Physico-chemical Properties of 1‑Alkyl-3-methylimidazolium Propionate Ionic Liquids {[C_<i>n</i>mim][Pro](<i>n</i> = 3, 4, 5, 6)} from 288.15 K to 328.15 K

Four propionic acid ionic liquids (PrAILs) [C_<i>n</i>mim]­[Pro] (<i>n</i> = 3, 4, 5, 6) (1-alkyl-3-methylimidazolium propionate) were prepared by the neutralization method and characterized by ¹H NMR spectroscopy and differential scanning calorimetry. The values of their density, surface tension, and refractive index were measured in the temperature range of 288.15–328.15 K. Since the PrAILs can strongly form hydrogen bonds with water, the small amount of water is difficult to remove from the PrAILs by conventional methods. To eliminate the effect of the water impurity, the standard addition method (SAM) was applied to these measurements. The dependence of volumetric properties, surface properties, and molar refraction on temperature were discussed. In terms of Kabo’s method the molar enthalpies of vaporization, Δ<i>H</i>^V, for the PrAILs were estimated. As a new idea, it was put forward that Δ<i>H</i>^V can be assumed to consist of two parts: one is the contribution from induced dipole, Δ<i>H</i>^V_<i>n</i>, and the other is the contribution from the permanent dipole of ion pair in ILs, Δ<i>H</i>^V_μ. The values of Δ<i>H</i>^V_<i>n</i> can be estimated in terms of refractive index data so that the values of Δ<i>H</i>^V_μ can be obtained and Δ<i>H</i>^V_μ may be seen as the measure of polarity for ILs