Cooperative dynamics and speciation in deep eutectic solvent + DMSO mixtures

Dielectric relaxation (DR) spectra within the frequency range of 0.05 ≤ ν/GHz ≤ 89 were measured at T = 308.15 K to reveal structure and dynamics of mixtures of deep eutectic solvents (DESs) with dimethylsulfoxide (DMSO). Studied DESs were reline, glyceline and ethaline, ie. 1:2 molar ratio mixtures of choline chloride (ChCl) and the hydrogen bond donors (HBDs) urea, glycerol and ethylene glycol. The obtained DR spectra were best fit by a superposition of relaxations associated with the reorientation of dipolar DES components and individual DMSO molecules. In contrast to previously studied mixtures of glyceline and reline with water, the analysis of the obtained relaxation amplitudes revealed that the present DES/DMSO systems cannot be simply regarded as mixtures of individual Ch+, Cl−, HBD and DMSO units over the entire mixture range. Instead, a significant amount of ChCl contact ion pairs (CIPs) is formed at moderate DES concentrations. Due to their high dipole moment these CIPs cause pronounced maxima at ChCl:HBD:DMSO ratios of ~1: 2: 9 in the static permittivity of the mixtures. Pronounced solute-solvent interactions slow down the rotational motion of solvating DMSO molecules. At infinite dilution the derived total effective solvation numbers, Zt, extrapolate to ~8 − 9 DMSO per ethaline, ~16 per glyceline and ~7 − 8 per reline unit but numbers decrease rapidly with rising DES mole fraction. For the latter two DESs Zt could be split into ChCl and HBD contributions. Finally, careful analysis of the relaxation times provided evidence for pronounced micro-heterogeneity of the studied mixtures