Nonlinear Scaling of Water–Ion Interactions and Dynamics in Alkaline Solutions

Water–ion interactions govern the physicochemical properties of aqueous solutions, impacting the structure of the hydrogen bonding network and ion diffusivities. To elucidate these effects under alkaline conditions relevant to diverse application spaces, we examined NaOD-D₂O solutions using two-dimensional infrared spectroscopy (2D-IR), small-angle X-ray scattering (SAXS), and nuclear magnetic resonance spectroscopy (NMR). Vibrational energy transfer between the donor anion SeCN^–, used as a 2D-IR probe, and the acceptor anion OD^– was used to track the average separation distance of ions in the D₂O solutions, while SAXS and NMR experiments measured the structure of the bulk D₂O solvent. We observed consistent nonlinear scaling in the SeCN^– and OD^– average separation distance as a function of NaOD concentration, while bulk solution D₂O-to-D₂O average separation distance remained linear. The findings suggest that the SeCN^– and OD^– anions participate in a water–ion network that significantly reduces the degrees of freedom for the distribution of ions in solution relative to the stochastic ion distributions typical of the dilute limit.