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This study investigates the coupled dynamics of ions and water molecules in their first hydration shell, focusing on alkali and alkaline earth metals along with chloride, and how these dynamics are influenced by isotopic mass. By validating results against existing measurements and ab initio calculations, the authors reveal that the vibrational power spectra of ions in water consistently show multiple peaks, indicating the presence of distinct rattling frequencies. These frequencies, which vary with isotopic mass, provide insights into ion solvation processes and their implications for phenomena such as ion pairing and metal-ligand interactions.
Ions in water exhibit multiple rattling frequencies that reveal their solvation dynamics and could influence selective ion transport and binding behaviors.
The coupled dynamics of ions and water molecules in their first hydration shell impact a variety of processes including ion diffusion, selective ion transport in water-filled nanopores, and the kinetics of ion-pairing, ion adsorption, and metal-ligand binding reactions. In this work, we study these coupled dynamics for alkali metals (Li, Na, K, Rb, Cs), alkaline Earth metals (Mg, Ca, Sr, Ba), and chloride through the lens of their dependence on ion isotopic mass. Results are validated against previous measurements of the isotopic mass-dependence of ion diffusion coefficients in water and previous ab initio calculations of ion high-frequency dynamics in water. We find that the vibrational power spectra of ions in water consistently exhibit either two or three peaks, i.e., ions have several rattling frequencies within their solvations shells as previously reported for a subset of the species examined here. These frequencies have different sensititivies to isotopic mass that may serve as signatures of ion solvation processes (such as the tendency of ions to orient their first-shell water molecules) and that also may relate to Hofmeister-like effects including the relative affinity of different metals for ribonucleic acid (RNA).
