Difference in “Supercooling” Affinity between (Acetamide + Na/KSCN) Deep Eutectics: Reflections in the Simulated Anomalous Motions of the Constituents and Solution Microheterogeneity Features

Abstract

Deep depression of freezing points of ionic amide deep eutectic solvents (DESs) is known to exhibit a significant dependence on the identity of ions present in those systems and the nature of the functional group attached to the host amide. This deep depression of the freezing point is sometimes termed as “supercooling”. For (acetamide + electrolyte) DESs, experiments have revealed signatures of ion-dependent spatiotemporal heterogeneity features. The focus of this work is to provide microscopic explanations of these experimentally observed macroscopic system properties in terms of particle jumps and insights about the origin of the cation dependence. For this purpose, extensive molecular dynamics simulations have been performed employing (acetamide + Na/KSCN) deep eutectics as representative ionic systems at 303, 318, 333, and 348 K. The individual translational motions of acetamide and the ions are followed, and their connections to solution heterogeneity are explored. The center-of-mass motion for Na+ has been found to be more anomalous than that for K+. This difference corroborates well with experimental reports on heterogeneous relaxations in these systems. Simulated viscosity coefficients and dynamic heterogeneity features also reflect this difference. Moreover, simulated reorientational relaxations of acetamide molecules in these ionic DESs suggest that a Na+-containing DES is more heterogeneous than the corresponding K+-containing system. Estimated void and neck distributions for acetamide molecules differ as the alkali metal ions differ. In brief, this study provides a detailed microscopic view of the cation dependence of the microheterogeneous relaxation dynamics of these DESs reported repeatedly by different experiments.

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

“…Average dielectric relaxation times too showed fractional viscosity dependence, while the fs-RIKES experiments detected spectral signatures that were caused by the ion dependent microheterogeneity of these ionic DESs. Anomalous particle motions, such as, orientational 29 and translational 31 jumps have been detected in computer simulations and these might be recognised as heterogeneity markers for these media. Interestingly, these relatively more recent studies have essentially reconfirmed the micro heterogeneous nature of ionic DESs suggested already in several previous studies that reported ultrasonic and other measurements.…”

Enhanced fluidity of water in superhydrophobic nanotubes: estimating viscosity using jump-corrected confined Stokes–Einstein approach

“…An alternative method, based on the confined Stokes-Einstein (CSE) relation, 22,35 was proposed. Unfortunately, the CSE relation assumed complete validity of the SE relation, which is less applicable in supercooled water, [36][37][38][39][40][41] pressurized superheated water, 42 water under confinement, 13,14,33 the presence of impurity (e.g., solutes and other solvents), [43][44][45][46] etc. In our previous work, 34 we have proposed a novel approach, namely, the jump-corrected confined Stokes-Einstein (JCSE) relation, which takes into account both the system size effect and the breakdown of the SE relation to predict the viscosity of nanoconfined water.…”