[AAE]X composed of amino acid ester cations is a sort of typically “bio-based” protic ionic liquids (PILs). They possess potential Brønsted acidity due to the active hydrogens on their cations. The Brønsted acidity of [AAE]X PILs in green solvents (water and ethanol) at room temperature was systematically studied. Various frameworks of amino acid ester cations and four anions were investigated in this work from the viewpoint of structure–property relationship. Four different ways were used to study the acidity. Acid dissociation constants (pKa) of [AAE]X determined by the OIM (overlapping indicator method) were from 7.10 to 7.73 in water and from 8.54 to 9.05 in ethanol. The pKa values determined by the PTM (potential titration method) were from 7.12 to 7.82 in water. Their Hammett acidity function (H0) values (0.05 mol·L−1) were about 4.6 in water. In addition, the pKa values obtained by the DFT (proton-transfer reactions) were from 7.11 to 7.83 in water and from 8.54 to 9.34 in ethanol, respectively. The data revealed that the cationic structures of [AAE]X had little effect and the anions had no effect on the acidity of [AAE]X. At the same time, the OIM, PTM, Hammett method and DFT method were reliable for determining the acidic strength of [AAE]X in this study.

中文翻译：

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生物质子离子液体的质子解离：[AAE]X 氨基酸离子液体

由氨基酸酯阳离子组成的 [AAE]X 是一种典型的“生物基”质子离子液体 (PIL)。由于其阳离子上的活性氢，它们具有潜在的布朗斯台德酸度。系统研究了 [AAE]X PIL 在绿色溶剂（水和乙醇）中室温下的布朗斯台德酸度。本工作从结构-性能关系的角度研究了氨基酸酯阳离子和四种阴离子的各种框架。四种不同的方法被用来研究酸度。[AAE]X 的酸解离常数 (pKa) 由 OIM（重叠指示剂法）测定，在水中为 7.10 至 7.73，在乙醇中为 8.54 至 9.05。由 PTM（电位滴定法）测定的 pKa 值在水中为 7.12 至 7.82。他们的哈米特酸度函数 (H0) 值 (0.05 mol·L-1) 约为 4。6 在水中。此外，通过 DFT（质子转移反应）获得的 pKa 值在水中分别为 7.11 至 7.83，在乙醇中为 8.54 至 9.34。数据显示[AAE]X的阳离子结构几乎没有影响，阴离子对[AAE]X的酸度没有影响。同时，OIM、PTM、Hammett 法和 DFT 法可用于确定本研究中 [AAE]X 的酸强度。