Influence of temperature on molecular interactions of imidazolium-based ionic liquids with acetophenone: thermodynamic properties and quantum chemical studies

Abstract

The physicochemical properties namely: densities (ρ), sound velocities (u), viscosities (η), and refractive indices (n_D) of a series of alkyl imidazolium-based ionic liquids (ILs) with same cation and different anion and vice versa of ILs: 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM]⁺[BF₄]⁻, 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM]⁺[PF₆]⁻, 1-ethyl-3-methylimidazoium ethyl sulphate [EMIM]⁺[EtSO₄]⁻ and 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM]⁺[BF₄]⁻, with acetophenone over the wide range of composition and at (293.15, 303.15, 313.15, 323.5 and 333.15) K under atmospheric pressure is reported in this study. The excess molar volumes, (V^E_m), deviation in isentropic compressibilities (Δκ_s), deviation in viscosities (Δη) and deviation in refractive indices (Δn_D) were derived from experimental results. The V^E_m, Δκ_s and Δn_D values for the mentioned systems are both negative and positive over the entire composition range while the Δη values are negative under the same experimental conditions. The derived properties were fitted to the Redlich–Kister polynomial equation to check the accuracy of experimental results. Furthermore, the inter-ionic interactions between the cations and anions of the ILs both in vacuo and in acetophenone (using continuum solvation) were confirmed using quantum chemical technique such as [Density Functional Theory (DFT)]. The quantum chemical results are in good agreement with the experimental results suggesting that there exist appreciable interactions between the ILs and acetophenone. The theoretical and measured data were interpreted in terms of intermolecular interfaces and structural effects between similar and dissimilar molecules upon mixing in order to obtain more information on the thermophysical and thermodynamic properties of ILs and their binary mixtures. This study will contribute to the data bank of thermodynamic properties of IL mixtures, so as to establish principles for the molecular design for chemical separation processes and to enhance the applications of ILs in certain aspects of research or industrial application.