Translational and reorientational dynamics in carboxylic acid-based deep eutectic solvents

TL;DR

This study investigates the glassy and reorientational dynamics in carboxylic acid-based deep eutectic solvents using dielectric and rheological spectroscopy, revealing their influence on conductivity and mechanical properties.

Contribution

It provides new insights into the glass formation and dipolar motions in DESs and their impact on ionic conductivity and rheology, which were previously overlooked.

Findings

Glassy freezing observed in dielectric and rheological data.

Decoupling of dynamics near the glass transition.

Conductivity related to dipolar relaxation via power law.

Abstract

The glass formation and the dipolar reorientational motions in deep eutectic solvents (DESs) are frequently overlooked, despite their crucial role in defining the room-temperature physiochemical properties. To understand the effects of these dynamics on the ionic conductivity and their relation to the mechanical properties of the DES, we conducted broadband dielectric and rheological spectroscopy over a wide temperature range on three well-established carboxylic-acid-based natural DESs: oxaline, maline, and the eutectic mixture of choline chloride with phenylacetic acid (phenylaceline). In all three DESs, we observe signs of glassy freezing in the temperature dependence of their dipolar reorientational and structural dynamics, as well as varying degrees of motional decoupling between the different observed dynamics: Maline and oxaline display a breaking of the Walden rule near the glass-transition temperature, while the relation between the dc conductivity and dipolar relaxation time in both maline and phenylaceline is best described by a power law. The glass-forming properties of the investigated systems not only govern the orientational dipolar motions and rheological properties, which are of interest from a fundamental point of view, but they also affect the dc conductivity, even at room temperature, which is of high technical relevance.