Emergence of local slow dynamics of water molecules induced by sodium chloride*

We report on the real-space correlated motion of water molecules in sodium chloride aqueous solution. Measurements of high-resolution inelastic X-ray scattering spectra, S(Q, E), over wide energy transfer (E) and momentum transfer (Q) enabled us to determine the Van Hove function, G(R, t), by Fourier transform of S(Q, E) over Qand E. The results show that the addition of sodium chloride induces the emergence of a slow decay component at the first neighbor peak in the Van Hove function. The intensity of the slow decay and that of the fast decay are proportional and inversely proportional to the salt molality, respectively, whereas their decay time is independent of the molality. This indicates that the slowed correlated dynamics of water molecules is localized in the hydrated shell around ions whereas the dynamics of bulk water is not influenced by the salt.