Effect Of Organic Molecules On The Structure And Properties Of Ionic Liquids

Ionic liquids (ILs) have attracted increasing attention due to their excellent physical and chemical properties such as low melting point, negligible vapor pressure, wide liquidus range, well electrical conductivity and remarkable dissolution capacity. In the synthesis and purification processes of ILs, a lot of organic solvents are inevitably used and other organic impurities are produced in the course of side reaction. The presence of these organic impurities may greatly affect the structure, physical and chemical properties of ILs, and also have a large effect on their solvent properties and transport properties, and then affect the reaction rate and selectivity of the reaction process in ILs. Therefore, it is very important to study the effect of organic solvents on the structure and properties of ILs.The effect of methanol on the structure and properties of1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM][BF4]) were studied by molecular dynamics (MD) simulations. It was found that the first peak position of cation-anion radial distribution functions (RDFs) increases whereas the distribution area of BF4" around [BMIM]+reduces, and the first peak values of RDFs for anion-methanol and methanol-methanol increase with increasing methanol mole fraction. It was shown that the interaction between cation and anion is weakened whereas those between anion-methanol and methanol-methanol are enhanced. With increasing methanol mole fraction, the self-diffusion coefficients of the cation, anion and methanol increase gradually, the viscosity of [BMIM][BF4] decreases and the electrical conductivity increases at first and then decreases.The simulation study of the effect of methanol on the structure and properties of1-alkyl-3-methylimidazolium tetrafluoroborate ([CnMIM][BF4]) and1-butyl-3-methyl-imidazolium ILs ([BMIM][X]) were performed. It was found that the first peak value of cation-anion RDFs increases with the increase of alkyl side chain length. It was shown that the strength of hydrogen bonding interaction between cation and anion is enhanced, the self-diffusion coefficients of the cation, anion and methanol increase, the viscosity of [CnMIM][BF4] decreases whereas the electrical conductivity increases. It was found that the first peak position value of Cl--methanol RDFs is the biggest whereas that of methanol molecules is the smallest. It was shown that the strong hydrogen bonding between Cl-and methanol disrupted the polymerization between methanol molecules. Spatial distribution functions (SDFs) illustrates that the distribution areas of Cl-, and PF6-around the cation are larger than BF4-and NTf2-. When the anions are BF4-and NTf2-,[BMIM][X] has low viscosity and high electrical conductivity. With the increase of temperature, the distribution area of BF4-around [BMIM]+reduces. It was shown that the interaction of cation and anion is weakened, the self-diffusion coefficients of the cation, anion and methanol increase, the viscosity of [BMIM][BF4] decreases whereas the electrical conductivity increases.MD simulations were carried out to study the effect of ethanol on the structure and properties of [BMIM][BF4]. It was found the first peak position of cation-anion RDFs, the first peak values of RDFs for cation-ethanol and anion-ethanol increase gradually with increasing ethanol mole fraction. It was shown that the interaction between cation and anion is weakened whereas those between cation-ethanol and anion-ethanol are enhanced, which leads to the excess molar volumes of the system are negative. With increasing ethanol mole fraction, the self-diffusion coefficients of the cation, anion and ethanol increase gradually, the viscosity of [BMIM][BF4] decreases and the electrical conductivity increases at first and then decreases. The effect of ethanol on the microstructure and properties of [CnMIM][BF4] and [BMIM][X] were as same as methanol.The simulation study of the effect of acetonitrile on the structure and properties of [BMIM][BF4] were performed. It was found that the first peak value of cation-anion RDFs and cation-acetonitrile RDFs increase with increasing acetonitrile mole fraction. It was shown that the addition of acetonitrile stabilize ion pairs, although they destroy greater ion aggregates, the growing ion-dipole interaction between cation and acetonitrile weakened the interaction between cation and anion, the excess molar volumes are negative. With increasing acetonitrile mole fraction, the self-diffusion coefficients of the cation, anion and acetonitrile increase gradually, the viscosity of [BMIM][BF4] decreases and the electrical conductivity increases at first and then decreases.The effect of acetonitrile on the structure and properties of [CnMIM][BF4] and [BMIM][X] were studied by MD simulations. It was found that with the increase of alkyl side chain length the interaction between cation and anion is weakened. The self-diffusion coefficients of the cation, anion and acetonitrile increase, the viscosity of [CnMIM][BF4] decreases whereas the electrical conductivity increases. It was shown that the interaction between cation and anion is larger when the anions were Cl-and PF6-,[BMIM][X] has high viscosity and low electrical conductivity. With the increase of temperature, the distribution area of BF4-around [BMIM]+and acetonitrile molecules around other acetonitrile decrease. It was shown that the interaction between cation-anion and acetonitrile molecules are weakened, the self-diffusion coefficients of the cation, anion and ethanol increase, the viscosity of [BMIM][BF4] decreases whereas the electrical conductivity increases.