Simultaneous correlation of mean ionic activity coefficient and osmotic coefficient of electrolyte solutions by a new local composition model

A new model for representation of the excess Gibbs energy of electrolyte solutions is proposed. The excess Gibbs energy of an electrolyte solution is expressed as a sum of contributions of a long-range and a short-range excess Gibbs energy term. The Pitzer–Debye–Hückel model is used as a long-range contribution to the excess Gibbs energy. A new expression based on the local composition concept, which is the modified electrolyte Wilson model, is developed to account for the short-range contribution to the excess Gibbs energy, in which an energy correction term has been introduced empirically, based on calculation results from statistical thermodynamics. The new model contains four adjustable parameters per binary electrolyte–solvent system. The utility of the model is demonstrated with the successful representation of the mean ionic activity coefficient and osmotic coefficient of several aqueous electrolyte solutions. The results show that the model is valid for the whole range of electrolyte concentration, from dilute solution up to saturation. The results are compared with those obtained from the NRTL and Wilson models. The model presented in this work, which requires four adjustable parameters, yields better results than the NRTL and Wilson models which require two adjustable parameters.