Countercations Control Local Specific Bonding Interactions and Nucleation Mechanisms in Concentrated Water-in-Salt Solutions

One of the continuing challenges presented in salt solutions is understanding ion association reactions driving dynamic demixing from solvation, complexation, and solute clustering. The problems understanding this phenomenon are exacerbated in the highly concentrated water-in-salt solutions, where the deficiency of water leads to a dramatic retardation of water solvent and formation of extended solvent-solute clustering networks. By probing microscopic dynamics of water and prenucleation clusters using quasi-elastic neutron scattering and proton nuclear magnetic resonance spectroscopy, we observed contrasting mechanistic specifics of ion-water mobilities in highly concentrated Na⁺- versus K⁺-based aluminate solutions (diffusion coefficients of 0.2 vs 2.6 × 10^-10 m² s^-1 at 293 K, respectively). The magnitude of the differences is far beyond countercations acting as simple innocent charge-balancing species or water solvents functioning as a simple medium for ion diffusion. The distinct crystallization mechanisms observed further imply that different prenucleation cluster dynamics can either frustrate or promote crystallization, as described by nonclassical nucleation theory.