Abstract A new model for representation of the excess Gibbs energy of polymer–electrolyte solutions is proposed. The excess Gibbs energy of a polymer–electrolyte solution is expressed as a sum of contributions of a combinatorial, a long-range and a short-range excess Gibbs energy term. The Flory–Huggins expression and the Pitzer’s extension of the Debye–H¨uckel function are used respectively as a combinatorial and a longrange contribution to the excess Gibbs energy. A new expression based on the local composition concept, which is the modified Wilson model, is developed to account for the short-range contribution to the excess Gibbs energy. The model provides a versatile and flexible thermodynamic framework for both correlating and predicting the phase equilibrium of electrolyte solutions, polymer solutions and complex systems containing both electrolytes and polymers. The utility of the model is demonstrated with successful representation of vapor–liquid equilibrium of several PEG or PPG-salt–H2O systems at different polymer molar masses. It was found that, the absolute relative percentage deviations of water activity for polymer solutions, salt solutions and polymer + salt solutions are about 0.1%, 0.04% and 0.1%, respectively. Results are compared with those obtained from the Wilson model.
