Determination of vaporization enthalpy for ionic liquids [Cnmim][Lact] (n = 2, 3, 5) and applications of the molar surface Gibbs free energy

Lactic acid ionic liquids 1-alkyl-3-methylimidazolium [Cnmim][Lact] (n = 2, 3, 5) were prepared and confirmed. The enthalpy of vaporization, \( \Delta _{\text{l}}^{\text{g}} H_{\text{m}}^{\text{o}} \left( {T_{\text{av}} } \right) \), at the average temperature, T av, for the three ILs was determined using isothermogravimetrical analysis, and in terms of \( \Delta _{\text{l}}^{\text{g}} C_{\text{p,m}}^{\text{o}} \), the value of \( \Delta _{\text{l}}^{\text{g}} H_{\text{m}}^{\text{o}} \left( {T_{\text{av}} } \right) \) can be transformed into \( \Delta _{\text{l}}^{\text{g}} H_{\text{m}}^{\text{o}} \left( {298} \right) \); then, according to the scale of polarity, δ μ, the polarity of the three ILs was estimated, in which the order of the polarity is in accordance with our experiences. Moreover, a new thermodynamic quantity—molar surface Gibbs free energy, g s—is derived from the improved Tong’s vaporization enthalpy model. In terms of g s, the traditional Eotvos equation was improved to be a new Eotvos one, in which the parameters have clear physical meanings: Its slope is molar surface entropy, s, and the intercept is the molar surface enthalpy, h. In addition, combining g s with Lorentz–Lorenz equation, an expression for predicting surface tension of the ILs was obtained, and the predicted values of surface tension are in good agreement with the experimental ones.