On the coupling between ionic conduction and dipolar relaxation: Comparing ionic and nonionic deep eutectic solvent
Denis MORINEAU1, Claire D'HONDT1, Benjamin MALFAIT1, Aicha JANI1
1CNRS - Institute of Physics of Rennes, Rennes, France
We have established a comprehensive study of the molecular dynamics of two prototypical deep eutectic solvents (DES). On the one hand, ethaline (choline chloride/ethylene glycol 1:2) comprises both H-bonding molecules and molecular ions [1], while on the other hand, menthol-thymol mixture belongs to the new class of type V nonionic DES [2].
Dielectric spectroscopy and differential scanning calorimetry were combined to assess the ionic conductivity, dipolar relaxation and glassy dynamics over an extended range of temperatures, timescales (10-6 – 102 s), for different compositions of menthol-thymol binary liquids and different hydration levels of ethaline. For nonionic DES, positive deviations from ideal mixing approximation were demonstrated for both the solvent dielectric strength and the glass transition. They support the idea that preferential H-bond interactions between unlike molecules induce structural rearrangements in the DES mixtures [2].
For both types of DES mixtures, excellent glassforming capability was demonstrated for a broad range of compositions, i.e. xwater < 40 wt. % and xThymol = 0.4–0.7 for ionic ethaline DES solutions and nonionic menthol-thymol DES, respectively. In these cases, the dipolar dynamics could be assessed from the normal liquid to the deeply supercooled state. Many salient parameters of the relaxation functions, including fragility, non-Debye character point to the development of dynamic heterogeneities.
Interestingly, a decoupling between rotation and translation dynamics is additionally demonstrated for the nonionic menthol-thymol DES, which strongly supports the heterogeneous nature of the mixture. Surprisingly for ionic ethaline DES solutions, comparable decoupling is only achieved at high water content (DES-in-Water), when actually entering the biphasic region of the phase behavior.
[1] “On the coupling between ionic conduction and dipolar relaxation in deep eutectic solvents: Influence of hydration and glassy dynamics”, A. Jani, B. Malfait, D. Morineau, J. Chem. Phys. 154 164508 (2021).
[2] “Dynamics of type V menthol-thymol deep eutectic solvents: Do they reveal non-ideality?”, C. D’Hondt, D. Morineau, J. Mol. Liquids, 365, 120145 (2022).