Determination of the Enthalpy of Vaporization and Prediction of Surface Tension for Ionic Liquid 1‑Alkyl-3-methylimidazolium Propionate [C_<i>n</i>mim][Pro](<i>n</i> = 4, 5, 6)

With the use of isothermogravimetrical analysis, the enthalpies of vaporization, Δ^g_l<i>H</i>^o_m(<i>T</i>_av), at the average temperature, <i>T</i>_av = 445.65 K, for the ionic liquids (ILs) 1-alkyl-3-methylimidazolium propionate [C_<i>n</i>mim]­[Pro]­(<i>n</i> = 4, 5, 6) were determined. Using Verevkin’s method, the difference of heat capacities between the vapor phase and the liquid phase, Δ^g_l<i>C</i>_p^o_m, for [C_<i>n</i>mim]­[Pro]­(<i>n</i> = 2, 3, 4, 5, 6), were calculated based on the statistical thermodynamics. Therefore, with the use of Δ^g_l<i>C</i>_p^o_m, the values of Δ^g_l<i>H</i>^o_m(<i>T</i>_av) were transformed into Δ^g_l<i>H</i>^o_m(298), 126.8, 130.3, and 136.5 for [C_<i>n</i>mim]­[Pro]­(<i>n</i> = 4, 5, 6), respectively. In terms of the new scale of polarity for ILs, the order of the polarity of [C_<i>n</i>mim]­[Pro]­(<i>n</i> = 2, 3, 4, 5, 6) was predicted, that is, the polarity decreases with increasing methylene. A new model of the relationship between the surface tension and the enthalpy of vaporization for aprotic ILs was put forward and used to predict the surface tension for [C_<i>n</i>mim]­[Pro]­(<i>n</i> = 2, 3, 4, 5, 6) and others. The predicted surface tension for the ILs is in good agreement with the experimental one