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 ∼0.5kBT. Relative radial and angular displacements during jumps describe the sequence ClO(4)– > 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 (∼0.7kBT) found for orientation jumps in molten acetamide. Jump time distributions exhibit a power law dependence of the type, P(tjump) ∝ A(tjump/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.

Deep eutectics and analogues as electrolytes in batteries

“…11,130 In all cases the AA−ion interactions are stronger than the AA−AA ones. 131 Comparing the different anions, MD results indicate that the change in displacement and angle of the orientational jumps follows the order Br − < NO3 − < ClO4 − , which in turn follows the ion size sequence. 131 This indicates larger affinity of perchlorate ion to amide group, which is in line with LiClO4 forming the least viscous DES at room temperature.…”

“…123,[125][126][127][128][129] Among the various HBDs, AA has been one of the most frequently used amides because of its unique solvating power, high molecular dipole moment (3.7 D), and fairly large static dielectric constant in molten state (ε ≈ 61 at ∼367 K). 130,131 Also, AA has been long-known to form liquids at or near room temperature with a remarkable tendency to supercool when mixed with inorganic salts. 132 Thanks to these interesting solvent properties, mixtures of AA and Li salts are one of the best-in-classes in terms of specific conductivity.…”

Microstructures and their lifetimes in acetamide/electrolyte deep eutectics: anion dependence

“…3 , ClO − 4 were taken from the existing literature. [45][46][47] These force field parameters were used previously to study reorientational dynamics [32][33][34] and density relaxation of these DESs. 22,23 The force field was constructed using DL_FIELD.…”

“…22,23,31 These studies have reported and explained fractional viscosity dependence of the translational and rotation times for a dissolved solute in these molten mixtures. The orientational 940 Suman Das et al mechanism of molten acetamide 32 as well as acetamide in presence of these electrolytes have been analysed via simulations 33 where prominent signatures of angular jumps have been detected. Also, the orientational timescale and its connection to the hydrogen bond dynamics in ionic amide deep eutectics has been explored just recently.…”

“…Note, CH 3 CONH 2 molecules are connected via a hydrogen bond network joining O(−CONH 2 ) atom of one CH 3 CONH 2 and H(−CONH 2 ) of the other. This H-bond between two CH 3 CONH 2 molecules has been defined previously32,33 based on the well-accepted distance (R) and angle criteria (deg) and validity of the definition also has been justified successfully. Two CH 3 CONH 2 molecules are considered to be H-bonded if (i) the distance between the oxygen and the nitrogen atoms of two different CH 3 CONH 2 molecules, R ON , is less than the first minimum of the corresponding RDF…”