Molecular dynamics simulations of the aqueous interface with the [BMI][PF6] ionic liquid:: comparison of different solvent models

We report a Molecular Dynamics (MD) study of the interface between water and the hygroscopic room temperature Ionic Liquid "IL'' [BMI][PF6] (1-butyl-3-methyl-imidazolium hexafluorophosphate), comparing the TIP3P, SPC/E and TIP5P models for water and two IL models where the ions are +/- 1 or +/- 0.9 charged. A recent MD study (A. Chaumont, R. Schurhammer and G.Wipff, J. Phys. Chem. B, 2005, 109, 18964) showed that using TIP3P water in conjunction with the IL +/- 1 model led to water-IL mixing without forming an interface, whereas a biphasic system could be obtained with the IL +/- 0.9 model. With the TIP5P and SPC/E models, the juxtaposed aqueous and IL phases are found to remain distinct for at least 20 ns. The resulting IL humidity, exaggerated with the IL +/- 1 model, is in better agreement with experiment using the IL +/- 0.9 model. We also report demixing simulations on the "randomly mixed'' liquids, using the IL +/- 0.9 model for the ionic liquid. With the three tested water models, the phases separate very slowly (approximate to 20 ns or more) compared to "classical'' chloroform-water mixtures (less than 1 ns), leading to biphasic systems similar to those obtained after equilibration of the juxtaposed liquids. The characteristics of the interface (size, polarity, ion orientation, electrostatic potential) are compared with the different models. Possible reasons why, among the three tested water models, the widely-used TIP3P model exaggerates the inter-solvent mixing, are analyzed. The diffculty in computationally and experimentally equilibrating water-IL mixtures is attributed to the slow dynamics and micro-heterogeneity of the IL and to the different states of water in the IL phase.