natural Science Foundation of China（NO.21773058）,this thesis focuses on three types of microemulsions including high temperature stable,temperature responsive,and surfactant-free microemulsion.By using molecular simulation method,the microstructure and aggregation behavior of these systems were studied.The specific research work is as follows:1.The microemulsion[C₄mim][PF₆]/[C₁₂mim]Cl/EOAN system with high temperature stability was simulated by molecular dynamics simulation,and the interaction energy,radial distribution function,combined distribution function,spatial distribution function and coordination number of various ions in the system were calculated.The results show that after adding ionic liquid[C₄mim][PF₆],the ionic liquid[C₄mim][PF₆]is surrounded by the hydrophobic end of the surfactant.Then the[C₄mim][PF₆]/[C₁₂mim]Cl/EOAN high-temperature microemulsion system was compared with the traditional microemulsion[C₄mim][PF₆]/[C₁₂mim]Cl/H₂O,and the sizes of microemulsion aggregates with different[C₄mim][PF₆]contents were calculated and analyzed.The results show that with the increase of ionic liquid[C₄mim][PF₆],the size of the traditional microemulsion gradually increased,while the size of the high-temperature microemulsion[C₄mim][PF₆]/[C₁₂mim]Cl/EOAN firstly increased,and then decreased,lastly increased.In addition,by comparing the radial distribution function,combined distribution function,spatial distribution function and coordination number of these two microemulsion systems,it is found that as the content of[C₄mim][PF₆]increases,the[C₄mim][PF₆]/[C₁₂mim]Cl/EOAN microemulsion system of the anions[PF₆]^-,[NO₃]^-,and Cl^-around the cations distribution changed,so that the microemulsion still maintains a steady state.This is also an important reason why the microemulsion can keep its stable structure at higher temperatures without breaking the emulsion.2.By using molecular dynamics simulation and dynamic light scattering technology,the microstructure and aggregation behavior of temperature-responsive microemulsion[P_444,n]Br（n=8,10,12,14,16,18）/[C12mim]Br/H₂O were studied.By analyzing the trajectory files of different microemulsion systems,it is found that as the alkyl chain length of[P_444,n]⁺increases from 8,12 to 16,the non-polar regions of[P_444,n]⁺and[C₁₂mim]⁺became closer and closer,the interwinding of the non-polar chains became stronger,the molecular arrangement changes from loose to tight,and the cluster size becomes smaller.Then by analyzing the interaction energy,radial distribution function and coordination number of different systems,the results show that the long alkyl chain of temperature-responsive ionic liquids can enhance the van der Waals interaction between cations and anions,and weaken the temperature-responsive ionic liquids and water molecules,resulting in the reduction of cluster size.3.Molecular dynamics simulation was used to simulate the DMSO/H₂O/Butanol surfactant-free microemulsion.By comparing the interaction energy and spatial distribution function of the three different microstructures of the microemulsion system O/W,W/O and BC,the results show that butanol acts as the non-polar phase,H₂O as a polar phase and DMSO as a surfactant in the surfactant-free microemulsion system.During the formation of the microemulsion,the change of hydrogen bond between DMSO and H₂O plays a leading role in the system.By analyzing the changes in the radial distribution function,spatial distribution function,coordination number,hydrogen bond and interaction energy between the molecules in the mixed system under different temperature,the results show that the hydrogen bond between DMSO and H₂O in the system is weakened to a relatively large extent with the change of temperature,resulting in a reduction in the size of the aggregates.Therefore,we can easily achieve emulsification and demulsification without the presence of surfactants.