Predicting activity coefficients with the Debye‐Hückel theory using concentration dependent static permittivity

In order to investigate the impacts of a concentration dependent static permittivity in the Debye‐Hückel theory, two electrostatic Helmholtz free energy models and four activity coefficient models, with the ion‐solvent interactions naturally included, are derived under different assumptions. The effects of static permittivity and model parameters are analyzed by predicting the mean ionic activity coefficients. It is found out that it is reasonable to assume a constant static permittivity in deriving the electrostatic Helmholtz free energy model but it is highly recommended to take the concentration dependence of static permittivity into account in subsequent calculations of thermodynamic properties. The activity coefficient model derived in this way accurately predicts the mean ionic activity coefficients of the investigated systems up to 0.1mol/kg H₂O, which indicates that the size parameters in the Debye‐Hückel theory might be determined in advance before it is integrated into a more complete thermodynamic model.