Understanding Diffusion Of Water In Ionic Liquids By Molecular Simulations

Unlike the conventional organic solvents, ionic liquids have been widely applied to the research and industrial area because of its excellent features. There are two main methods for researchers to know the properties of the ionic liquids. Which respectively are experiments and computer simulations.However, high cost and long synthetic cycle of ionic liquids limite the ues of experimental methods. By contrast, computer simulation has become an important tool to study the diffusion of water in ionic liquids with advantages of its low cost, high accuracy, and high performance of computing technology.It is inevitable that some water will permeate into the ionic liquids in the process of practical application. Previously, reseachers only focused on the properties of pure ionic liquids. But there were just a few studies about the mixtures of ionic liquids and water. According to this situation, this paper uses molecular dynamic simulations to study the dynamic, thermodynamic and microstructure properties of mixtures of three ionic liquids:1-butyl-3-methyl imidazolum acetate ([BMIM][AC]),1-butyl-3-methylimidazolum N-(2-aminoethyl) maleimide trifluoraceta ([BMIM][TFA]),1-butyl -3-methylimidazolium bis(trifluoromethylsulfonyl)imide and water at the entire concentration as a function of the mole fraction of water.Ionic liquids have been greatly restricted in their industrial application due to the high viscosity and the low diffusion coefficients. By adding water to the ionic liquids can reduce the viscosity and accelerate the diffusion of the ionic liquids. So in this work, we mainly discuss the diffusion of the mixtures of ionic liquids and water at the entire concentration. Thus we find that the diffusions of the two hydrophilic ionic liquids [BMIM][AC] and [BMIM][TFA] have the same rules with the increasing of the mole fraction of water. In the mixtures of [BMIM][TFA] and water, water is more likely to aggregate and form a tetrahedral structure similar to that in bulk water. In the two hydrophilic ionic liquids both anions ([AC]’and [TEA]") have strong interactions with water and play the major role to affect water diffusion. But in the less hydrophilic ionic liquid [BMIM][TF2N], the diffusions have more complex situations. In the two hydrophilic ionic liquids, two turning points are found for diffusion of water in ILs. When the mole fraction of water (x2<0.4), most of the water molecules distribute independently in the polar network. With the increasing of the concentration (0.4<x2<0.8), water begin to form clusters via self-assembly between themselves. When the mole fraction of water (x2>0.8), ionic liquids are percolated by water molecules.In this work, we use the visualization tools SDF (Space distribution functions) to study microscopic structures of the mixtures of ionic liquids and water. Combined with RDF (Radial Distribution Functions, RDFs), we can accurately elucidate the microstructure properties of mixtures of ionic liquids and water.