Study on thermodynamic properties and estimation of polarity of ionic liquids {[Cnmmim][NTf2] (n = 2, 4)}

Abstract

Two bis(trifluoromethyl sulfonyl)imide ionic liquids [C_nmmim][NTf₂] (n = 2, 4) {1-alkyl-2,3-dimethyimidazolium-N,N-bis(trifluoromethyl sulfonyl)imide} were prepared and characterized by ¹H NMR spectroscopy and differential scanning calorimetry (DSC). The values of their density, surface tension and refractive index were measured in the temperature range of (298.15 to 338.15 ± 0.01) K and the average contributions to the density, surface tension, and refractive index per methyl group in the alkyl chain and the addition of a methylene group in the imidazolium ring for the ILs were discussed. The dependence of volumetric properties, surface properties and molar refraction on temperature was discussed. Based on Kabo's method and Verevkin's experimental values, the molar enthalpies of vaporization, ΔH^v, for [C_nmmim][NTf₂] (n = 2, 4) were estimated. As a new idea, it was put forward that ΔH^v can be assumed to consist of two parts: a part corresponds with the induced energy, ΔH^v_n, and another part corresponds with orientation energy from the permanent dipole moment of the ion pair in ILs, ΔH^v_μ. The values of ΔH^v_n were calculated in terms of the Lawson–Ingham equation so that the values of ΔH^v_μ could be estimated. Using the values of ΔH^v, ΔH^v_n and ΔH^v_μ, cohesive energy density, δ² (δ is Hildebrand solubility parameter), the contribution of induced energy, δ_n², and the contribution of orientation energy, δ_μ², were obtained. If a liquid only has δ_n then it is a non-polar liquid and if a liquid not only has δ_n, but also has δ_μ then it is a polar liquid. Since the ion pairs in ILs have a permanent dipole moment, the ionic liquid has a certain polarity. Therefore, using δ_μ as the polarity scaling of ILs is very convenient because the values of δ_μ are very easy to calculate from the enthalpy of vaporization and refractive index data.