Temperature-Dependent Dielectric Relaxation in Ionic Acetamide Deep Eutectics: Partial Viscosity Decoupling and Explanations from the Simulated Single-Particle Reorientation Dynamics and Hydrogen-Bond Fluctuations

Intermolecular Vibrational and Orientational Dynamics of Deep Eutectic Solvents Composed of Lithium Bis(trifluoromethylsulfonyl)amide and Organic Amides Revealed by Dynamic Kerr Effect Spectroscopy

In this study, we investigated the intermolecular dynamics, including intermolecular vibration and orientational dynamics, of five deep eutectic solvents (DESs) consisting of lithium bis(trifluoromethylsulfonyl)amide and organic amides, such as acetamide, propanamide, N-methylacetamide, butyramide, and urea, at a mole ratio of 1:3 using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and subpicosecond optical Kerr effect spectroscopy (ps-OKES). The fs-RIKES results showed that the line shape of the low-frequency band of the N-methylacetamide was trapezoidal, while that of the other organic amide-based DESs was bimodal. The peak and first moment of the intermolecular vibrational band appearing in the frequency range less than 250 cm-1 for the acetamide- and urea-based DESs were in a higher-frequency region compared to the other three DESs, indicating stronger intermolecular interactions. Furthermore, analysis of the intramolecular vibrational bands of the bis(trifluoromethylsulfonyl)amide anion showed that the population of the transoid conformer of the anion was slightly higher in the urea-based DES than in the other organic amide-based DESs, suggesting that urea solvate lithium cations more than the other organic amides. The slow relaxation dynamics of all five DESs were captured for up to 1 ns using ps-OKES. The slow relaxation dynamics also depended on the organic amide species. However, the slow relaxation time constant did not show a clear correlation with the viscosity; therefore, the relaxation dynamics of the DESs did not follow the Stokes-Einstein-Debye hydrodynamic model. The densities, viscosities, surface tensions, and electrical conductivities of the DESs were also measured for comparison with spectroscopic results.

Hydrogen-bond induced non-linear size dependence of lysozyme under the influence of aqueous glyceline

Natural deep eutectic solvents (NADESs) are environmentally friendly green solvents and hold great promise in the pharmaceutical industry. The secondary structure of a protein, lysozyme, follows a non-monotonous behavior in aqueous glyceline (choline chloride + glycerol) as the wt. % of water is increased. However, it is unclear how the hydration affects the stability of the protein in a non-linear way. In this work, we have performed all-atom molecular dynamic simulations for 1 μs with the lysozyme protein in an aqueous glyceline deep eutectic solvent (DES) by varying the wt. % of water. The simulated radius of gyration, Rg, values can qualitatively reproduce the protein behavior such that the Rg increases initially with an increase in wt. % of water, reaches the peak at 40 wt. %, and then gradually decreases with dilution. Several other properties, including root mean square deviation, root-mean square fluctuation, secondary structure of the protein, and solvent accessible surface area, are examined to explore the NADES effect on the protein structure. Next, we analyze the hydrogen bond profile of intra-protein and among various interspecies, e.g., protein-DES, DES-DES, protein-water, and water-water. The variation in protein-protein hydrogen bonds with concentrations can qualitatively explain the non-linear conformational dependence of the protein. The radial distribution function analyses show various microscopic structures formed due to the DES and water interaction, which play a critical role in protein behavior. This study indicates that at lower wt. % of water, the protein is constrained in a strong hydrogen bond network formed by glycerol and water molecules, resulting in a lower Rg. As the wt. % of water increases, the protein-water interaction drives the protein to expand, reflecting an increasing Rg. At sufficiently higher wt. % of water, the DES constituent and the water molecules interact strongly with the protein, resulting in a decrease in Rg. Overall, the investigation offers a microscopic insight into the protein conformation in DES.

Temperature Dependence of Intermolecular Dynamics and Liquid Properties of Deep Eutectic Solvent, Reline

We investigated the temperature dependence of the intermolecular dynamics, including intermolecular vibrations and collective orientational relaxation, of one of the most typical deep eutectic solvents, reline, using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), subpicosecond optical Kerr effect spectroscopy (ps-OKES), and molecular dynamics (MD) simulations. According to fs-RIKES results, the temperature-dependent intermolecular vibrational band peak at ∼90 cm-1 exhibited a redshift with increasing temperature. The density-of-state (DOS) spectrum of reline by MD simulations reproduced this fs-RIKES spectral feature. The decomposition analysis of the DOS spectra showed that all constituent components, including urea, cholinium cation, and chloride anion, also exhibited similar magnitudes of redshifts upon heating, indicating that the three species intermolecularly interact one another. The temperature sensitivity of the intermolecular vibrational frequency was high compared to that of ionic liquids. According to ps-OKES results, the slow orientational relaxation rate increased with increasing temperature; however, this phenomenon was not well explained by the Stokes-Einstein-Debye hydrodynamic model. Analysis of the orientational relaxation time based on the Stokes-Einstein-Debye model indicates that the decoupling between the orientational relaxation time and viscosity occurs at temperatures below ∼330 K. Quantum chemistry calculations of urea and the cholinium cation based on the MP2/6-311++G(d,p) level of theory confirmed that the contribution of intramolecular vibrational bands to low-frequency bands below 200 cm-1 was minimal. The densities, viscosities, electrical conductivities, and surface tensions of reline at various temperatures were also estimated and compared with the dynamics data.

