Vibrational memory of anion-modulated water molecules in ultrahigh-concentrated electrolytes

Abstract

The properties of aqueous (aq.) salt solutions are consequential to various scientific and technological domains as the ion–water and inter–water interactions modulate their properties. In this study, we studied the vibrational, structural, and molecular dynamics properties and correlations for a wide range of molal concentrations of aq. LiCl and LiNO₃ solutions (deuterated; five different concentrations ranging from 1.0 m to ultrahigh concentrations) using classical molecular dynamics simulations and a flexible water model. The more charge-dense Cl⁻ anions induce greater structural rigidity and sluggish dynamics. The time evolution of vibrational frequencies of probe OD modes of dil. HOD/H₂O was also investigated. The orientation dynamics of water dominate the slow vibrational spectral diffusions of this mode. The correlations of dynamic properties are more monotonous for aq. LiNO₃ solutions across the studied concentrations. In contrast, for aq. LiCl systems, the influence of Cl⁻ became more prominent, leading to more heterogeneous trends. For both anions, the duration of the tethering of water in their solvation strongly correlates linearly with the observed spectral diffusion, contributing to the memory effect. At ultrahigh concentrations, the Cl⁻–water hydrogen bonding directly modulates it, thereby playing a direct role in preserving the vibrational memory of water. In aq. LiNO₃, the inter–water interactions appear to determine the timescale of spectral diffusion. When considered together with the strong correlation of its hydration shell, a more subtle influence is observed from NO₃⁻ anions.