Theoretical And Simulation Studies Of The Cluster Formation And Its Effect On The Properties Of Mixed Ionic Liquid Systems

As a kind of promising solvent,ionic liquids（ILs）can be used in both the traditional chemical process and new energy field due to their unique physical and chemical properties,such as new extractants in extractive distillation and electrolytes in supercapacitors.In the first principle,all of the properties are determined by their structure.It was confirmed that various clusters exist in the systems containing ionic liquids by many experimental measures,which plays an important role in forming their unique properties.However,it is difficult to extract their information by current experimental means in details and dynamically.In constrast,molecular simulation,as an inherently atomistic-level method,can be used to conformational analysis,interaction energy breakdown analysis,cluster structure analysis,ion dynamic migration,etc.,which is not only the perfect complement to experiments,but also a well-developed predictive tool and indispensable method to investigate the mechanisms of physical phenomena.In this thesis,molecular dynamics simulations are used to study a few systems containing ionic liquids,with the following results:（1）The role of ionic liquid as entrainers in the separation of azeotropic systems.Ionic liquids can be widely used in extractive distillation processes of various azeotropic systems,while the related mechanisms at atomic level are not well explianed.We studied the interaction energies and the structure of clusters in the systems of acetone-methanol,a typical azeotropic mixture,with and without the ionic liquid of 1,3-dimethylimidazolium dimethyl phosphate（[C₁mim][DMP]）.It is found that the anions interact strongly with methanol molecules,forming tight cluster structures connected by hydrogen bonds,stabilizing them in the system,and finally eliminating the azeotrope of the mixture.The energy calculation shows that the interaction between the anion and methanol is stronger than the interaction methanol-methanol.The spatial distribution functions intuitively indicate that the two interactions are competing on occupying the interaction sites,so that the addition of DMP^-will destroy the structure of methanol clusters in the original system.After further cluster classification and statistics,it was found that DMP-methanol clusters with anions as the core would be formed in the mixed system,and the two O atom with double bonds of the anions could both connect with two methanol molecules.It is proved that the addition ratio of ionic liquid extractant is related to the number of hydrogen bond electron acceptors of its ions.In the separation of similar azeotropic systems,the appropriate anion type can be selected according to the theoretical hydrogen bond ratio,therefore the addition range of the extractant can be calculated.（2）The mechanism of ionic liquid+water systems as the"water-in-salt"electrolytes for supercapacitors.Ionic liquids are widely used as electrolytes in electric double-layer capacitors due to their good electrochemical and thermal properties.However,their hygroscopic nature can affect seriously the stable electrochemical window（EW）of the electrolyte.We found that the influence of water on the EW can be alleviated by modulating the structure and distribution around the water molecules in the mixtures,simply by chaning its hydrophobicity/hydrophilicity through appropriate combinations of anion/cation.In this study,molecular dynamics simulations of the[C₄mim][Tf₂N]/[C₄mim][OAc]/hybrid IL+water systems were performed to investigate the behavior of water molecules.The decomposition of the interaction energy proves that the strong interaction between H₂O and the hydrophilic anion OAc^-leads to a decrease in the excess enthalpy of the mixture.Sturetural and dynamic analysis revealed that the diffusion and distribution of H₂O molecules are strongly affected by the anion hydrophilicity.The presence of hydrophilic anions was found to reduce the formation of large-sized H₂O clusters,which is the mechanism for increasing the upper limit of the EW.In addition,simulation results also show that only a low percentage of OAc^-is required to effectively influence the environment around the water molecules and their cluster structure.Thus,the hybrid IL systems combines the advantages of higher mobility in Tf₂N^-type ILs with the wider EW of hydrophilic ILs.（3）The mechanisms of high conductivity in ionic liquid solid-state batteries.The computational simulations were conducted to study the solid-state electrolyte of polyborane B_nH_n/CB_nH_n+1 anions,which possess high ionic conductivity.It is found by ab initio molecular dynamics simulations that the hopping of Li⁺is accompanied by the rotation of anions at a temperature of liquid-like state.Among these electrolytes,Li CB₉H₁₀ has the highest lithium ionic conductivity for its low phase-transition temperature,so that it is the promising material for popularization and application and analyzed in this paper.The transition energy barrier of Li⁺is calculated through fixing the initial configuration of the anion before the transition and the final configuration after the transition.The energy barrier fixed to the initial configuration was found to be the highest,suggesting that the cage rotation facilitates the Li ion to hop easier.The energy barrier of anion rotation is calculated through fixing the Li⁺.It is found that the energy barrier is no more than 0.25 e V despite the various occupation configurations of Li⁺,which is a good reproduce of the experimental data.The two transition energy barriers of Li+are calculated by fixing the initial configuration of the anion before the transition and the final configuration after the transition,respectively.The lowest energy barrier is found when fixing the anion to the final configuration,which proves that the rotation and orientation of the anion contribute to the Li⁺transition.