Sensitizing Hydration Shells of Ions by Analyzing Water Dynamics Using High Sensitive Dielectric Spectroscopy

A variety of measurement techniques have provided evidence that ions and other solute molecules affect the structure and dynamics of water directly surrounding them. Most experiments use infrared spectroscopy to study the vibrational relaxation of hydration shells, which observe intramolecular vibrations. In response, we have employed a highly sensitive, high-resolution, frequency domain, MHz to THz dielectric spectrometer that is sensitive to intermolecular dynamics. We have confirmed that water dynamics over this range are best described by three Debye relaxation processes with three-time constants of 8.56, 1.1ps and 179fs. Our argument is also supported by studies of dielectric relaxation of aqueous salt solutions, which produce the same three-time constants but amplitudes that vary with solute concentration. The amplitude of each process provides information about the structure of hydration shells. While the amplitude of the first process is related to the structure of the first hydration layer, the amplitude of the second and the third are linked to the second and/or the third hydration layers. Our results shed light on the dynamics of hydration shells around solute molecules in a biologically relevant environment.