Ion dissociation in aqueous 1-alkyl-3-methyl-imidazolium chlorides and the impact of microstructure formation

We study ion dissociation in four aqueous 1-alkyl-3-methyl-imidazolium chloride ([amim][Cl]) (where a = alkyl) ionic liquids (ILs) focusing on the effects of microstructure formation. We calculate ion dissociation using precise density, viscosity, and ionic conductivity measurements taken in this work as well as values reported by Rogac et al. (M. Bešter-Rogač, M. V. Fedotova, S.E. Kruchinin, and M. Klähn, Mobility and association of ions in aqueous solutions: The case of imidazolium based ionic liquids, Phys. Chem. Chem. Phys. 18, 28594–28605 (2016); R. Toma, A. Tot, J. Kuhar, and M. Be, Interactions in aqueous solutions of imidazolium chloride ionic liquids [C n mim][Cl] (n=0, 1, 2, 4, 6, 8) from volumetric properties, viscosity B-coefficients and molecular dynamics simulations, 254, 267–271 (2018)) in conjunction with our estimation framework presented previously (O. Nordness, L.D. Simoni, M.A. Stadtherr, and J.F. Brennecke, Characterization of Aqueous 1-Ethyl-3-Methylimidazolium Ionic Liquids for Calculation of Ion Dissociation, J. Phys. Chem. B 123, 1348–1358 (2019); L.D. Simoni, Predictive Modeling of Fluid Phase Equilibria for Systems Containing Ionic Liquids (University of Notre Dame, Notre Dame, IN, 2009)). We first consider ion dissociation at dilute IL concentrations below the critical aggregate concentrations (cac). We introduce a combined Stokes radii term calculated from limiting molar conductivity data, which more accurately estimates ion dissociation in dilute IL systems than radii estimated from a group contribution method. Incorporating the new radii term in the existing model, we study ion dissociation across a wide IL concentration range. Even if prior knowledge of aggregation were not available, as is the case for aqueous [amim][Cl] systems, aggregation is apparent from this analysis because the increased viscous forces of microstructure formation lead to unrealistic estimates of ion dissociation.