Why are ionic liquid ions mainly associated in water? A Car-Parrinello study of 1-ethyl-3-methyl-imidazolium chloride water mixture

Abstract

In this study we present the results of a first principles mol. dynamics simulation of a single 1-ethyl-3-methyl-imidazolium chloride [C2C1i.m.][Cl] ion pair dissolved in 60 water mols. We observe a preference of the in plane chloride coordination with respect to the cation ring plane as compared to the energetic slightly more demanding on top coordination. Evaluation of the different radial distribution functions demonstrates that the structure of the hydration shell around the ion pair differs significantly from bulk water and that no true ion pair dissocn. in terms of completely autonomous solvation shells takes place on the timescale of the simulation. In addn., dipole moment distributions of the solvent in distinct solvation shells around different functional parts of the [C2C1i.m.][Cl] ion pair are calcd. from maximally localized Wannier functions. The anal. of these distributions gives evidence for a depolarization of water mols. close to the hydrophobic parts of the cation as well as close to the anion. Examn. of the angular distribution of different OH(H2O)-X angles in turn shows a linear coordination of chloride accompanied by a tangential orientation of water mols. around the hydrophobic groups, being a typical feature of hydrophobic hydration. Based on these orientational aspects, a structural model for the obvious preference of ion pair assocn. is developed, which justifies the assocg. behavior of solvated [C2C1i.m.][Cl] ions in terms of an energetically favorable interface between the solvation shells of the anion and the hydrophobic parts of the cation. (c) 2008 American Institute of Physics.