A Perturbed Hard-Sphere Model with Mean Spherical Approximation for the Activity Coefficients of Amino Acids in Aqueous Electrolyte Solutions

A perturbed hard-sphere thermodynamic model with an electrostatic term has been developed to correlate the activity coefficients of amino acids in aqueous electrolyte solutions. The model considers the amino acid molecules as hard spheres with embedded dipole moments and the ions as hard spheres with embedded charges. The electrostatic term, which accounts for the electrostatic interactions of ions in a water−electrolyte system, is represented by a mean spherical approximation model. The energy of interaction terms incorporated in the perturbed term of the model are those due to the effect of the dispersion forces, angle-averaged dipole−dipole, dipole−charge-induced dipole, and angle-averaged charge−dipole interactions. The model can predict the experimental data of activity coefficients of amino acids in aqueous solutions at low electrolyte concentrations and can accurately correlate the results at higher electrolyte concentrations.