Molecular dynamics investigation of thermo-physical properties and hydrogen-bonds of 1-ethyl-3-methylimidazolium dimethylphosphate-water system

Aqueous solution of ionic liquid 1-ethyl-3-methylimidazolium dimethylphosphate ([Emim][Dmp]) have the potential to become the new working fluids for absorption heat pump. The thermo-physical properties, such as density, self-diffusion coefficient and viscosity of [Emim][Dmp] and its aqueous solution relying strongly on their molecular structures, are investigated at different water mole fractions and at the temperature range from 293.15 K to 373.15 K. The structures of [Emim][Dmp] and water are initially optimized by density functional theory and atomic charges were obtained by CHELPG method. The simulated density for researched systems is quite in agreement with the experimental data. The calculated self-diffusion coefficients of [Emim][Dmp] and water increase rapidly with temperature increase. When water mole fraction is less than 0.6, the self-diffusion coefficients of [Emim][Dmp] and water vary little with water mole fraction and the binary solution is more ionic liquid like because the hydrogen-bonds (H-bonds) between IL and water are stronger than that between water molecules. In this case, water molecules are restricted by IL. However, when water mole fraction is more than 0.8, the self-diffusion coefficients of [Emim][Dmp] and water increase obviously due to the fact that some free water molecules can form H-bonds with themselves rather than IL. The weak H-bonds between H2 in cation and O atoms in anion and strong hydrogen bond between O21 in anion and H34 in water are also found from radial distribution functions (RDFs). The strong interaction between [Dmp]− and water may also explain why [Emim][Dmp] and water have good solubility and affinity. Moreover, the H-bonds between [Dmp]− and water become weaker with temperature increase.