Molecular dynamics insights into the dynamical behavior of structurally modified water in aqueous deep eutectic solvents (ADES)

Recent studies have demonstrated that the presence of water in deep eutectic solvents (DESs) significantly affects their dynamics, structure, and physical properties. Although the structural changes due to the addition of water are well understood, the microscopic dynamics of these changes have been rarely studied. Here, we performed molecular dynamics simulation of 30% (v/v) (∼0.57 molar fraction) water mixture of DES containing CH3CONH2 and NaSCN/KSCN at various salt fractions to understand the microscopic structure and dynamics of water. The simulated results reveal a heterogeneous environment for water molecules in aqueous DES (ADES), which is influenced by the nature of the cation. The diffusion coefficients of water in ADESs are significantly lower than that in neat water and concentrated aqueous NaSCN/KSCN solution. When Na+ ions are replaced by K+ ions in the ADES system, the diffusion coefficient increases, which is consistent with the measured nuclear magnetic resonance data. Self-dynamic structure factor for water and other simulated dynamic quantities, such as reorientation, hydrogen-bond, and residence time correlation functions, show markedly slower dynamics inside ADES than in the neat water and aqueous salt solution. Moreover, these dynamics become faster when Na+ ions in ADES are replaced by K+ ions. The results suggest that the structural environment of water in Na+-rich ADES is rigid due to the presence of cation-bound water and geometrically constrained water. The medium becomes less rigid as the KSCN fraction increases due to the relatively weaker interaction of K+ ions with water than Na+ ions, which accelerates the dynamical processes.

Temperature-Dependent Dielectric Relaxation Measurements of (Betaine + Urea + Water) Deep Eutectic Solvent in Hz-GHz Frequency Window: Microscopic Insights into Constituent Contributions and Relaxation Mechanisms

A combined experimental and simulation study of dielectric relaxation (DR) of a deep eutectic solvent (DES) composed of betaine, urea, and water with the composition [Betaine:Urea:Water = 11.7:12:1 (weight ratio) and 9:18:5 (molar ratio)] was performed to explore and understand the interaction and dynamics of this system. Temperature-dependent (303 ≤ T/K ≤ 343) measurements were performed over 9 decades of frequency, combining three different measurement setups. Measured DR, comprising four distinct steps with relaxation times spreading over a few picoseconds to several nanoseconds, was found to agree well with simulations. The simulated total DR spectra, upon dissection into three self (intraspecies) and three cross (interspecies) interaction contributions, revealed that the betaine-betaine self-term dominated (∼65%) the relaxation, while the urea-urea and water-water interactions contributed only ∼7% and ∼1%, respectively. The cross-terms (betaine-urea, betaine-water, and urea-water) together accounted for <30% of the total DR. The slowest DR component with a time constant of ∼1-10 ns derived dominant contribution from betaine-betaine interactions, where betaine-water and urea-water interactions also contributed. The subnanosecond (0.1-0.6 ns) time scale originated from all interactions except betaine-water interaction. An extensive interaction of water with betaine and urea severely reduced the average number of water-water H-bonds (∼0.7) and heavily decreased the static dielectric constant of water in this DES (εs ∼ 2). Furthermore, simulated first rank collective single particle reorientational relaxations (C1(t)) and the structural H-bond fluctuation dynamics (CHB (t)) exhibited multiexponential kinetics with time scales that corresponded well with those found both in the simulated and measured DR.

Temperature-dependent dielectric relaxation measurements of (acetamide + K/Na SCN) deep eutectic solvents: Decoding the impact of cation identity via computer simulations

The impact of successive replacement of K+ by Na+ on the megahertz-gigahertz polarization response of 0.25[fKSCN + (1 - f)NaSCN] + 0.75CH3CONH2 deep eutectic solvents (DESs) was explored via temperature-dependent (303 ≤ T/K ≤ 343) dielectric relaxation (DR) measurements and computer simulations. Both the DR measurements (0.2 ≤ ν/GHz ≤ 50) and the simulations revealed multi-Debye relaxations accompanied by a decrease in the solution static dielectric constant (ɛs) upon the replacement of K+ by Na+. Accurate measurements of the DR response of DESs below 100 MHz were limited by the well-known one-over-frequency divergence for conducting solutions. This problem was tackled in simulations by removing the zero frequency contributions arising from the ion current to the total simulated DR response. The temperature-dependent measurements revealed a much stronger viscosity decoupling of DR times for Na+-containing DES than for the corresponding K+ system. The differential scanning calorimetry measurements indicated a higher glass transition temperature for Na+-DES (∼220 K) than K+-DES (∼200 K), implying more fragility and cooperativity for the former (Na+-DES) than the latter. The computer simulations revealed a gradual decrease in the average number of H bonds (⟨nHB⟩) per acetamide molecule and increased frustrations in the average orientational order upon the replacement of K+ by Na+. Both the measured and simulated ɛs values were found to decrease linearly with ⟨nHB⟩. Decompositions of the simulated DR spectra revealed that the cation-dependent cross interaction (dipole-ion) term contributes negligibly to ɛs and appears in the terahertz regime. Finally, the simulated collective single-particle reorientational relaxations and the structural H-bond fluctuation dynamics revealed the microscopic origin of the cation identity dependence shown by the measured DR relaxation times.

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

Frequency-dependent dielectric relaxation in three deep eutectic solvents (DESs), (acetamide+LiClO4/NO3/Br), was investigated in the temperature range, 329 ≤ T/K ≤ 358, via molecular dynamics 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 (ɛs ∼ 20 to 30) for acetamide (ɛs ∼ 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" molecule. The dielectric decrement is, therefore, largely static in origin. This provides a new insight into the ion dependence of the dielectric behavior of these ionic DESs. A good agreement between the simulated and the experimental timescales was also noticed.

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

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.