United Atom Force Field Development Andmolecular Dynamics Simulation of Ionic Liquids
|School||Beijing University of Chemical Technology|
|Course||Chemical Engineering and Technology|
|Keywords||ionic liquid molecular force field molecular dynamicssimulation structure-properties relationship viscositymicroscopic distribution|
Ionic liquids （ILs） are a class of novel environmental benign “greensolvents”, which is developed in the framework of green chemistry. ILs arenon-flammable, pollution-free, easy separation and easy recovery, which makethem are regarded as environment-friendly green solvent. In recent years, thestudies of ionic liquid mainly focus on the chemistry and separation process.Moreover, their properties can be tailored for specific applications by differentcombinations of cations and anions or altering their structure, which makethem as “designer green solvents”. However, the traditional trial-and-errorway to find new ILs or optimize their combinations leads to the large cost onthe syntheses and experiments. The understanding of how the structure and thechemical constitute of ILs effect their properties is very important. Molecularsimulations on the atomistic level provide an effective way in screening alarge number of candidate materials. In this work, several ILs was studied bymolecular dynamics simulations. The main research results are summarized asfollows:1. We proposed a cost-effective united-atom （UA） force field for ionicliquids （I Ls） composed by1-alkyl-3-methyl-imidazolium cations （[Cnmim]+, n=110） and seven kinds of anions, including tetrafluoroborate （[BF4]）,hexafluorophosphate （[PF6]）, methylsulfate （[CH3SO4]-）,trifluoromethylsulfonate （[CF3SO3]-）, acetate （[CH3CO2]）, trifluoroacetate（[CF3CO2]-）, and bis（trifluoromethylsulfonyl）amide （[NTf2]-）. The chargedistribution was derived from the RESP fitting to the ab initio calculations ofion pair dimers in this work. Thus, the charge transfer and polarizability isaccounted for in an effective way. In addition, take the COM of the UA groupas the center of UA, and then fitting other parameters. It was found wellreproduce the liquid densities of all the ILs studied in this work.2. Molecular dynamics （MD） simulations were performed over a widerange of temperatures to validate the force field. The liquid densities werepredicted very well for all of the ILs, with typical deviations less than1%. Thesimulated enthalpies of vaporization, Hvap, are also in good agreement withexperimental values, with slight overestimation about5kJ/mol. More criticalvalidations were made by comparing the transport properties, including theself-diffusion coefficient and shear viscosity, between simulation andexperiments. The simulated self-diffusion coefficients and viscosities for allthe ILs studied in this work are in good agreement with experiments. Theforce field also showed good performance on the temperature and alkyl chainlength dependence of these properties. The viscosities of ILs is graduallyincreased with the alkyl length of cations, with a minimum of the viscosity atn=2, which good agreement with experiments. 3.Extensive molecular dynamics simulations for [C4mim][BF4]/water mixtures are performed using above force field. Cross parametersare obtained by the Lorentz Berthelot （LB） combining rules withoutfurther optimization. The positive excess molar volume and excess molarenthalpy, as well as their dependence on temperature, are well reproducedin our simulations.个In addition, the local structure, as characterized byradial distribution functions （RDFs）, spatial distribution functions （SDFs）and water clustering analysis, is used to elucidate the connection betweenstructure evolution and the macroscopic observations in simulations. Whenx2（mole fraction of water）<0.2, most of water molecules are isolated eachother and experience a local environment in the polar network nearly thesame as that in neat IL. In the concentration range of0.2<x2<0.8, watermolecules tend to form clusters by self-aggregation. When x2≥0.8, ILis percolated by water molecules.4.Using the above method of developing force field molecularsimulation well reproduce the decreased densities, self-diffusioncoefficients and increased heats of vaporization and viscosities with themethylation at position of C2. In addition, it is pointed that the increasedviscosity is caused by the flexibility of alkyl chains of cation. In order tounderstanding the mechanism of the increased viscosity of the C2methylation, the author assume that there exists another two aritificial ILs.It was found that, only increase rotation energy barrier will not lead to an obvious increase of viscosity, which account for only13%of the totalincrement of viscosity. The increased viscosity mainly caused by theredistribution of charges （account for41%） and the Van der Waalsinteraction （account for46%）.