Modeling of activity coefficients of amino acid and electrolyte in aqueous solutions

The amino acid exists in the zwitterion form with positive (amino group, − NH3+) and negative (carboxyl group, COO−) charge in water, and it is able to form the ion-pair complexes physically bonded with ions. The ion-pair complex between amino acid and ions has a strong effect on the variation of activity coefficient of amino acid in aqueous electrolyte solution. In this work, the PC-SAFT EOS combined with primitive mean spherical approximation (PM-MSA) model is applied to the 32 amino acid-electrolyte aqueous systems (a total of 1305 data points) for modeling the activity coefficients of amino acids and electrolyte in aqueous solutions. The ion-pair complex is considered as a cross-association between amino acid and ions, and this method accurately represents the activity coefficient of an amino acid and electrolyte over a wide range of concentrations. Based on the calculation of activity coefficient ratio of amino acid, the solubility of an amino acid in electrolyte-water system and density of solution are predicted, and the predicted results show qualitative agreement with experimental results. Therefore, the model proposed in this work is expected to be useful for describing activity coefficients of an electrolyte and amino acid in amino acid-electrolyte-water system.