How special are the dynamics of deep eutectic solvents? A look at the prototypical case of ethaline

Abstract

We investigated the molecular dynamics of the prototypical deep eutectic solvent (DES) ethaline. We disentangled the different motions of its two constituents, namely choline chloride and ethylene glycol, on a spatiotemporal range that extends from sub-nanometer to micrometer distances and from picoseconds to milliseconds. This was achieved by a combination of pulsed-field-gradient NMR, time-of-flight, and backscattering quasielastic neutron scattering experiments with isotopically labeled samples. On the micrometer scale, we observe that the translational motions of the two DES constituents obey classical hydrodynamics, with distinct diffusivities that reflect their different hydrodynamic radii. This is no longer valid at the nanometer scale, where the two DES components present similar short-ranged diffusivities, indicating a significant effect of their supramolecular association. The sub-nanometer scale motions include jumps that precede Fickian diffusion and localized dynamics that precede the breaking of the transient cage formed by neighboring molecules. Therein, the spatial amplitude of the localized motions mirrors their different molecular sizes and chemical structures, while their respective correlation times contrast with observations made for other choline-based DESs such as glyceline. This result underlines the importance of more subtle effects, such as the different H-bond propensities of the polyol donor, and demonstrates the difficulty in anticipating the nanoscale dynamic behavior of DESs from knowledge of their macroscopic properties.