Solubility of Amino Acids: Influence of the pH value and the Addition of Alcoholic Cosolvents on Aqueous Solubility

The solubility of dl-methionine in pure water has been measured in a temperature range of 293.15-353.15 K by means of on-line attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. These experimental data, as well as literature data for the solubility of glycine and dl-alanine in aqueous and alcohol solutions, were modeled using the PC-SAFT equation of state. Purecomponent parameters for glycine, dl-alanine, and dl-methionine were fitted to the vapor pressures and to the densities of their aqueous solutions. Only one temperature-independent binary parameter is required for each system. The solubility of amino acids in water-alcohol mixtures (ternary systems) was then predicted without fitting any additional parameters. The results of the prediction correspond well to literature data. To retrieve the pH dependency, the solubility of dl-methionine in different HCl and NaOH solutions in the pH range of 1.5-9.5 was determined via gravimetric measurements. Using the knowledge of the acid-base behavior (pKa values) of the respective amino acids, the solubility of glycine, dl-alanine, and dl-methionine in aqueous electrolyte solutions can be predicted in good agreement with the experimental data.