Dielectric relaxation of deep eutectic solvent + water mixtures: structural implications and application to microwave heating

We studied the dielectric response of deep eutectic solvents (DESs) composed of choline chloride (ChCl) and such hydrogen bond donors (HBDs) as glycerol (glyceline) and urea (reline) mixed with water at T = 298.15 K and frequencies varying from 0.05 to 89 GHz. The dielectric loss data were used to calculate normalized heating rates for these systems upon electromagnetic irradiation at operating frequencies of domestic (ν = 2.45 GHz) and industrial (ν = 900 MHz) microwave ovens. We show that due to slow dynamics and substantial Ohmic-loss contributions DES/water mixtures constitute promising solvents for microwave synthesis. Their dielectric spectra can be best fit by a superposition of relaxation processes assigned to the reorientation of dipolar DES components and water molecules. Static permittivities were found to smoothly decrease from the value of neat water (78.4) to 22.8 for glyceline and 41.2 for reline. The analysis of the obtained relaxation amplitudes suggests that the studied systems can be viewed as mixtures of individual choline, HBD and water dipoles without pronounced dipole–dipole correlations and negligible ChCl ion pairs. However, rotational motions of the dipoles are partly synchronized, leading to the slow-down of 22 water molecules for glyceline and 9.2 for reline at infinite dilution. At vanishing DES concentration ChCl–HBD interactions appear to be negligible. Relaxation times as a function of viscosity show a break point at the ChCl : HBD : H2O ratio equal to 1 : 2 : 4. This supports the suggestion of a structural transition from homogeneous electrolyte solution to a micro-heterogeneous mixture already discussed in the literature.