Study on Vapor–Liquid Equilibrium of l‑Lysine, l‑Arginine, and l‑Threonine Aqueous Solutions

The vapor–liquid equilibrium (VLE) data of amino acids hold immense significance in the realm of chemical engineering. In a meticulous experimental setup, the VLE data of l-lysine + H₂O, l-arginine + H₂O, and l-threonine + H₂O were accurately measured. The research results indicate that as the temperature increases, the vapor pressure in the amino acid system also increases accordingly. Furthermore, systems with high solubility exhibit lower vapor pressures compared to those with low solubility and pure solvents. The validity of these data was thoroughly examined by using the NRTL-Xu model, and the system parameters were precisely calculated. Then, the COSMO-RS theory was employed to calculate the interaction energy within the amino acid system. It was found that as the temperature increased, the absolute value of the total average interaction energy decreased, indicating that water molecules became more prone to escaping. This observation aligns well with the conclusions drawn from the VLE experiments. It is noteworthy that l-arginine exhibited behaviors that differed from those reported in previous studies. To explain this phenomenon, a comprehensive analysis was conducted using σ-profiles and quantum chemical calculations. These investigations revealed that l-arginine possesses a stronger hydrogen bond receptor ability and possesses a greater number of potential hydrogen bond sites, facilitating the formation of hydrogen bonds. This unique characteristic may explain the unique behavior observed in this study.