Characterization of water in (glucose+urea+water) deep eutectic solvent through dielectric relaxation spectroscopy:...

We report here experiments and computer simulations of dielectric relaxation (DR) in (glucose+urea+water) deep eutectic solvent (DES), where one glucose and one urea molecule are statistically competing for interactions with each of the water molecules that are present as a minor but integral component in this naturally abundant three-component DES. This means that for two H-bond donor and two H-bond acceptor sites in each water molecule, there exist, on average, five donor and 12 acceptor sites from a glucose molecule and four donor and two acceptor sites from a urea molecule. These severe competitive H-bond interactions were highlighted in an earlier simulation study [Baksi et al., Phys. Chem. Chem. Phys. 23, 12191, (2021)] through a predicted substantial disruption of the conventional water-water H-bond network structure in this DES. Temperature dependent DR measurements at 20 Hz ≤ ν ≤ 50 GHz and subsequent comparison to the simulated DR spectra and decomposition into component contributions support, as predicted earlier, an extensive interaction of water molecules with the other two component molecules and substantial break-down of the tetrahedral water-water network. In addition, our simulations predict a drastic reduction in the polarity of these water molecules (static dielectric constant, ɛs≤5) along with a nano-second relaxation timescale. Further support for the lengthening of the relaxation times has been provided by the simulated continuous [SHB(t)] and structural [CHB(t)] H-bond dynamics. The separation of the component contributions has also indicated that the DR response of this DES is dominated by the urea-urea interaction contribution, followed by glucose-urea and glucose-glucose contributions.