A study of free energy relationships in some amino acid-sodium chloride-water systems.

Four systems of the type, amino acid-sodium chloridewater, have been investigated over wide solute molality ranges using the isopiestic vapor pressure method. The amino acids used were glycine, (3-alanine, y-aminobutyric acid, and +aminocaproic acid. Equations were obtained in terms of the molalities of the solutes for the activity coefficients of the amino acids in the salt solutions. The trace activity coefficients of the amino acids were all slightly negative at low salt molalities. At higher salt molalities, the trace activity coefficients for glycine and p-alanine remained negative whereas those for ,&aminobutyric acid and Eaminocaproic acid became positive. The limiting interaction parameters were calculated for these systems using the Kirkwood ion-dipole expression and an empirical term for the salt effect on the nonpolar portion of the molecule. Good agreement was obtained between the calculated values and those determined experimentally.