  The glass formation and the dipolar reorientational motions in deep eutectic
solvents (DESs) are frequently overlooked, despite their crucial role in
defining the room-temperature physiochemical properties. To understand the
effects of these dynamics on the ionic conductivity and their relation to the
mechanical properties of the DES, we conducted broadband dielectric and
rheological spectroscopy over a wide temperature range on three
well-established carboxylic-acid-based natural DESs: oxaline, maline, and the
eutectic mixture of choline chloride with phenylacetic acid (phenylaceline). In
all three DESs, we observe signs of glassy freezing in the temperature
dependence of their dipolar reorientational and structural dynamics, as well as
varying degrees of motional decoupling between the different observed dynamics:
Maline and oxaline display a breaking of the Walden rule near the
glass-transition temperature, while the relation between the dc conductivity
and dipolar relaxation time in both maline and phenylaceline is best described
by a power law. The glass-forming properties of the investigated systems not
only govern the orientational dipolar motions and rheological properties, which
are of interest from a fundamental point of view, but they also affect the dc
conductivity, even at room temperature, which is of high technical relevance.
