Interaction and dynamics of (alkylamide + electrolyte) deep eutectics: Dependence on alkyl chain-length, temperature, and anion identity

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.

Solvation Shell Anatomy of H2S and CO Dissolved in Reline and Ethaline Deep Eutectic Solvents

Rising atmospheric concentrations of anthropogenic hydrogen sulfide (H2S) and carbon monoxide (CO) as a result of industrialization have encouraged researchers to explore innovative technologies for capturing these gases. Deep eutectic solvents (DESs) are an alternative media for mitigating H2S and CO emissions. Herein, we have employed ab initio molecular dynamics simulations to investigate the structures of the nearest-neighbor solvation shells surrounding H2S and CO when they are dissolved in reline and ethaline DESs. We aim to delineate the structural arrangement responsible for favorable H2S and CO capture by analyzing the key interactions between H2S and CO solutes with various components of the DESs. We observe that in the reline-H2S system, chloride and carbonyl oxygen of urea are found to have the closest distance interaction with hydrogen atoms of the H2S solute. The sulfur atom of H2S is found to be predominantly solvated by hydrogen and oxygen atoms of urea molecules and the hydroxyl hydrogen of choline cations. The chloride ions and ethylene glycol molecules predominantly govern the solvation of H2S in the ethaline-H2S system. In both the DESs, H2S is solvated by the hydroxyl group of the choline cations rather than by their ammonium group. In the reline-CO system, all the atoms of urea along with chloride dominate the immediate solvation shell around CO. In the ethaline-CO system, hydroxyl oxygen and hydrogen atoms of ethylene glycol are found in the nearest solvation structure around CO. Both the DESs exhibit a stronger solvent-solute charge-transfer tendency toward the H2S solute compared to CO.

Noncanonical Relationship between Heterogeneity and the Stokes-Einstein Breakdown in Deep Eutectic Solvents

The relationship between Stokes-Einstein breakdown (SEB) and dynamical heterogeneity (DH) is of paramount importance in the physical chemistry of complex fluids. In this work, we employ neutron scattering to probe the DH and SEB in a series of deep eutectic solvents (DESs) composed of acetamide and lithium salts. Quasielastic neutron scattering experiments reveal SEB in the jump diffusion of acetamide, represented by a fractional Stokes-Einstein relationship. Among these DESs, lithium perchlorate exhibits the most pronounced SEB while lithium bromide displays the weakest. Concurrently, elastic incoherent neutron scans identify that bromide DES is the most heterogeneous and perchlorate is the least. For the first time, our study unveils a counterintuitive incommensurate relationship between DH and SEB. Further, it reveals the intricate contrasting nature of the SEB-DH relationship when investigated in proximity to the glass-transition temperature and further away from it.

Does Viscosity Decoupling Guarantee Dynamic Heterogeneity? A Clue through an Excitation and Emission Wavelength-Dependent Time-Resolved Fluorescence Anisotropy Study

Traditionally, deviation from Stokes-Einstein-Debye (SED) relation in terms of viscosity dependence of medium dynamics, i.e., τ x ∝ ( η T ) p with p ≠ 1, is taken as a signature of dynamic heterogeneity. However, it does not guarantee medium heterogeneity, as the decoupling may also originate from the deviation of the basic assumption of SED. Here, we developed a method to find a stronger relation between viscosity decoupling (p ≠ 1) and dynamic heterogeneity in terms of rotational motion. Our approach exploited the fact that in heterogeneous media, a solvatochromic probe will be solvated to a different extent at different microdomains (subpopulations), and photoselection of these subpopulations can be achieved by excitation or emission wavelength-dependent measurements. We hypothesized that the dynamics of a homogeneous system might show viscosity decoupling, but the extent of decoupling at different excitations (or at different emissions) should not be different. On the other hand, in a heterogeneous medium, this extent of viscosity decoupling (p-value) should be different at different excitations (or at different emissions). As proof of concept, we investigated three versatile solvent media: squalane (viscous molecular liquid), 1-ethyle-3-methylimidazolium ethyl sulfate ionic liquid (IL), and [0.78 acetamide + 0.22 LiNO3] deep eutectic solvent (DES). We found that squalane is homogeneous, although it shows fractional viscosity dependence (p ≠ 1). Interestingly, mild heterogeneity in IL and significant heterogeneity in the DES were observed. Overall, we conclude that the difference in the p-value as a function of excitation (or emission) wavelength-dependent might be a superior way for the detection of dynamic heterogeneity.

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.

Solvation Shell Structures of Ammonia in Reline and Ethaline Deep Eutectic Solvents

Because of increasing atmospheric anthropogenic ammonia (NH₃) emission, researchers are devising new techniques to capture NH₃. Deep eutectic solvents (DESs) are found as potential media for NH₃ mitigation. In the present study, we have carried out ab initio molecular dynamics (AIMD) simulations to decipher the solvation shell structures of an ammonia solute in reline (1:2 mixture of choline chloride and urea) and ethaline (1:2 mixture of choline chloride and ethylene glycol) DESs. We aim to resolve the fundamental interactions which help stabilize NH₃ in these DESs, focusing on the structural arrangement of the DES species in the nearest solvation shell around NH₃ solute. In reline, the hydrogen atoms of NH₃ are preferentially solvated by chloride anions and the carbonyl oxygen atoms of urea. The nitrogen atom of NH₃ renders hydrogen bonding with hydroxyl hydrogen of the choline cation. The positively charged head groups of the choline cations prefer to stay away from NH₃ solute. In ethaline, strong hydrogen bonding interaction exists between the nitrogen atom of NH₃ and hydroxyl hydrogen atoms of ethylene glycol. The hydrogen atoms of NH₃ are found to be solvated by hydroxyl oxygen atoms of ethylene glycol and choline cation. While ethylene glycol molecules play a crucial role in solvating NH₃, the chloride anions remain passive in deciding the first solvation shell. In both the DESs, choline cations approach NH₃ from their hydroxyl group side. We observe slightly stronger solute-solvent charge transfer and hydrogen bonding interaction in ethaline than those in reline.

