Dielectric relaxation and dielectric decrement in ionic acetamide deep eutectic solvents: Spectral decomposition and comparison with experiments

Frequency dependent dielectric relaxation in the three deep eutectic solvents (DESs), (acetamide+LiClO4/NO3/Br), was investigated in the temperature range, 329T/K358, via molecular dynamics (MD) simulations. Subsequently, decomposition of the real and the imaginary components of the simulated dielectric spectra was carried out to separate the rotational (dipole-dipole), translational (ion-ion) and ro-translational (dipole-ion) contributions. The dipolar contribution, as expected, was found to dominate all the frequency dependent dielectric spectra over the entire frequency regime, while the other two components together made tiny contributions only. The translational (ion-ion) and the cross ro-translational contributions appeared in the THz regime in contrast to the viscosity dependent dipolar relaxations that dominated the MHz-GHz frequency window. Our simulations predicted, in agreement with experiments, anion dependent decrement of the static dielectric constant (_s ~ 20 - 30) for acetamide (_s ~66) in these ionic DESs. Simulated dipole-correlations (Kirkwood g factor) indicated significant orientational frustrations. The frustrated orientational structure was found to be associated with the anion dependent damage of the acetamide H-bond network. Single dipole reorientation time distributions suggested slowed down acetamide rotations but did not indicate presence of any rotationally frozen molecules. The dielectric decrement is, therefore, largely static in origin. This provides a new insight into the ion dependence of the dielectric behaviour of these ionic DESs. A good agreement between the simulated and the experimental timescales was also noticed.