Study Of The Force Field And Molecular Dynamic Simulation Of Imidazolium Ionic Liquids

Room temperature ionic liquids(ILs)are special class of molten salts composed solely of ions(cations and anions),which are in the liquid state at temperature less than 373K.As novel solvents,it is widely believed that ILs are low volatility,high thermal stability,non-flammability,high conductivity,designable characteristic and they have been successfully used as catalysts,solvents,and electrolytes.The unique properties of ILs are decided by their special structures,the various interactions between cation and anion,such as electrostatic interaction,hydrogen bond interaction and dispersion interaction.Molecular dynamic simulation,as a method based on atom or molecular model,play an important role in recent studies of ILs.Molecular dynamic could supply help for investigation on the interaction and structure-properties relationship between ILs.In this work,we have developed an all-atom force field with three-body hydrogen bond model for 1-alkyl-3-methyl-imidazolium chloride([Rmim][Cl]),predicted the experimental dynamic viscosity successfully and calculated the relationship between dynamic properties and ion pair lifetime by using molecular dynamics simulation.And the effect of local hydrogen bonding structure of ILs on the viscosities was studied.For the application of ILs in water system,the aggergation structures were simulated and the formation mechanism and transport properties of vesicle were further analyzed.Finally,a coarse grain model of[C12mim][Br]-water was developed and the potential profiles were optimized.The main contents and results are as follow:(1)Designing of force field with three-body hydrogen bond model and the study on the relationship between ion pairs and dynamic properties of ILs.A refined non-polarizable force field was proposed,which combined the mean polarizable effect on the scaled charge strategy and local polarizable effect on the hydrogen bond model.Ab initio molecular dynamic simulation of 32 ion pairs was carried out and then fitted the charges distribution,the statistics of various configurations and different ions surround environment for a long real time enable to represent the mean polarizability much more reality.Considering the local unique hydrogen bonding interactions,a modified Morse Potential Hydrogen Bond interaction model was adopted into force field for describing the hydrogen bond interaction,and the parameters were optimized by calculated the hydrogen bonding ion pair structure.The force field was verified by comparing the radial distribution function of hydrogen atom in imidazolium ring and anions with the AIMD results.And the calculated densities were agreement with the experimental values.The structure of[Emim][Cl]was further studied by radial distribution function and space distribution functions,finding the ?-? stack of cations.Furthermore,the distributions of anions around the cation were displayed.The shear viscosity was calculated by using Green-Kubo relations,considering statistics based on the time decomposition method.And the shear viscosities computed by using the force fields with hydrogen bond model showed very good agreement with experimental data.The lifetime of ion pairs were also calculated and the results were in linear relation with dynamic properties(2)Study on the influence of the local hydrogen bond structure on the shear viscosity of ILs by using molecular dynamics and quantum mechanism.The shear viscosity was calculated by using periodic perturbation method,and the parameters for obtaining accurate calculated viscosity value was evaluated.The results indicated that the perfect cosine-like velocity profile was the key to get the reasonable calculated viscosity.And the amplitude of perturbation acceleration is directly related to the z-direction length of the simulated box.And the accurate viscosities were obtained correspond to the experimental values.Furthermore,the calculated viscosities correlated to the lifetime of ion pairs.The structure of three ionic liquids were analyzed by Radial distribution functions and space distribution functions.And the structural distribution differences could be used to explain viscosities,the local aggregation of anions in certain site of cation prevent the ion movement and increase the viscosity The hydrogen bond interactions were further analyzed to reveal the relationship between viscosities and local structure.The multiple interaction site and modest strength of interacting energy difference lead the lower viscosities of[Bmim][NTf2]This is an indication for the design of lower viscosity ionic liquids(3)Study on the aggregation structure of ionic liquid in aqueous system.It was found that[C12mim][Sal]ionic liquid could form the aggregation structure as the micelle and vesicle in aqueous by using molecular dynamic simulations.The structure and interaction energy of the ionic liquid unilamellar vesicle were then investigated and indicated that the vesicle was a perfect bilayer structure and interaction energy of cation and anion played an important role in stabilizing the structure.Then the entire forming process of vesicle from the random distribution of ILs were simulated and reveal the formation mechanism.For application,the permeability of vesicle were further analyzed by computing time correlation function of water molecules inside the vesicle at a series different temperatures.The results indicated that water could be rapid exchange as the temperature increase,while the vesicle structure still maintain.Finally,the fusion simulation of two vesicles were further carried out.(4)Study on coarse grain force field and further molecular dynamic simulation.For large scale and length time simulation,the coarse grain force field were developed by using Iterative Boltzmann Inversion method.The potential energy function for coarse grain model were optimized after 120 iteration,and the RDF obtained from the coarse grain model were consistent with the all-atom force field results.The molecular dynamic simulations based on coarse grain force field were carried out,and the densities at different temperature were in good agreement with the all-atom force field results.The self-diffusion coefficient was a little larger as the coarse grain model was much simpler,and the freedom was decreased.The further simulation on larger system and found the aggregation structure of ionic liquids.