Abstract
We performed rheological measurements of the typical deep eutectic solvents (DESs) glyceline, ethaline, and reline in a very broad temperature and dynamic range, extending from the low-viscosity to the high-viscosity supercooled-liquid regime. We find that the mechanical compliance spectra can be well described by the random free-energy barrier hopping model, while the dielectric spectra on the same materials involve significant contributions arising from reorientational dynamics. The temperature-dependent viscosity and structural relaxation time, revealing non-Arrhenius behavior typical for glassy freezing, are compared to the ionic dc conductivity and relaxation times determined by broadband dielectric spectroscopy. For glyceline and ethaline, we find essentially identical temperature dependences for all dynamic quantities. These findings point to a close coupling of the ionic and molecular translational and reorientational motions in these systems. However, for reline, the ionic charge transport appears decoupled from the structural and reorientational dynamics, following a fractional Walden rule. In particular, at low temperatures, the ionic conductivity in this DES is enhanced by about one decade compared to expectations based on the temperature dependence of the viscosity. The results for all three DESs can be understood without invoking a revolving-door mechanism previously considered as a possible charge-transport mechanism in DESs.
Highlights
Deep eutectic solvents (DESs) are promising alternatives to ionic liquids in a variety of applications, e.g., as “green solvents” in material synthesis or as electrolytes in electrochemical devices such as batteries or solar cells.1–11 Many of them can be produced at low cost and are superior concerning environmental friendliness and renewability
In addition to the translational dynamics of the ionic charge carriers, in most DESs reorientational dynamics should exist because the hydrogen-bond donor (HBD) and the ion species of the added salt often are asymmetric molecules with rotational degrees of freedom. This type of dynamics can be investigated by dielectric spectroscopy because asymmetric molecules and ions usually possess dipole moments, whose reorientations lead to typical features in spectra of the complex dielectric permittivity
The corresponding results for ethaline and reline were treated in detail in Ref. 13
Summary
Deep eutectic solvents (DESs) are promising alternatives to ionic liquids in a variety of applications, e.g., as “green solvents” in material synthesis or as electrolytes in electrochemical devices such as batteries or solar cells. Many of them can be produced at low cost and are superior concerning environmental friendliness and renewability. In addition to the translational dynamics of the ionic charge carriers, in most DESs reorientational dynamics should exist because the HBDs (e.g., glycerol or urea) and the ion species of the added salt (e.g., the cation of the frequently used choline chloride) often are asymmetric molecules with rotational degrees of freedom This type of dynamics can be investigated by dielectric spectroscopy because asymmetric molecules and ions usually possess dipole moments, whose reorientations lead to typical features in spectra of the complex dielectric permittivity.. It should be noted that, in contrast to one-component supercooled liquids, the translation– rotation coupling or decoupling in this case of dielectrically investigated DESs mainly refers to relations between the dynamics of different particles, namely, the HBD molecules (rotation only) and the ions (translation only for the spherical Cl− ions and both translation and rotation for the charged and dipolar choline+ ions). We will check this notion for the investigated DESs
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