A new model for predicting activity coefficients in aqueous solutions of amino acids and peptides

Abstract

A new two-parameter model based on the perturbation of a hard-sphere reference has been developed to correlate the activity coefficients of several amino acids and simple peptides in aqueous solutions. The hard-sphere equation of state used as the reference in the model was proposed recently by Ghotbi and Vera. The perturbation terms coupled with the reference hard-sphere equation of state are attributed to the dispersion forces and the dipole–dipole interactions. The Lennard-Jones and Keesom potential functions are used to represent the dispersion and dipole–dipole interactions, respectively. The results of the new model are compared with those obtained by other models. It is shown that the new model can more accurately correlate the activity coefficients of amino acids and peptides in comparison with the other available models in the literature. The model was also used to correlate the solubility of several amino acids in aqueous solutions. The results show that the model can accurately correlate the solubility of the experimental data over a wide range of temperatures with only two adjustable parameters. New values for Gibbs free energy change, Δ g, and enthalpy change, Δ h, of the solute, i.e., amino acid for transferring one mole of solute from a saturated solution to a hypothetical aqueous solution with an activity of one molal at temperature 298.15 K are also reported.