Thermodynamic modeling of salt-water systems up to saturation concentrations based on solute speciation: CuCl2–MCln–H2O at 298 K (M = Li, Mg, Ca)

A reaction model was developed to describe the thermodynamic properties of aqueous electrolyte solutions. In the model various types of association (ion–solvent, ion–ion) are incorporated using as much as possible structure information. In the framework of the model, an electrolyte aqueous solution is treated as a mixture of charged or neutral associated species consisting of cations, anions and the solvent water, among the species the short range interactions are assumed to be equal. The abundance of each species is determined by its specific Gibbs energy of formation related to the assumed master species. The total Gibbs energy of mixing consists of a long range electrostatic term and short range interaction terms, the latter are sum of Gibbs energy of all species. Based on the total Gibbs energy of mixing, activity expressions for each species were derived. The Gibbs energy of formation of each associated species is correlated by its stepwise formation Gibbs energies, thus reducing the number of necessary adjustable parameters. At the example systems CuCl2–MCln–H2O (M = Li, Ca, Mg) model parameters were determined by fitting experimental data of water activities and solubilities on the basis of ion associates in agreement with available structure information. Component activities, solubility isotherms and species abundances were calculated and compared with experimental results. This facilitates an understanding of structure–property relationships in the titled systems.