Modeling Self-diffusion in Multicomponent Aqueous Electrolyte Systems in Wide Concentration Ranges

A comprehensive model has been developed for calculating self-diffusion coefficients in multicomponent aqueous electrolyte systems. The model combines contributions of long-range (Coulombic) and short-range interactions. The long-range interaction contribution, which manifests itself in the relaxation effect, is obtained from the dielectric continuum-based meanspherical approximation (MSA) theory for the unrestricted primitive model. The short-range interactions are represented using the hard-sphere model. In the combined model, aqueous species are characterized by effective radii, which depend on the ionic environment. For multicomponent systems, a mixing rule has been developed on the basis of phenomenological equations of nonequilibrium thermodynamics. The effects of complexation are taken into account by combining the diffusivity model with thermodynamic speciation calculations. The model accurately reproduces self-diffusivities of ions and neutral species in aqueous solutions ranging from infinitely dilute to concentrated (up to ca. 30 mol/kg of H2O). Also, the model makes it possible to predict diffusivities in multicomponent solutions using data for single-solute systems.