Prediction of Thermophysical Properties of Acetate-Based Ionic Liquids Using Semiempirical Methods

Five acetate-based ionic liquids (AcAILs) [C_nmim][OAc](n = 2, 3, 4, 5, 6) (1-alkyl-3-methylimidazolium acetate) were prepared by the neutralization method and characterized by ¹H NMR spectroscopy and differential scanning calorimetry (DSC). Their density, surface tension, and refractive index were measured at 298.15 to 338.15 K. Since the AcAILs can form strong hydrogen bonds with water, small amounts of water are difficult to remove by common methods. In order to eliminate the effect of the impurity water, the standard addition method (SAM) was applied to these measurements. On the basis of the experimental data, the molecular volume (sum of positive and negative ion volumes), V_m, of [C_nmim][OAc](n = 2, 3, 4, 5, 6), the entropy of surface formation, S_a, the Gibbs energy of surface formation E_a, the parachor, P, and the molar refraction, R_m, the polarization coefficient, α_p, were calculated. The results show that E_a, P, and R_m are approximately temperature-independent and the contribution to these properties per methylene (?CH₂?) on alkyl chain of [C_nmim][OAc](n = 2, 3, 4, 5, 6) was discussed. According to both Kabo’s and Rebelo’s methods, the molar enthalpy of evaporation of the [C_nmim][OAc](n = 2, 3, 4, 5, 6) was estimated using surface tension and molar volume. In the same time, the surface tension of the AcAILs may be estimated using the parachor and the refractive index. The estimated values of the surface tension and the corresponding experimental one are extremely similar.