Prediction of Fick Diffusion Coefficients in Binary Electrolyte Mixtures

The present work reviews different approaches from the literature and suggests an empirical correlation scheme for calculating Fick diffusion coefficients in binary electrolyte mixtures. The mixtures consist of either an electrolyte component dissolved in a molecular solvent or of two electrolyte components sharing a common ion. For both types of mixtures, the diffusive mass transport is characterized by a single Fick diffusion coefficient D₁₁. Prediction models for electrolytes and non-electrolytes mixtures are evaluated, considering experimental D₁₁ data in the literature from dynamic light scattering experiments for binary mixtures, which have a solute amount fraction of 0.05 and include a systematic variation of the solute and solvent components. It could be shown that including information about the fluid structure obtained from molecular dynamics (MD) simulations, such as the formation of solvation shells, the predictive performance of the models can be greatly improved. In general, most models are able to predict D₁₁ in mixtures where the ions are well dissociated by the solvent molecules, but can fail for mixtures where the ionic species tend to aggregate. For binary electrolyte mixtures based on a molecular solvent, an empirical correlation is developed, which can predict D₁₁ for all mixtures with an average absolute relative deviation (AARD) of 21% from the experimental values. For binary mixtures consisting of two electrolyte components sharing a common ion, it could be shown that D₁₁ can be predicted by using only the self-diffusivities of the three ions with an AARD of 15% from experimental data.