Charge Environment and Hydrogen Bond Dynamics in Binary Ionic Liquid Mixtures: A Computational Study

The extent of charge transfer between the cation and the anion in a room-temperature ionic liquid depends on the basicity of the anion. Ion charges determined in the condensed state via density functional theory calculations capture this effect rather well, and charges derived in such a manner have been employed in force field-based molecular dynamics simulations to quantitatively reproduce several physical properties of the liquids. However, the issue of transferability of cation charges in mixtures of ionic liquids, say with one type of cation and two different anion types needs to be addressed. Herein, we demonstrate that the cation charge in such a mixture varies linearly with anion composition, a result that ties in rather well with X-ray photoelectron spectroscopic experiments. The variation in cation charge with bulk anion composition is shown to be a result of changes in its coordination environment. Cations surrounded by a higher proportion of more basic anions possess lower charges than those surrounded by less basic anions. Time scales for the exchange of anion types for the occupation of hydrogen bonding sites around the cation have been determined and are seen to be constituted by three processes breakage of existing hydrogen bond, diffusion to the hydrogen bonding site and displacement of the incumbent anion from its site in the cation coordination shell. These time scales explain the differences observed between infrared and NMR spectroscopic experiments in ionic liquid mixtures rather well.