Effects of Water Concentration on the Structural and Diffusion Properties of Imidazolium-Based Ionic Liquid-Water Mixtures

We have used molecular dynamics simulations to study the properties of three ionic liquid (IL)-water systems: 1-butyl-3-methylimidazolium chloride ([bmim]Cl), 1-ethyl-3-methylimidazolium acetate ([emim][Ac]), and 1,3-dimethylimidazohum dimethylphosphate ([dmim][DMP]). We observe the transition of those mixtures from pure IL to aqueous solution by analyzing the changes in important bulk properties (density) and structural and bonding properties (radial distribution functions, water clustering, hydrogen bonding, and cationic stacking) as well as dynamical properties (diffusion coefficients) at 12 different concentration samplings of each mixture, ranging from 0.0 to 99.95 mol % water. Our simulations revealed across all of the different structural, bonding, and dynamical properties major structural changes consistent with a transition from IL water mixture to aqueous solution in all three ILs at water concentrations around 75 mol %. Among the structural changes observed were rapid increase in the frequency of hydrogen bonds, both water water and water-anion. Similarly, at these critical concentrations, the water clusters formed begin to span the entire simulation box, rather than existing as isolated networks of molecules. At the same time, there is a sudden decrease in cationic stacking at the transition point, followed by a rapid increase near 90 mol % water. Finally, the diffusion coefficients of individual cations and anions show a rapid transition from rates consistent with diffusion in IL's to rates consistent with diffusion in water beginning at 75 mol % water. The location of this transition is consistent, for [bmim]Cl and [dmim][DMP], with the water concentration limit above which the ILs are unable to dissolve cellulose.