Monte Carlo simulations of water solubility in ionic liquids: A force field assessment

Gibbs ensemble Monte Carlo simulations were used to calculate absorption isotherms of water in three imidazolium-based ionic liquids (ILs) with different degrees of hydrophobicity: 1-n-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C4mim][Tf2N]) and 1-n-butyl-3-methylimidazolium chloride ([C4mim][Cl]). An evaluation of several water models and various IL force fields was conducted. The selected water models are the TIP5P, TIP4P2005 and a collection of three site models based on the SPC geometry but with different sets of fixed partial charges. These partial charges are consistent with water surrounded by different electrostatic environments, and crudely account for polarizability. In the case of the IL force fields, net partial charges of ±1.0e were used. In addition, scaled charges of ±0.9e and ±0.8e were used to account for polarization and charge transfer. It is found that most water models tested with the full charge [C4mim][PF6] and [C4mim][Cl] force field overestimate the solubility, but the agreement is reasonably good with [C4mim][Tf2N]. Attempts at improving the results by either scaling the charges on the IL or on water were made, but no combination was able to reproduce experimental phase behavior satisfactorily. The main conclusion of the study is that, to consistently model water absorption in ILs, force fields that include effects such as polarization are likely necessary.