An Improved Four-Site Ionic Liquid Model

Computer simulations provide insight into the molecular-level details responsible for the unique properties of ionic liquids. Due to the sluggish dynamics and nanostructured nature of many ionic liquids, coarse-grained models are an important complement to fully atomistic simulations because they enable simulation of much larger system sizes and much longer times, which are often of interest. This paper reports a four-site, coarse-grained model for studying ionic liquids and their solutions. It is intended to be a generic model representative of common ionic liquids currently in use, but it is parametrized to fit the properties of 1-buty1-3-methylimidazolium hexafluorophosphate, [Im(41)][PF6] The present model is a variant of one introduced in J. Phys. Chem. B 114, 8410 (2010). Reduction of ion charges to +/- 0.78e and fine-tuning Lennard-Jones parameters from the original model leads to a remarkable improvement in the realism of the model and surprisingly good agreement between simulation and experiment for a variety of static and dynamic properties of [Im(41)][PF6]. This idealized model should prove valuable for studies of solute-based dynamics and other phenomena occurring on nanosecond and longer time scales, which are not feasible with all-atom simulations.