Measurement of vaporization enthalpy for amino acid ionic liquids [Cnmim][Thr](n = 3, 5) using the isothermogravimetrical analysis

Abstract

The amino acid ionic liquids (AAILs) [Cnmim][Thr](n = 3, 5) were prepared, and their structures were confirmed by NMR spectroscopy and element analysis. The evaporation enthalpy of AAILs [Cnmim][Thr](n = 3, 5) and the sublimation enthalpy of benzoic acid were determined by the isothermogravimetrical analysis at their average temperature, respectively. Using Verevkin’s method, the difference of heat capacities between the vapor phase and the liquid phase, $$\Delta^{\text{g}} _{\text{l}} C_{\text{p}} ^{\text{o}} {}_{\text{m}}$$ , for AAILs [Cnmim][Thr](n = 3, 5) was calculated and in terms of $$\Delta^{\text{g}} _{\text{l}} C_{\text{p}}^{\text{o}} {}_{\text{m}}$$ , $$\Delta^{\text{g}} _{\text{l}} H^{\text{o}} _{\text{m}} \left({T_{\text{av}}} \right)$$ can be transformed into $$\Delta^{\text{g}} _{\text{l}} H^{\text{o}} _{\text{m}} \left(T \right)$$ at different temperatures, T, where the evaporation enthalpy, $$\Delta^{\text{g}} _{\text{l}} H^{\text{o}} _{\text{m}}$$ (298.15), at 298.15 K was included. The difference between the value of $$\Delta^{\text{g}} _{\text{l}} H^{\text{o}} _{\text{m}}$$ (298.15) and the corresponding one predicted by Tong’s model is less than the experimental error of 3.0 kJ mol−1, so that it is shown that the model has some reasonableness. According to Rebelo et al., the hypothetical normal boiling point, Tb, was estimated so that the vaporization entropy, $$\Delta^{\text{g}} _{\text{l}} S$$ (Tb) was estimated also. In terms of $$\Delta^{\text{g}} _{\text{l}} C_{\text{p}}^{\text{o}} {}_{\text{m}}$$ , the vaporization entropy, $$\Delta^{\text{g}} _{\text{l}} S\left(T \right)$$ and the evaporation Gibbs free energy, $$\Delta^{\text{g}} _{\text{l}} G^{\text{o}} _{\text{m}} \left(T \right)$$ , of the AAILs were determined at different temperatures. The results show that $$\Delta^{\text{g}} _{\text{l}} G^{\text{o}} _{\text{m}} \left(T \right)$$ decreases with the temperature rise until the boiling point temperature is zero, and $$\Delta^{\text{g}} _{\text{l}} S\left(T \right)$$ increases with the temperature rise until Tb is the maximum so that this indicted that the vaporization entropy is the driving force of the evaporation process of AAIL [Cnmim][Thr](n = 3, 5).