Neutron Inelastic Scattering Investigations of Hydration Complexes and Associated Diffusive Kinetics in Ionic solutions
Authors: Safford G. J. , Leung P. S. 
Published: August 2023

The dependences on concentration, on cation, on anion, and on temperature of frequencies (below 900 cm−1) characteristic of H2O molecules in hydration complexes and the solvent structure and of the associated diffusive kinetics have been investigated by neutron inelastic and “quasi‐elastic” scattering. With increasing concentration the frequencies characteristic of water are rapidly lost, and frequencies characteristic of librational modes of H2O's and of the metal‐oxygen stretching and bending modes for ion hydration complexes appear. For small or multiply charged cations such frequencies are primarily determined by the cation and only secondarily by – 1 anions. With increasing temperature these frequencies broaden and become more poorly resolved as the hydration complexes are disrupted thermally. In contrast, for larger, singly charged cations, the influence of anions is more pronounced. In addition, the ion‐water frequencies initially sharpen with increasing temperature as the result of a more rapid thermal disruption of remnant solvent coordinations than that of the ion‐water coordinations. Below 25°C, the angular and temperature dependences of the widths and of the areas of the diffusively broadened quasi‐elastic components are in accord with a simple delayed diffusion approximation. For solutions containing small or multiply charged ions, a decrease in the self‐diffusion coefficient, D, and an increase in the residence time, relative to water occurs (a “positive hydration” behavior). The values of D and for positively hydrating ions decrease and increase respectively with increasing concentration and are primarily determined by the cations and only secondarily influenced by – 1 anions. Larger, singly charged ions at lower temperatures, initially increase D and decrease relative to water (a “negative hydration” behavior). To study the relationship between diffusive kinetics in ionic solutions and glass formation upon supercooling, neutron spectra have been measured at 1°C for solutions of LiCl, LiNO3, LiNO2, CD3COOLi, CrCl3 and Cr(NO3)3 where solubility allows sufficient concentrations to be attained so that the majority of the H2O molecules present are coordinated in hydration complexes. In addition, measurements were made on concentrated CrCl3 and La(NO3)3 solutions which could be held in the supercooled state so that spectral comparisons could be made just above and below the glass transition. At these high concentrations, departures from a delayed diffusion behavior involving individual H2O molecules were observed and the diffusive kinetics were in accord with the “classical” diffusion involving heavy masses and low values of D. For such strongly hydrating cations, the exchange times for primary H2O's should be long compared to the neutron interaction time, and so such motions would not contribute to spectra. For a 15.02m solution a change from a classical diffusion behavior toward a delayed diffusion behavior occurs as the temperature increases from 1 °C to 25 °C and the for primary waters decreases. In contrast, for a 7.2 m CD3COOLi solution a classical diffusion behavior persists to 50 °C, due to a stabilization of primary H2O's by the acetate ion. The observed diffusive behavior appears in accord with diffusive motions of hydrated cations. The self‐diffusion coefficients decrease rapidly both with “supercooling” and with anions in the progression DCl‐ > D > D > D (in correspondence to increases in the glass transition temperatures). The results indicate that glass formation may primarily arise from a gradual “freezing out” of such diffusive motions upon supercooling these solutions.