Density (ρ), dynamic viscosity (η), and electrical conductivity (κ) of the deep eutectic solvent (DES) reline, composed of choline chloride (ChCl) and urea in a 1:2 molar ratio, and its mixtures with water, covering the entire miscibility range, were studied at T = (293.15 to 338.15) K. Compared to many previous studies, reline purity was significantly improved by using ultrapure urea and ChCl recrystallized from ethanol. For the investigated DES samples the mass fraction of residual water was <0.00035. This allowed checking the influence of water traces and impurities on the physicochemical properties of pure reline. It was found that the presence of small amounts of water (w(H 2 O) < 0.0081) only negligibly decreased reline density, not exceeding 0.14% compared to the dry sample. However, for the same amount of water η decreased by ∼36% at 298.15 K. The temperature dependence of ρ was well fitted by a quadratic expression, whereas η(T) and κ(T) were found to follow the empirical Vogel−Fulcher−Tammann equation. For the aqueous mixtures excess properties of molar volume (V E ) and viscosity (η E ) showed only minor variation with composition, suggesting rather weak interactions between water and the constituents of reline. However, V E and η E depended significantly on temperature, indicating a significant contribution of H-bonding to the inherent reline structure. Similar to conventional ionic liquids, the conductivity of aqueous reline showed a broad maximum at the reline mole fraction of x 1 ≈ 0.18 associated with the border between aqueous solutions of individual reline components and reline/water mixtures. The Walden plot classifies reline as a poor ionic liquid.

We report results on urea hydration obtained by dielectric relaxation spectroscopy (DRS) in a broad range of concentrations and temperatures. In particular, the effective hydration number and dipole moment of urea have been determined. The observed changes with composition and temperature were found to be insignificant and mainly caused by the changing number density of urea. Similarly, solute reorientation scaled simply with viscosity. In contrast, we find that water reorientation undergoes substantial changes in the presence of urea, resulting in two water fractions. The first corresponds to water molecules strongly bound to urea. These solvent molecules follow the reorientational dynamics of the solute. The second fraction exhibits only a minor increase of its relaxation time (in comparison with pure water) which is not linked to solution viscosity. Its activation energy decreases significantly with urea concentration, indicating a marked decrease of the number of H-bonds among the H2O molecules belonging to this fraction. Noncovalent interactions (NCI) analysis, capable to estimate the strength of the interactions within a cluster, shows that bound water molecules are most probably double-hydrogen bonded to urea via the oxygen atom of the carbonyl group and a cis-hydrogen atom. Due to the increased H-bond strength compared to the water dimer and the rigid position in the formed complex the reorientation of these bound H2O molecules is strongly impeded.

The ethaline composition lies in the ChCl-saturated region of the phase diagram which explains why ChCl precipitates in this solvent.

The influence of the amphiphile 1,3-dimethylurea (1,3-DMU) on the dynamic properties of water was studied using dielectric relaxation spectroscopy. The experiment provided evidence for substantial retardation of water reorientation in the hydration shell of 1,3-DMU, leading to a separate slow-water relaxation in addition to contributions from bulk-like and fast water as well as from the solute. From the amplitudes of the resolved water modes effective hydration numbers were calculated, showing that each 1,3-DMU molecule effectively freezes the reorientation of 1-2 water molecules. Additionally, a significant amount of solvent molecules, decreasing from ∼39 at infinite dilution to ∼3 close to the solubility limit, is retarded by a factor of ∼1.4 to 2.3, depending on concentration. The marked increase of the solute amplitude indicates pronounced parallel dipole alignment between 1,3-DMU and its strongly bound HO molecules. Molecular dynamics (MD) simulations of selected solutions revealed a notable slowdown of water rotation for those solvent molecules surrounding the methyl groups of 1,3-DMU and strong binding of ∼2HO by the hydrophilic carbonyl group, corroborating thus the experimental results. Additionally, the simulations revealed 1,3-DMU self-aggregates of substantial lifetime.