Molecular Dynamics Simulation Study On The Liquid Or Super-critical Carbon Dioxide Microemulsion Constructed By 4FG（EO）₂ Surfactant

Water in liquid or supercritical carbon dioxide（sc CO₂）microemulsion,i.e.w/c microemulsion,is a thermodynamically stable mixed system having nano-scale water core structure,which has great potential in synthesizing nanomaterials,intensifying chemical reactions,ex-tractions,and dry-cleaning processes etc.However,a relatively high pressure is often required to form the w/c microemulsion and restricts its industrial implementation.Adding additives is an effective way to reduce the pressure,and an in-depth understanding of its mechanism is prerequisite for promoting the industrialization of microemulsions.On the other hand,increas-ing the water content（W₀）of w/c microemulsions can bring more opportunities for developing new applications,but the current knowledge concerning the formation and stability of high-W₀microemulsions is limited.In addition,the green solvent ionic liquid（IL）has the virtues of extremely low vapor pressure,non-flammability,and high thermal stability.To construct a new type of IL/c microemulsion can vigorously combine the desirable properties of both ILs and CO₂,but studies focused on its microscopic properties are only handful,which is not favorable for developing applications.The content of this thesis was designed based on the above three aspects,and the molecular dynamics（MD）simulation method was adopted to investigate the w/c and IL/c microemulsion system formed by 1,5-bis（1H,1H,2H,2H-perfluoroalkoxy）-1,5-di-oxolane-2-sodium sulfonate（n FG（EO）₂,n=4 denotes the length of the fluorinated chain）.The purpose is to achieve an atom-level comprehension about the thermodynamic properties of dif-ferent systems,which can serve as invaluable inputs for guiding theoretical and experimental studies,as well as developing novel applications.The main contents are as following:（1）For understanding the microscopic mechanism underlying the macroscopic phenomenon that sodium p-alkylbenzoate hydrogels（BA0,BA2 and BA8）with different branched alkyl lengths can reduce the pressure required to form the 4FG（EO）₂-based w/c microemulsion,the H₂O+4FG（EO）₂+sc CO₂ ternary and H₂O+4FG（EO）₂+BA0/BA2/BA8+sc CO₂ quaternary sim-ulation systems were designed.A systematic investigation was conducted on the self-assem-bling of the microemulsion droplet,its equilibrium morphology and detailed internal structure,the packing behavior of 4FG（EO）₂,the microscopic interactions within the interface,as well as the potential of mean force between two aggregates.The results show that the hydrogels can speed up the self-assembling process,and the acceleration is progressively intensified with the increase of the branched alkyl length,which complies with the pressure reducing effect ob-served by experiments.The hydrogels are distributed within the equilibrium aggregate and do not affect its shape or structure.However,each additive has distinct conformation behavior and distribution characteristics.To be specific,BA0 adopts a conformation perpendicular to the interface,while BA2 tends to self-associate and BA8 has a distribution featured large radial fluctuation.The different conformation and distribution characteristics of the three hydrogels mentioned above can effectively enhance the stiffness of 4FG（EO）₂ tail,thereby improve the stability of the microemulsion droplet,and ultimately lead to the positive synergistic（pressure reducing）effect between the hydrogel and 4FG（EO）₂ in constructing the w/c microemulsion.In particular,the interfacial fluctuation behavior of BA8 can best reinforce the stability of the microemulsion droplet.（2）To ascertain the essential reason for the experimental observation that 4FG（EO）₂ can successfully construct the w/c microemulsion with W₀ up to 80,the H₂O+4FG（EO）₂+sc CO₂ternary simulation systems with W₀ being equal to 10,20,40 and 80 were designed.A thorough comparative study was performed concerning the self-assembling of the microemulsion aggre-gate,its overall morphology and detailed structure.The focus was put on probing the structure of the interfacial region,where the distribution stability of H₂O and CO₂ were respectively analyzed as W₀ varies.The results show that a stable microemulsion droplet can be formed in each system.As W₀increases,the surface of water core is exposed to a larger extent and be-comes more corrugated,indicating a reduced interfacial tension and hence an improved stability of the aggregate.Both H₂O and CO₂ can insert between 4FG（EO）₂ to constitute a mixed inter-facial region,which is thickened as W₀ becomes larger.Furthermore,the radial density gradient of H₂O and CO₂ are maximized near the most probable radial position of the proximal carbonyl oxygen in 4FG（EO）₂.This indicates that the proximal carbonyl oxygen is an effective CO₂-philic group,which can attract CO₂ molecules and"squeeze out"water molecules,causing CO₂to accumulate at the interface.Both the accumulation amount and distribution stability of CO₂are enhanced as W₀ increases.This will intensify the molecular mixing between H₂O and CO₂,and hence strengthen their mutual attraction,which is beneficial to reduce the interfacial tension and increase the fluidity of the interfacial film.Consequently,the interfacial accumulation of CO₂ will soften the interface and enlarge the radius of spontaneous curvature,which promotes the formation and stability of w/c microemulsion droplets with extremely high W₀.（3）Given the excellent performance of 4FG（EO）₂ in constructing w/c microemulsions,it is further employed to form the IL/c microemulsion,which is compared with the system built by N-ethyl perfluorooctylsulfonamide（NEt FOSA）surfactant.1,1,3,3-tetramethylguanidinium ac-etate（[TMG][AC]）was selected as the IL component,and the binary[TMG][AC]+CO₂ as well as the ternary[TMG][AC]+4FG（EO）₂/NEt FOSA+CO₂ simulation system was devised here.The study was focused on investigating the microscopic properties of the microemulsion drop-let arising from[TMG][AC]split by the surfactant,such as the overall morphology,the internal structure,and the properties of[TMG][AC]within the inner core.The results show that the[TMG][AC]/c microemulsion droplet constructed by 4FG（EO）₂ and NEt FOSA are both ellipsoidal.Furthermore,the shape and size of the aggregate containing 4FG（EO）₂ are not much affected by the amount of solubilized ionic liquid,which exhibits a certain"shape memory"feature.Compared with the bulk property,the coordination strength between the[TMG]⁺and[AC]^-distributed in the 4FG（EO）₂-based droplet is significantly weaker,which surprisingly possesses certain crystalline structural characteristics.However,[TMG]⁺and[AC]^-in the NEt-FOSA-based droplet are more strongly associated,which evinces that NEt FOSA has a certain"confining effect"on the[TMG][AC].The aforementioned"shape memory"feature of the ag-gregate is related to the crystalline structural traits of[TMG][AC]in the core,which is essen-tially due to the fact that Na⁺and the sulfonate headgroup of 4FG（EO）₂ can form a very stable network structure.