The structure and dynamics of salt solutions with low water activity have increasingly important implications for energy storage, radioactive waste remediation and industrial chemical processing. Obtaining a microscopic view of solvent-solute interactions in concentrated water-in-salt solutions allows determination of reactant encounter rates, which influence chemical separation and nucleation processes. The enhanced viscosity in high salt solutions is microscopically related to retardation of ion-water mobilities, and formation of solvent-solute clustering networks. At high concentrations, counterions are no longer innocent charge-balancing/screening species, and instead greatly affect the local coordination responsible for distinct ion-water clustering dynamics. Neutron pair distribution function and quasi-elastic neutron scattering represent experimental tools for probing structure and dynamics of ion-water interactions, providing rigorous and quantitative validation to computational models. Here, we demonstrate the contrasting influence of sodium and potassium countercations on speciation equilibria and stabilization of polynuclear aluminate entities in concentrated solutions. Distinct crystallization mechanisms suggest that different prenucleation cluster dynamics and network packing morphologies can either frustrate (sodium) or promote (potassium) crystallization, as described by non-classical nucleation theory. Characterizing and controlling aluminum speciation in highly concentrated hydroxide solutions is a challenge in the treatment of the nation’s high-level legacy radioactive waste streams. Our studies demonstrate the potential to use ion-specific effects to promote greener, more energy-efficient aluminum processing technologies, as well as chemical strategies for the control of crystallization processes beyond the classical settings.

Advances in understanding concentrated water-in-salt solution: Structure, dynamics and nucleation mechanisms

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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