(127c) Microscopic State of Ionic Liquid Ion Pairs At Low Concentrations In Water As Studied From Molecular Dynamics Simulations

Inorganic salts are known to undergo dissociation in water due to large free energy gain upon hydration of the individual ions.  In the case of ionic liquids, however, the question of association/dissociation of ions when dissolved in water remains to date largely unexplored. In this study, we apply techniques of molecular simulations to investigate the microscopic state of the ionic liquid ion pair under extremely low aqueous concentration (~0.0003 mole fraction). To this end, we calculate the potential of mean force (PMF) and hence the work required to bring the ions together as a function of the separation distance. We carry out molecular dynamics calculations on three ionic liquids of varying hydrophobicity: 1-ethyl-3-methylimidazolium chloride [C₂mim]Cl, [C₂mim] ethylsulfate [C₂H₅SO₄] and 1-n­-butyl-3-methylimidazolium [C₄mim] bis(trifluoromethylsulfonyl)imide [NTf₂]. The PMFs of the three ionic liquids are characterized by two free energy minima corresponding to contact pair and solvent separated pair while the long-range behavior is different. The most hydrophobic ionic liquid [C₄mim][NTf₂] exhibits the deepest free energy minima and suggests that the most favorable thermodynamic state of ion pairs is either association through direct ion-ion interaction or that with a single layer of water separating the ions. In addition to similar characteristics, the PMF of the ionic liquid [C₂mim][C₂H₅SO₄] also shows features indicative of dissociation. In the case of the ionic liquid [C₂mim]Cl, the PMF clearly suggests that the most preferred thermodynamics state is that in which ions are completely dissociated. We explain the results in terms of hydrophobicity of these ionic liquids. Further insight into the solvation of the ions is provided by the organization of water molecules around the ions.