On the coupling between ionic conduction and dipolar relaxation in deep eutectic solvents: Influence of hydration and glassy dynamics

Abstract

We have studied the ionic conductivity and the dipolar reorientational dynamics of aqueous solutions of a prototypical deep eutectic solvent (DES), ethaline, by using dielectric spectroscopy on a broad range of frequency (MHz-Hz) and for temperatures ranging from 128 to 283 K. The fraction of water in the DES was varied systematically to cover different regimes, starting from pure DES and its water-in-DES mixtures to the diluted electrolyte solutions. Depending on these parameters, different physical states were examined, including low viscosity liquid, supercooled viscous liquid, amorphous solid and freeze-concentrated solution. The ionic conductivity and the reorientational relaxation both exhibited characteristic features of glassy dynamics that could be quantified from the deviation from Arrhenius temperature dependence and non-exponential decay of the relaxation function. A transition occurred between the water-in-DES regime, (< 40 wt %), where the dipolar relaxation and ionic conductivity remained inversely proportional to each other, and the DES-in-water regime, (> 40 wt %), where a clear rotation-translation decoupling was observed. This suggests that for low water content, on the timescale covered by this study (~10 -6 s to 1 s), the rotational and transport properties of ethaline aqueous solutions obey classical hydrodynamic scaling despite these systems being presumably spatially microheterogeneous. A fractional scaling is observed in the DES-in-water regime, due to the formation of a maximally freeze-concentrated DES aqueous solution coexisting with frozen water domains at sub-ambient temperature.

Electron Transfer Kinetics in Ethaline/Water Mixtures: An Apparent Non‐Marcus Behavior in a Deep Eutectic Solvent

“…It justifies the use of DES for improving the solubility of a redox couple without affecting the characteristics of the device. It is also noticeable that macroscopic quantities as viscosity, conductivity or self‐diffusion coefficients follow the expected trends when water is added [12,14] . However, it is not surprising in a complex mixture presenting different levels of organization that the local solvation of the dissolved molecules affects the charge transfers in a different manner than it changes the macroscopic quantities.…”

“…It is also noticeable that macroscopic quantities as viscosity, conductivity or self-diffusion coefficients follow the expected trends when water is added. [12,14] However, it is not surprising in a complex mixture presenting different levels of organization that the local solvation of the dissolved molecules affects the charge transfers in a different manner than it changes the macroscopic quantities. This could explain the apparent disagreement with the Marcus Model and reflects the complexity of the mixture of Ethaline with another component that is not a simple solvent.…”

“…[9][10][11][12][13][14] Recent published works involve theoretical calculations [9,10,12] and/or experimental investigations of the evolution of the physical-chemistry properties of the DES among them detailed spectroscopic and thermodynamics studies. [12][13][14] In these works, it was concluded that the structures in a water-containing DES mixture are governed by a subtle balance of hydrogen bond networks, that water mainly competes for associations with the anions and that for most of them, there is no "magic" composition that may lead to remarkable properties. [10,12] Besides the variations on basic properties like viscosity or conductivity, spectroscopic studies show the considerable influence of the presence of water on the solvation dynamics that is especially visible on the solvent time dipolar relaxations.…”

“…[10,12] Besides the variations on basic properties like viscosity or conductivity, spectroscopic studies show the considerable influence of the presence of water on the solvation dynamics that is especially visible on the solvent time dipolar relaxations. [12,14] It was concluded that water addition (1-10 %) to the DES may even be beneficial in electrochemistry because water accelerates the relaxation and solvation and that drying a DES may be rather counterproductive. [12] Other works provide similar comments notably how the controlled addition of water could generate desired results [15,16] or about controlling the strength of the H-Bonding by adding a third component in the DES.…”

“…[6][7][8] Because it is very difficult to totally avoid the presence of water in such hydrophilic DES in practical situations, it strengthens the recent efforts for describing the consequences of the presence of water on the DES properties. [9][10][11][12][13][14] Recent published works involve theoretical calculations [9,10,12] and/or experimental investigations of the evolution of the physical-chemistry properties of the DES among them detailed spectroscopic and thermodynamics studies. [12][13][14] In these works, it was concluded that the structures in a water-containing DES mixture are governed by a subtle balance of hydrogen bond networks, that water mainly competes for associations with the anions and that for most of them, there is no "magic" composition that may lead to remarkable properties.…”