Study on Vapor–Liquid Equilibrium and Microscopic Properties of Glycine + H2O, l-Histidine + H2O, and l-Tryptophan + H2O

Vapor–liquid equilibrium (VLE) is essential for separation processes, such as distillation and absorption in chemical production. In this work, the VLE of glycine, l-histidine, and l-tryptophan aqueous solutions was measured at pressures ranging from 5.3 to 101.1 kPa. Xu’s model was used, and the model was based on the nonrandom two-liquid (NRTL) model, which has been successfully correlated with VLE data at temperatures between 298.15 and 373.85 K, and the model parameters were obtained. In addition, the absorption peaks of three amino acid aqueous solutions were analyzed by Fourier transform infrared (FTIR) spectroscopy from a microscopic perspective. The results showed that the higher the solubility of amino acids in water, the stronger the hydrogen-bonding interaction with water. Furthermore, the total mean interaction energy of water in the systems was studied by using a COSMOthermX 2021 tool. As inferred from the calculations, the total mean interaction energy was slightly affected by the concentration. The structure–activity relationship was established between the microscopic interactions and the macroscopic VLE.