HKF model for aqueous electrolytes

Helgeson and co-workers (Helgeson and Kirkham, 1974, 1976 Helgeson et al, 1981) developed an equation of state for aqueous electrolytes based on this continuum model. The model, known as HKF, has two contributions to the standard partial volume an electrostatic part given by Equation (2.78) and the nonelectrostatic part having an intrinsic term, temperature and pressure independent, and a short-range term related to the electrostriction of water around the ion, equivalent to a change of density and dielectric constant of the continuum near the ion. This last contribution was considered to be dependent on temperature and pressure. 

For most electrolyte solutions at or near room temperature, there have been two main approaches, exemplified by the Pitzer and HKF models to be described below. (These two approaches were also mentioned on page 304.) In one, referred to as ion-interaction theory and embodied in the Pitzer equations, no attempt is made (except if there are very strong complexes present) to identify species. Components are treated stoichiometrically, and all ion interactions are accounted for in the form of fit coefficients in some form of equation. Advocates of this approach point to the rather uncertain nature of our knowledge of aqueous species. Molecular dynamics simulations of these solutions... 

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