  We report relaxation dynamics of glycerol-water mixtures as probed by
megahertz-to-terahertz dielectric spectroscopy in a frequency range from 50 MHz
to 0.5 THz at room temperature. The dielectric relaxation spectra reveal
several polarization processes at the molecular level with different time
constants and dielectric strengths, providing an understanding of the
hydrogen-bonding network in glycerol-water mixtures. We have determined the
structure of hydration shells around glycerol molecules and the dynamics of
bound water as a function of glycerol concentration in solutions using the
Debye relaxation model. The experimental results show the existence of a
critical glycerol concentration of ~7.5 mol %, which is related to the number
of water molecules in the hydration layer around a glycerol molecule. At higher
glycerol concentrations, water molecules dispersed in a glycerol network become
abundant and eventually dominate and four distinct relaxation processes emerge
in the mixtures. The relaxation dynamics and hydration structure in
glycerol-water mixtures are further probed with molecular dynamics simulations,
which confirm the physical picture revealed by the dielectric spectroscopy.
