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

Dielectric relaxation (DR) measurements in the frequency range 0.2 <= nu/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 (epsilon') and loss (epsilon '') spectra of these DESs at similar to 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 (similar to 354 K) depict 2-D relaxation with time constants similar to 50 ps and similar to 5 ps. For both the neat and ionic systems, the undetected dispersion, e(infinity) - n(D)(2), remains to be similar to 3-4. Upon comparison, measured DR dynamics reveal pronounced anion and cation effects. Estimated static dielectric constants (epsilon(0)) from fits for these DESs cover the range 12 < epsilon(0) < 30 and are remarkably lower than that (epsilon(0) similar to 64) measured for molten acetamide at similar to 354 K. Hydrodynamic effective rotation volumes (V-eff) 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 epsilon(0) and V-eff 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.