Measurements and modelling of the solubility of a mixture of two amino acids in aqueous solutions

Measurements were performed to determine the solubilities at 298.2 K of two pairs of amino acids in aqueous solutions. The systems studied were water + glycine + DL-aspartic acid and water + glycine + DL-phenylalanine. The results show that for these systems the solubility of an amino acid increases as the concentration of the other amino acid increases. The solubility of DL-phenylalanine passes through a maximum with an increase in the glycine concentration. Comparison of the results obtained in this work with those reported in previous studies, indicates that the solubility of an amino acid in the presence of another amino acid depends not only on the concentration of the second amino acid, but also on the chemical structure of both amino acids. A model has been developed to correlate the solubilities of an amino acid in aqueous solutions of another amino acid. The activity coefficients of the amino acids, required to correlate the solubility were represented by two different models. An NRTL and a perturbed hard sphere model, with two amino acid-amino acid adjustable interaction parameters, were used. The water-amino acid interaction parameters in the NRTL model for the amino acids with low solubilities, namely DL-aspartic acid and DL-phenylalanine, were set to zero. Both activity models can accurately correlate the solubility of an amino acid in aqueous solutions of another amino acid. It was found that, at amino acid concentrations higher than the saturation in pure water, the NRTL model can extrapolate the activity coefficients of amino acids better than the perturbed hard sphere model. (C) 1999 Elsevier Science B.V. All rights reserved.