Temperature-Dependent Dielectric Relaxation in Ionic Acetamide Deep Eutectics: Partial Viscosity Decoupling and Explanations from the Simulated Single-Particle Reorientation Dynamics and Hydrogen-Bond Fluctuations

Abstract

We report here temperature dependent (293 ≤ T(K) ≤ 336) dielectric relaxation (DR) measurements of (acetamide + LiBr/NO3 −/ClO4 −) deep eutectic solvents (DESs) in a frequency window, 0.2 ≤ ν(GHz) ≤ 50, and explore, via molecular dynamics simulations, the relative roles for the collective single particle reorientational relaxations, and the H-bond dynamics of acetamide in the measured DR response. In addition, DR measurements of neat molten acetamide have been performed. Recorded DR spectra of these DESs require multi-Debye fits and produce well-separated DR timescales spreading over several picoseconds to about a nanosecond. Hydrodynamic molecular rotation of acetamide cannot explain the temperature dependence of the measured average DR times. Simulations suggest DR timescales derive contributions from both the collective reorientational (Cℓ(t)) relaxation and structural H-bond (CHB(t)) dynamics of acetamide. A good correlation between the measured and simulated activation energies further reveals a strong connection between the measured DR, and the simulated Cℓ(t) and CHB(t). Average DR times exhibit a strong fractional viscosity dependence, suggesting substantial microheterogeneity in these media. Simulations of Cℓ(t) and CHB(t) reveal strong stretched exponential relaxations with stretching exponent, 0.4 ≤ β ≤ 0.7. The ratio between the average reorientational correlation times of first and second ranks, ⟨τ⟩ℓ=1 ⟨τ⟩ℓ=2⁄ , deviates appreciably from the Debye’s ℓ(ℓ + 1) law for homogeneous media. Importantly, a pronounced translation-rotation decoupling between the simulated reorientation and centre-of-mass diffusion times has been observed. All these provide microscopic insight into the relaxation processes in these media and provide support to the interpretation of the experimentally observed fractional viscosity dependence of the measured average DR times in terms of dynamic heterogeneity.