Molecular interactions in binary mixtures of n-alkylmethylimidazolium trifluoromethanesulfonate + methanol: Thermophysical and density functional theory studies

Investigations on the thermophysical properties of ionic liquids (ILs) with molecular organic solvent has been performed at temperature ranges 298.15–323.15 K with an interval of 5 K and 0.1 MPa pressure as functions of mole fraction ($x_1)$ of ILs. We have experimentally measured densities (ρ), speeds of sound (u) and viscosities (η) of pure components and the binary mixtures of 1-butyl-/1-ethyl-3-methylimidazolium trifluoromethanesulfonate {[BMIM]/[EMIM][CF₃SO₃]} with methanol (MeOH) at studied temperatures and pressure using a high-precision vibrating tube densitometer and an automated falling ball microviscometer. Using ρ, u and η, the excess molar volumes $(V^E$), excess molar isentropic compressibilities ($K_{s,m}^E$) and deviations in viscosities (Δη) have been evaluated and correlated it with extended form of Redlich-Kister polynomial equation. We have found the experimental and correlated values of excess/deviations parameters are good agreements to each other. Interactions between the atoms/molecules of ILs-solvents are well discussed in terms of possible forces of attractions occurred. Density Functional Theory (DFT) were performed to show the possible interactions between the ion-pair of ILs $({\left(BMIM\right)}^{+}$/${\left(EMIM\right)}^{+}$ and ${\left({\mathrm{CF}}_3{\mathrm{SO}}_3\right)}^{-})$ and ILs-solvent components, revealing interactions at the D3-B3LYP/6–311++G(d,p) level of theory. Moreover, various molecular properties were obtained are well discussed at the same level of theory. Thereafter, the natural bond orbital (NBO) analyses were performed to see all the interactions between donor–acceptor atoms at molecular level.