Hydration water and ionic aggregation in aqueous solutions of imidazolium-based protic ionic liquids

Water molecules, present as additive or as contaminant of Protic Ionic Liquids (PILs), can compete for the hydrogen bond sites leading to important modifications of the local order of these liquids and to the modulation of their physical–chemical properties. In this work, aqueous solutions of a set of N-methylimidazolium-based PILs [MIM][X] (X = NO3−, TfO−, HSO4−, and Cl−) were investigated by deep UV Resonance Raman (UVRR) spectroscopy in the water-rich domain where ionic aggregates and bulk-like water coexist. A differential method was used to analyze the OH stretching profile to extract the so-called solute-correlated (SC) spectrum, which is particular informative of the hydration features of the PILs. Moreover, specific bands of the cation, sensitive to the hydrogen bonding, were comparatively investigated. The progressive evolution from solvent-separated ion pairs (SIP) and/or solvent-shared ion pairs (SSIP) to contact ion pairs (CIP) and/or larger ionic aggregates can be monitored as a function of the hydration level, in the water-rich domain. Our approach showed that, in the high-diluted regime, the hydration environment around the [MIM] cation does not depend on the type of anion. Moreover, [MIM][NO3] and [MIM][TfO] showed cation-water (ionic) H-bonds at the NH site stronger than the cation–anion (double-ionic) ones. The analysis of SC Raman spectra points out the formation of cation–anion H-bonds (through the CH ring groups), stronger than cation-water ones, upon PILs concentration increase, especially evident in the case of [MIM][Cl]. The H-bond strength between the anion and hydration water is found to decrease following the order: [Cl] ∼ [HSO4] > [NO3] > [TfO]. Chloride ions tend to perturb a larger number of water molecules than the other anions. The number of perturbed water molecules decreases at increasing PIL concentration, showing a larger dependence for [MIM][Cl], consistently with its larger propensity to form ionic aggregates. The unique response of [MIM][Cl] to hydration found by SC-UVRR data is related to the synergy of different factors such as the anion reduced size (higher charge density), spherical symmetry, and high H-bond basicity.