Rapidly Converging Activity Expansions for Representing the Thermodynamic Properties of Fluid Systems: Gases, Non-electrolyte Solutions, Weak and Strong Electrolyte Solutions

Abstract

For dilute gases and non-electrolyte solutions in the McMillan–Mayer standard state, an activity expansion due to Mayer has great advantages over the normal concentration expansion (virial equation) for strongly associating species. For weakly interacting systems, both approaches are suitable. The activity expansion eliminates the need to differentiate between strong “chemical” interactions and weak “physical” interactions since the same equation is used in each situation.

The equation has been modified to represent electrolyte solutions in the McMillan–Mayer standard state by requiring that it be consistent with the Debye–Hückel and higher order limiting laws for strong electrolytes and that it be equivalent to a chemical association model for weak electrolytes. The result is a compact equation which contains no arbitrary ion-size parameters and which does not require the classification of an electrolyte as strong or weak. For 2:2 electrolytes, the equation gives a very good fit to the anomalous low concentration region.

For practical thermodynamic calculations, similar equations for molal activity coefficients are proposed; good fits of the data are obtained.