用恒温热重法测定1-己基-3-甲基咪唑苏氨酸离子液体C6mimThr蒸汽压和蒸发焓

摘要

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AAIL[C6mim][Thr]的蒸汽压和在平均温度下(Tav= 438.15 K)的蒸发焓(△glH?m(Tav) =128.5 ± 6.0 kJ·mol?1).利用Verevkin等人提出的方法计算得到AAIL[C6mim][Thr]气态和液态的恒压热容差(△glC?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△glH?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AAIL[C6mim][Thr]的假想的正常沸点Tb= 522.07 K,以及沸点的蒸发熵△glS?m(Tb) = 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△glG?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Tb时变为零,而蒸发熵则随着温度上升而增大,是AAIL[C6mim][Thr]蒸发过程的驱动力.%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 △glH?m(Tav) of [C6mim][Thr] were determined by isothermogravimetric analysis at the average temperature (Tav= 438.15 K) and △glH?m(Tav) 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, △glC?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, △glH?m(298.15 K) = 138.4 kJ·mol?1. This value is 1.6 kJ·mol–1higher 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, Tb, was estimated to be 522.07 K. Thus, the evaporation entropy, △glS?m(T), and the evaporation Gibbs free energy, △glG?m(T), of [C6mim][Thr] could be determined for different temperatures. These results showed that △glG?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].