Dielectric Relaxations of (Acetamide + Electrolyte) Deep Eutectic Solvents in the Frequency Window, 0.2 ≤ ν/GHz ≤ 50: Anion and Cation Dependence

Abstract

Dielectric relaxation (DR) measurements in the frequency range 0.2 ≤ ν/GHz ≤ 50 have been carried out for neat molten acetamide and six different (acetamide + electrolyte) deep eutectic solvents (DESs) for investigating ion effects on DR dynamics in these ionic DESs. Electrolytes used are lithium salts of bromide (LiBr), nitrate (LiNO3), and perchlorate (LiClO4); sodium salts of perchlorate (NaClO4) and thiocyante (NaSCN); and potassium thiocyanate (KSCN). With these electrolytes acetamide forms DESs approximately at an 80:20 mol ratio. Simultaneous fits to the measured permittivity (ε′) and loss (ε″) spectra of these DESs at ∼293 K require a sum of four Debye (4-D) processes with relaxation times spread over picosecond to nanosecond regime. In contrast, DR spectra for neat molten acetamide (∼354 K) depict 2-D relaxation with time constants ∼50 ps and ∼5 ps. For both the neat and ionic systems, the undetected dispersion, ε∞ – n(D)2, remains to be ∼3–4. Upon comparison, measured DR dynamics reveal pronounced anion and cation effects. Estimated static dielectric constants (ε0) from fits for these DESs cover the range 12 < ε0 < 30 and are remarkably lower than that (ε0 ∼ 64) measured for molten acetamide at ∼354 K. Hydrodynamic effective rotation volumes (Veff) estimated from the slowest DR relaxation time constants vary with ion identity and are much smaller than the molecular volume of acetamide. This decrease of ε0 and Veff is attributed respectively to the pinning of acetamide molecules by ions and orientation jumps and undetected portion to the limited frequency coverage employed in these measurements

Ionic conductivity of deep eutectic solvents: the role of orientational dynamics and glassy freezing

“…Similar relaxation processes were previously only detected in few other DESs, moreover in rather restricted temperature and/or frequency ranges only. 35,37,53 In principle, the ' spectra shown in the inset of Fig. 1 closely resemble those of conventional dipolar liquids under supercooling, 12,20,48,55 which could be expected when considering that 2/3 of the samples indeed consist of such a dipolar liquid.…”

Linking Diffusion–Viscosity Decoupling and Jump Dynamics in a Hydroxyl‐Functionalized Ionic Liquid: Realization of Microheterogeneous Nature of the Medium

“…In such ac ase, the hydrodynamicd escriptioni se xpected to provide results that may either be inconsistent or bury the subtle ef-fects of environmental complexity. [76] In light of these discussions, the presentinapplicability of the hydrodynamics description in describing the solute rotationi nahydroxyl-functionalized IL perhaps indicatest he involvement of orientational jumpsa nd the role of hydrogen-bond formationi nt hat event.…”

“…[42] Details of the calculation procedure have been described in our earlier publications. [55] Recent theoreticali nvestigations by Biswas et al [76] have also demonstrated that the jumpd ynamics and hydrogen-bond fluctuations are closely related in highly viscous media such as IL. [32] The slip and stick boundary limits that were assigned with the help of SED hydrodynamic theory are shown in Figure 7w ith the experimentally measuredr eorientation times of C153 in the presentI Ls.…”

“…Interestingly,w hen we fitted the log-logp lot of t r versus h/T by means of the relation log ht r i = a + p log(h/T)[ Equations (5) and (6) [54] The degree of decoupling is significantly higher for [OH-Emim][FAP].T he observed nonlinear relationship between the rotational time of the solute and the bulk viscosity of the mediumi ndicates that the hydroxyl-functionalized IL is more heterogeneous and structurally organized than the other.I n this context,i th as been recently demonstrated that decoupling of rotational motion from the viscosity of the medium can arise due to non-Brownianm ovement such as orientational jumps and the retention of inertia driven motion even much after the onset of the typicald iffusion timescale. [55] Recent theoreticali nvestigations by Biswas et al [76] have also demonstrated that the jumpd ynamics and hydrogen-bond fluctuations are closely related in highly viscous media such as IL. In such ac ase, the hydrodynamicd escriptioni se xpected to provide results that may either be inconsistent or bury the subtle ef-fects of environmental complexity.…”

“…[32] The van der Waals volume, shape factor,a nd calculated slip boundary condition parameter for C153 are 243 3 ,1 .5, and 0.18 respectively. [76] Figure S7, respectively.I nR OSEY spectra two types of peaks are obtained, that is, cross-peaks and diagonal peaks. With the help of Equation (4), the experimentally observed rotationalc oupling constant (C obs = t exp r /t stk r ), which is am easure of the extent of departures from normalh ydrodynamic behavior of as olute due to specific interactions,w as calculated and is listed in Ta ble3.…”