  We have studied the ionic conductivity and the dipolar reorientational
dynamics of aqueous solutions of a prototypical deep eutectic solvent (DES),
ethaline, by using dielectric spectroscopy on a broad range of frequency
(MHz-Hz) and for temperatures ranging from 128 to 283 K. The fraction of water
in the DES was varied systematically to cover different regimes, starting from
pure DES and its water-in-DES mixtures to the diluted electrolyte solutions.
Depending on these parameters, different physical states were examined,
including low viscosity liquid, supercooled viscous liquid, amorphous solid and
freeze-concentrated solution. The ionic conductivity and the reorientational
relaxation both exhibited characteristic features of glassy dynamics that could
be quantified from the deviation from Arrhenius temperature dependence and
non-exponential decay of the relaxation function. A transition occurred between
the water-in-DES regime, (< 40 wt %),where the dipolar relaxation and ionic
conductivity remained inversely proportional to each other, and the
DES-in-water regime, (> 40 wt %), where a clear rotation-translation decoupling
was observed. This suggests that for low water content, on the timescale
covered by this study (~10-6 s to 1 s), the rotational and transport properties
of ethaline aqueous solutions obey classical hydrodynamic scaling despite these
systems being presumably spatially microheterogeneous. A fractional scaling is
observed in the DES-in-water regime, due to the formation of a maximally
freeze-concentrated DES aqueous solution coexisting with frozen water domains
at sub-ambient temperature.
