用恒温热重法测定1-己基-3-甲基咪唑苏氨酸离子液体[C6mim][Thr]蒸汽压和蒸发焓

用中和法合成了氨基酸离子液体(AAIL)1-己基-3-甲基苏氨酸盐C₆mim]Thr]，并用核磁共振氢谱(¹H NMR)和核磁共振碳谱(¹³C NMR)进行了表征。以苯甲酸为参考物质，用恒温热重法确定了AAILC₆mim]Thr]的蒸汽压和在平均温度下(T_av= 438.15 K)的蒸发焓(Δ^g_lH_m^? (T_av) =128.5 ± 6.0 kJ·mol^-1)。利用Verevkin等人提出的方法计算得到AAILC₆mim]Thr]气态和液态的恒压热容差(Δ^g_lC_pm^? = -70.8 J·K^-1·mol^-1)，进而计算了不同温度的蒸发焓，其中参考温度(298.15 K)下的蒸发焓Δ^g_lH_m^? (298.15 K) = 138.4 kJ·mol^-1，只比应用我们提出的蒸发焓理论模型估算值大1.6 kJ·mol^-1，小于恒温热重法的实验误差3.0 kJ·mol^-1，说明这个蒸发焓的理论模型有一定的合理性。借助Clausius-Clapeyron方程估算了AAILC₆mim]Thr]的假想的正常沸点T_b= 522.07 K，以及沸点的蒸发熵Δ^g_lS_m^? (T_b) = 228.5 J·K^-1·mol^-1，进一步得到了不同温度的蒸发熵和蒸发自由能Δ^g_lG_m^? (T)，其结果表明蒸发自由能随着温度的上升而减小，达到沸点温度T_b时变为零，而蒸发熵则随着温度上升而增大，是AAILC₆mim]Thr]蒸发过程的驱动力。

Measurement of Vapor Pressure and Vaporization Enthalpy for Ionic Liquids 1-Hexyl-3-methylimidazolium Threonine Salt[C6mim][Thr]by Isothermogravimetric Analysis

The amino acid ionic liquid (AAIL) 1-hexyl-3-methylimidazolium threonine salt, C₆mim]Thr], was prepared by the neutralization method and its structure was confirmed by ¹H and ¹³C NMR spectroscopy. Using benzoic acid as the reference material, the vapor pressure and evaporation enthalpy Δ^g_lH_m^? (T_av) of C₆mim]Thr] were determined by isothermogravimetric analysis at the average temperature (T_av = 438.15 K) and D^g_lH_m^? (T_av)was found to be (128.5 ± 6.0) kJ·mol^-1. Using Verevkin's method, the difference between the heat capacities of the vapor and liquid phases, Δ^g_lC_pm^?, was calculated to be -70.8 J·K^-1·mol^-1. Subsequently, the enthalpy of vaporization for AAIL C₆mim]Thr] at different temperatures was determined based on the reference enthalpy of vaporization at 298.15 K, Δ^g_lH_m^? (298.15 K) = 138.4 kJ·mol^-1. This value is 1.6 kJ·mol^-1 higher than that predicted by our theoretical model and less than the experimental error (±3.0 kJ·mol^-1) of the isothermogravimetric method. These results show that our theoretical model for determining the evaporation enthalpy of ILs is reasonable. In terms of the Clausius-Clapeyron equation, the hypothetical normal boiling point, T_b, was estimated to be 522.07 K. Thus, the evaporation entropy, Δ^g_lS_m^? (T), and the evaporation Gibbs free energy, Δ^g_lG_m^? (T), of C₆mim]Thr] could be determined for different temperatures. These results showed that D^g_lG_m^? (T) decreases as the temperature increases, the evaporation entropy increases with increaseing temperature. Furthermore, the latter is the driving force in the evaporation process of the AAIL C₆mim]Thr].