Determination of Ion–Water Correlated Motions in Aqueous Salt Solutions*

We report on the real-space correlated motion of water molecules and ions in an aqueous salt solution. The Van Hove functions of aqueous salt solution were determined by high-resolution inelastic X-ray scattering (IXS) spectra and molecular dynamics simulation. (Pseudo-) Partial Van Hove function was determined to identify the element-specific correlated motions. Our results depict the distance-dependent correlated dynamics in the picosecond time-scale and identify the changes in the anion–water correlations. It is found that the anion–water correlations show a two-step relaxation. The fast term depends on the anion type, while the slow term is hardly dependent on the anion type. This result indicates that the process governing the molecular/ionic connectivity between the water molecules and the anions is almost independent of the type of anions.