Residual water in ionic liquids: clustered or dissociated?

How do residual water molecules in ionic liquids (ILs) interact with themselves, as well as with the ions? This question is crucial in understanding why the physical properties of ILs--and chemical reactions performed in them--are strongly affected by the residual water content. There have been three conflicting hypotheses regarding the structure and behaviour of the residual water: (i) water molecules are separated from one another, while interacting strongly with the ions, and dispersed throughout the medium; (ii) water molecules self-associate or form clusters in the ILs; (iii) residual water weakens ion-ion interactions. A satisfactory resolution of these conflicting suggestions has been hindered by the complexity and long range of the interactions in the water-IL mixture and by the often profound differences in physical structure between various different ILs. Here we present a route to resolve this question through a combination of a statistical thermodynamic theory (Kirkwood-Buff theory) with density and osmotic data from the literature. The structure of water-IL mixtures is shown to be water content dependent; at the lowest measured water concentration, strong water-IL interaction and water-water separation are observed in accordance to (i), whereas water in a more hydrophobic IL environment seems to self-associate at moderately low water concentrations, in accordance with (ii).