The extended NRTL and Wilson local composition models for amino acid solutions have been modified for the representation of the phase equilibrium behavior of aqueous amino acid – electrolyte solutions by considering cells with random composition for the reference Gibbs energies or enthalpies of local composition cells with a central amino acid molecule and also with a central ion. These new local composition models, which have a molecular thermodynamic framework, have been used to model the vapor–liquid and solid–liquid equilibrium behavior of amino acids and small peptides in aqueous solutions as functions of temperature, ionic strength, and amino acid compositions. The utility of the models is demonstrated with a successful representation of the activity coefficients and the solubility of several amino acids in different aqueous solutions.Key words: amino acid, NRTL, Wilson, activity coefficient, solubility, aqueous solution.
