title: Residual water in ionic liquids: clustered or dissociated?
abstract: 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).
authors.affiliation: York Structural Biology Laboratory, Department of Chemistry, University of York, Heslington, York YO10 5DD, UK. seishi.shimizu@york.ac.uk.
authors.forename: Joshua E S J
authors.initials: JE
authors.lastname: Reid
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Walker
authors.forename: Seishi
authors.initials: S
authors.lastname: Shimizu
mesh_terms: D045424:Complex Mixtures / Q000737:chemistry*; D003198:Computer Simulation; D048288:Imidazolines / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D008956:Models, Chemical; D009997:Osmotic Pressure; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D045424:Complex Mixtures; D048288:Imidazolines; D052578:Ionic Liquids; D014867:Water