Investigation of Low-Frequency Motions of H2O Molecules in Ionic Solutions by Neutron Inelastic Scattering

Intermolecular frequencies below 900 cm⁻¹ of H₂O molecules in water and aqueous solutions of LaCl₃, CaCl₂, MgCl₂, CsI, CsCl, KSCN, KI, KBr, KCl, KF, NaCl, LiNO₃, and LiCl have been measured by slow neutron inelastic scattering. The diffusive kinetics of H₂O molecules in aqueous solutions of LaCl₃, MgCl₂, CsCl, KSCN, KCl, KF, NaCl, and LiCl have also been investigated. Changes were observed simultaneously in both the intermolecular frequencies and the diffusive motions of H₂O in ionic solutions relative to water, and they were specific to the size and charge of the ions, the concentrations, and the temperature. Ionic solutions containing a small and/or highly charged ion (e.g., LaCl₃, MgCl₂, LiCl, and KF) showed vibrational maxima at frequencies similar to rocking, wagging, and twisting librational modes and to ion–water stretching modes of H₂O molecules in the corresponding solid salt hydrates. These maxima intensify with increasing concentration. In general, these frequencies lose intensity and broaden with increasing temperature, but many persist to 75°C. Solutions containing large singly charged ions (e.g., CsCl, KCl) also show new frequencies at lower temperatures, but these are generally broader and weaker than those characteristic of smaller or highly charged ions. Many of these maxima appear better defined at higher temperatures. At lower temperatures (1° and 25°C) the diffusive motions of water molecules in most salt solutions are in accord with a delayed diffusion model. Small and/or highly charged ions decrease the self‐diffusion coefficient $D$ and increase the residence time ${\tau }_0$ relative to water. $D$ increases and ${\tau }_0$ decreases with increasing temperature, but they remain smaller and larger, respectively, than for pure water. At higher temperatures the diffusion kinetics depart from delayed diffusion behavior. The neutron spectra indicate that ions of high charge‐to‐radius ratio disrupt the water structure and form complexes having local ordering and bonding similar to that of H₂O molecules in solid salt hydrates. These strong ion–water interactions give rise to higher activation energies for the movement of H₂O molecules relative to pure water; thus, these salts act as “positive hydrators.” In contrast, salts of ions of low charge‐to‐radius ratio increase $D$ and decrease ${\tau }_0$ relative to water and act as “negative hydrators.” The values of the intermolecular frequencies, diffusion coefficients, and residence times obtained during this study are in agreement with those obtained by other techniques. A tentative explanation for the diffusion kinetics is given for cases where the delayed diffusion model is not valid.