Since water has a high impact on the chemical-physical characteristics of Ionic Liquids (IL), both as a contaminant or as cosolvent, the detailed knowledge of the intermolecular interactions in IL/water solutions is a crucial step for understanding and predicting the range of properties of these non-conventional solvents for applications in many fields including electrochemistry, biochemistry, and synthesis.
In this work, aqueous solutions of a prototypical set of methyl-imidazolium (MIM) - based ionic liquids is investigated by UV Raman spectroscopy and Small-Angle Neutron Scattering in the water-rich domain. Selected Raman signals in different wavenumber ranges provide insights into the local organization of cation-anion pairs as a function of the increasing amount of water in a wide range of concentrations. The high-frequency range of Raman spectra is analyzed by a differential method to extract from the OH stretching profile of water the solute-correlate (SC) spectra, which emphasize the molecular structuring of the interfacial water present in the hydration shells around the selected anions. The neutron scattering data show the water – IL segregation at nanoscale.
Peculiar solvation behavior is observed for the different MIM-based mixtures in connection with the hydrogen bonding features of the hydrating water molecules. Interestingly, the ionic liquid [MIM][Cl] seems more sensitive to hydration than [MIM][TfO] even in an extremely hydrated regime.