Studies On Mechanical Properties Of Ionic Liquids Basing On A Custom Force Field

As an organic salt with unique physical and chemical properties, ionic liquidshave caused the sustained attention of the research community and industry. Giventhe need for a computer simulation study, we developed a fully automated force fieldgenerater basing on our custom all-atom force field framework. In our practice, thecustom all-atom force field achieves good coincidence with the quantum calculation.With the helping of custom force field and molecular dynamics method, severalionic liquids system had been studied by computer simulation. All of them arerepresentative in industrial applications. Based on their simulation results, thenano-structure of fluid and flowing properties were systematically studied, especiallythe viscosity. We found that several factors have effects on the mechanical propertiesof ionic liquids, including the environmental factors (temperature or pressure) andfluid subjective factors (molecular structure or contained impurities). Furthermore,the mechanical mechanism causing these phenomenons were well discussed. Due tothe designability of ionic liquids and their relatively low cost, ionic liquids is playingan increasingly important role in industry. In view of the achievement which ionicliquid have made, such as ultra-lubrication, active electrode material and mechanicaloperations fluids, we have reason to believe that the usage of ionic liquids inmachinery must be growing prosperity. Therefore, our research has great value andimportance of scientific significance.Specifically, this paper includes the following three research:1, After a systematically discussion of the advantages and disadvantages ofmainstream molecular force field, we improve the framework of the existing forcefields and develop a fully automated force field generater. By introducing the1-neffect, dynamic torsion angle and quantum charge, we have significantly improvedthe accuracy of the molecular force field, so that the results of molecular mechanics and quantum mechanics highly consistent. By optimizing the force fielddevelopment process and introduction of new conformation perturbation approach,the molecular structure will no longer be a barrier. A highly accurate force field willbe provided any molecule we designated. It is a very powerful safeguard for ourfollow-up work.2, In order to simulate the case of ionic liquid in dye-sensitized solar cell, basedon our custom force field, we study the interaction between lkylimidazole-hexafluorophosphate ionic liquid and graphene. And the structural property of thismixture has been discovered. In addition, we find that the distance of graphene platesand the system temperature have effect on the fluid structure. Wherein the grapheneplates’ distance effects are decisive. We also find that a self-assembly process willshape the fluid’s nano-structure. But Contrary to all expectations, the trigger of thisbehavior is not the π πinteraction.3, In order to simulate the case of ionic liquid in machinery lubrication, basedon our custom force field, we study the fluid structure of alkylimidazole-hexafluorophosphate ionic liquid under different environments. In the study, we findthat fluid structure is less affected by the ionic liquid environental factors. In contrast,the temperature and pressure have significant effect on the flow properties, such asviscosity and diffusion. Its good temperature-sensitivity of viscosity makes it be anideal choice for mechanical lubricant. In addition, we also find that the cationic alkylchain length can also affect the flow properties of the ionic liquid.Accordingly, the innovation of this paper is as follows:1. In domestic area, for the first time, we complete the whole process of forefield development, begining with the calculation of quantum mechanics, ending withthe molecular dynamic simulation. The whole all-atom force field has the perfectself-consistency and the highest effeciency. We also developed a full-automatic forcefield generator all by our effort.2. We successfully modify the molecular mechanical framework by introducing1-n pair interaction, and this farmwork has been successful applicated for nearly20years. It prove that the applicability of the existing universal all-atom force field is less than the industry general view.And it also points out that many of the calculationerrors are actually derived by the defect of force field’s framework, rather than therough force field parameters themselves.3. For the first time, this custom all-atom force field is applied to the ionicliquid simulation, including mixture of ionic liquids and graphene, or pure ionicliquid system itself. With the study on nano-structure of ionic liquid system, we findthe dependence between graphene distance and fluid structure. With the study onflow properties of ionic liquid system, we find ionic the dependence between flowproperties and environment factors, which are temperature and pressure.For the wider application of ionic liquid in mechanical engineering field, ourstudy gives the guidance and advice.