Improving Molecular Force Fields For Room Temperature Ionic Liquids By United-Atom Selection Strategy

Boasting various fascinating features, ionic liquid (IL) serves as a favorable alternative solvent, replacing volatile organic solvents which are extensively used in chemical industry. Besides the environmental benefit, the properties of ILs can be changed dramatically by different combinations of anions and cations, which can be further designed by carefully tuning the functional groups. This’designable’feature attracted many researchers covered from synthetic chemists to chemical engineers. However, the tremendous number of possible ILs provides a big challenge to find the’right’IL suitable to a specific task. It is impractical to solved by traditional try-n-error experimental route, which is too costly.In virtue of the increasing computational power and advance parallel techniques, molecular simulation becomes a more and more practical method to predict many properties directly from structure of molecule and interactions between them. The latter is so-called force fields, which need to be carefully parameterized to improve the accuracy of predictions.In previous work, a series of a transferable cost-effective, classical united-atom (UA) force field has been established on the basis of1-alkyl-3-methyl-imidazolium cations combined with8kinds of anions. The thermophysical properties such as densities and Enthalpies of Vaporization are in good agreement with experimental values. The transport coefficients, such as viscosity and self-diffusion coefficients, are predicted quite well at higher temperatures. However, the simulated values become worse at lower temperature, typically2-3times of those in experiments.In this work, we proposed a different choice of UA and found it can improve the force field significantly. In details:1. In the previous version of our force field, all of the CH2/CH3are treated as UA to decrease the computational cost. Due to fact that they are different of the H atoms in CH2/CH3when connected to the electron-withdrawing N atoms in imidazolium ring, we treated them as all-atoms with explicit H. Responding to this change, we adjusted the atom charges, as well as some missing parameters. The total number of charges of anion/cation decreased from0.8e to0.7e to get better predictions of densities. The van der Waals radius of H atom were also adjusted by ab initio calculation of ion-pair configuration of [C1mim][Cl][C1mim][BF4] and [C1mim][PF6]. 2. The improved UA force field was used in the molecular dynamics simulations of IL series of [Cnmim][NTf2](n=1,2,4,6,8,10) and most of the properties are predicted very well. Especially the self-diffusion coefficients and zero-shear viscosities are in good agreements with experimental values at lower temperatures, which is a substantial improvements on our previous force field.3. The above success motivated us to proposed a general and practical strategy in developing force fields of ILs. In the final part of the thesis, we develop the force field of ILs based on other cations including ammonium and pyridinium.