Orientational Jumps in (Acetamide + Electrolyte) Deep Eutectics: Anion Dependence

Abstract

All-atom molecular dynamics simulations have been carried out to investigate orientation jumps of acetamide molecules in three different ionic deep eutectics made of acetamide (CH₃CONH₂) and lithium salts of bromide (Br^–), nitrate (NO₃^–) and perchlorate (ClO₄^–) at approximately 80:20 mole ratio and 303 K. Orientational jumps have been dissected into acetamide–acetamide and acetamide–ion catagories. Simulated jump characteristics register a considerable dependence on the anion identity. For example, large angle jumps are relatively less frequent in the presence of NO₃^– than in the presence of the other two anions. Distribution of jump angles for rotation of acetamide molecules hydrogen bonded (H-bonded) to anions has been found to be bimodal in the presence of Br^– and is qualitatively different from the other two cases. Estimated energy barrier for orientation jumps of these acetamide molecules (H-bonded to anions) differ by a factor of ∼2 between NO₃^– and ClO₄^–, the barrier height for the latter being lower and ∼0.5<i>k</i>_B<i>T</i>. Relative radial and angular displacements during jumps describe the sequence ClO₄^– > NO₃^– > Br^– and follow a reverse viscosity trend. Jump barrier for acetamide–acetamide pairs reflects weak dependence on anion identity and remains closer to the magnitude (∼0.7<i>k</i>_B<i>T</i>) found for orientation jumps in molten acetamide. Jump time distributions exhibit a power law dependence of the type, <i>P</i>(<i>t</i>_jump) ∝ <i>A</i>(<i>t</i>_jump/τ)^−β, with both β and τ showing substantial anion dependence. The latter suggests the presence of dynamic heterogeneity in these systems and supports earlier conclusions from time-resolved fluorescence measurements