The water dielectric relaxation in electrolyte solutions of simple salts is usually faster than that in pure water, even though the ions structure water in their hydration shell and restrict its mobility. As the electrolyte concentration increases, the water dielectric relaxation accelerates despite increasing solution viscosity – in contradiction to Debye theory.  <br/>Here, by analyzing the results of molecular dynamics simulations, dielectric relaxation, infrared and NMR relaxation spectroscopies we explain the origin of the fast dielectric response of water in a series of alkali-chloride, alkaline-Earth and transition metal solutions. Specifically, we show how the local, ion-induced perturbation to the hydrogen-bonding structure and dynamics affects macroscopic dielectric responses. The simulations allow us to decompose spectra into subsets of water in different states of interactions and to connect macroscopic observations with the molecular properties of water in these compartments.<br/>We conclude that if water molecules are symmetrically structured in ion hydration shells, then the vector sum of their dipole moments approaches zero, tetrahedral orientation promoting hydrogen bonding is lost and consequently the effective dielectric constant is low – dielectric saturation. These water molecules behave as defects in the hydrogen-bond network and therefore have much faster dielectric response than the bulk water. However, the hydration shells start to significantly overlap at higher concentrations, which reverse the trend in dielectric response time as a function of salt concentration. In this regime, the overlapping hydration shells start to restrict and slow each others dielectric responses.<br/>Our results are relevant for understanding properties of high-concentration aqueous solutions that are common in energy-relevant Earth, atmospheric and technological systems. Ranging from brines to high-salinity waters used for freshwater production or as heat-transfer media in cooling towers.

Origin of fast dielectric response of water in electrolytes

2019 Fall National Meeting

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