A Fresh Perspective on Water Dynamics in Aqueous Salt Solutions

Abstract

Molecular dynamics in pure water and aqueous salt solutions remain incompletely understood, partly due to the apparent contradictions between results from different spectroscopic techniques. In this work, we demonstrate, by detailed comparison of light scattering and dielectric spectroscopy data for pure water and aqueous lithium chloride solutions, that these apparent contradictions can be resolved by accounting for orientational cross-correlations of neighboring molecules. Remarkably, a single structural relaxation mode with largely temperature- and concentration-independent shape can be identified in all spectra, from room temperature down to the deeply supercooled regime. These results provide a new perspective for the study of molecular dynamics in aqueous salt solutions.

Figures (4)

• Figure 1: a) DDLS (combined TFPI and RS) spectra of pure water and aqueous lithium chloride solutions at 298K. Colored curves are model functions superimposed to describe the data (see text). The black dashed curves represent the total fit function. b) Peak relaxation times of the relaxation peak compared to literature data of the viscosity at 298K by Tanaka et al. tanaka1991physico.
• Figure 2: a) DDLS spectra of the 14.8% solution at 298K and the 1.8% at 263K normalized to the amplitude of the relaxation peak. b) DDLS spectra of pure water and the 0.5% solution at 298K.
• Figure 3: DDLS susceptibilities and dielectric loss spectra of pure water and some chosen aqueous lithium chloride solutions. The spectra are shifted in amplitude for better comparison.
• Figure 4: a) DDLS spectrum and dielectric loss of the 14.8% solution at 148K. The inset shows an Arrhenius plot of the DDLS peak relaxation times and the BDS peak relaxation times obtained in this work (blue empty squares) and by Lunkenheimer et al.lunkenheimer2023mysteries (green filled triangles) at the same lithium chloride concentration. The black curve represents a fit by the VFT equation. b) DDLS spectra of pure water and aqueous lithium chloride solutions at different concentrations and temperatures, normalized to the peak frequency and amplitude of the relaxation peak. The black dashed curve represents a generalized gamma distribution of relaxation times with width parameter of $\alpha=2$ and a high frequency power law parameter of $\beta=0.5$.