Understanding ion-ion and ion-solvent interactions in aqueous solutions of NMP based protic ionic liquids through partial molar properties and DFT calculations

•Apparent molar properties of aqueous protic ionic liquids have been studied.•Infinite dilution partial molar properties calculated using Redlich-Mayer type of equation.•Ion-solvent interactions weakening with increase in hydrophobic anionic chain length.•Observed intra-ionic and inter-ionic interactions were supported by DFT.

The precise and accurate thermophysical properties determination of ionic liquid (IL)-solvent binary system is needed for understanding the molecular interactions occurring between these components. The nature of ion-ion and ion-solvent interactions can be studied either experimentally on the basis of apparent and partial molar properties and corresponding infinite dilution properties or by computational studies. In this regard, we have measured the thermodynamic properties of binary aqueous solutions containing newly synthesized N-methylpyrrolidone based protic ionic liquids (PILs) in the temperature range (288.15-328.15) K and at ambient pressure. Experimentally measured density and speed of sound data have been used to calculate the infinite dilution partial molar properties and the corresponding empirical parameters through the Redlich-Mayer type of equation. It is observed that the studied PILs interacts strongly with water, however the ion-solvent interactions decreases with increase in anionic chain length and increase in temperature. Observed intra-ionic and inter-ionic interactions in studied aqueous ionic liquid mixtures were also supported by Density Functional Theory (DFT) calculations.

Graphical abstractIon-ion and ion-solvent interactions in aqueous solutions of NMP based protic ionic liquids.Download high-res image (312KB)Download full-size image