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.

Modulation of Diffusion Mechanism and Its Correlation with Complexation in Aqueous Deep Eutectic Solvents

Aqueous mixtures of deep eutectic solvents (DESs) have gained traction recently as an effective template to tailor their physicochemical properties. But detailed microscopic insights into the effects of water on the molecular relaxation phenomenon in DESs are not entirely understood. DESs are strong network-forming liquids due to the extensive hydrogen bonding and complex formation between their species, and therefore, water can behave as a controlled disruptor altering the microscopic structure and dynamics in DESs. In this study, the role of water in the diffusion mechanism of acetamide in the aqueous mixtures of DESs synthesized using acetamide and lithium perchlorate is investigated using molecular dynamics (MD) simulation and quasielastic neutron scattering (QENS). The acetamide dynamics comprises localized diffusion within transient cages and a jump diffusion process across cages. The jump diffusion process is observed to be strongly enhanced by about a factor of 10 as the water content in the system is increased. Meanwhile, the geometry of the localized dynamics is unaltered by addition of water, but the localized diffusion becomes significantly faster and more heterogeneous with increasing water concentration. The accelerating effects of water on localized diffusion are also substantiated by QENS experiments. The water concentration in the DES is observed to control the solvation structure of lithium ions, with the ions becoming significantly hydrated at 20 wt % water. The formation of interwater and water-acetamide hydrogen bonds is observed. The increase in water concentration is found to increase the number of H-bonds; however, their lifetimes are found to decrease substantially. Similarly, the lifetimes of acetamide-lithium complexes are also found to be diminished by increasing water concentration. A power-law scaling relationship between lifetimes and diffusion constants is established, elucidating the extent of coupling between diffusive processes and hydrogen bonding and microscopic complexation. This study demonstrates the ability to use water as an agent to probe the role of structural relaxation and complex lifetimes of diffusive processes at different time and length scales.

Solvent Organization around Methane Dissolved in Archetypal Reline and Ethaline Deep Eutectic Solvents as Revealed by AIMD Investigation

Because of the rising concentration of harmful greenhouse gases like methane in the atmosphere, researchers are striving for developing novel techniques for capturing these gases. Recently, neoteric liquids such as deep eutectic solvents (DESs) have emerged as an efficient means of sequestration of methane. Herein, we have performed ab initio molecular dynamics (AIMD) simulations to elucidate the solvation structure around a methane molecule dissolved in reline and ethaline DESs. We aim to elicit the structural organization of different constituents of the DESs in the vicinity of methane, particularly highlighting the key interactions that stabilize such gases in DESs. We observe quite different solvation structures of methane in the two DESs. In ethaline, chloride ions play an active role in solvating methane. Instead, in reline, chloride ions do not interact much with the methane molecule in the first solvation shell. In reline, choline cations approach the methane molecule from their hydroxyl group side, whereas urea molecules approach methane from their carbonyl oxygen as well as amide group sides. In ethaline, ethylene glycol and Cl^- dominate the nearest neighbor solvation structure around the methane molecule. In both the DESs, we do not observe any significant methane-DES charge transfer interactions, apart from what is present between choline cation and Cl^- anion.

Structure, Organization, and Heterogeneity of Water-Containing Deep Eutectic Solvents

The spectroscopy and structural dynamics of a deep eutectic mixture (KSCN/acetamide) with varying water content is investigated from 2D IR (with the C-N stretch vibration of the SCN^- anions as the reporter) and THz spectroscopy. Molecular dynamics simulations correctly describe the nontrivial dependence of both spectroscopic signatures depending on water content. For the 2D IR spectra, the MD simulations relate the steep increase in the cross-relaxation rate at high water content to the parallel alignment of packed SCN^- anions. Conversely, the nonlinear increase of the THz absorption with increasing water content is mainly attributed to the formation of larger water clusters. The results demonstrate that a combination of structure-sensitive spectroscopies and molecular dynamics simulations provides molecular-level insights into the emergence of heterogeneity of such mixtures by modulating their composition.

Absorption of Volatile Organic Compounds Toluene and Acetaldehyde in Choline Chloride-Based Deep Eutectic Solvents

Unrestricted emission of volatile organic compounds (VOCs)─a threat to human health and the environment─can be controlled to a large extent by the capturing mechanism. Few recent experimental studies explored the efficacy of the deep eutectic solvent (DES), a designer solvent with some fascinating properties, as a VOC-capturing medium. Through the partition coefficient measurement, it was found that the choline chloride-based DESs exhibit excellent VOC-capturing potencies. However, a molecular picture of the above absorption process is still lacking. Here, we study the molecular mechanism of the absorption of two commonly occurring VOCs, toluene and acetaldehyde, in two different choline chloride-based DESs with varying donor molecules, urea, and levulinic acid via the molecular dynamics simulation technique. Strong absorption of the VOCs is observed in both the DESs. The free energy profile for the absorption process has been explored using the umbrella sampling method. The VOCs are preferentially solvated near the liquid/vapor interface. The simulated partition coefficients for the VOCs from the vapor to the liquid phase show good agreement with the experimental results. Detailed analyses of the spatial and orientational structure of the VOCs and different components of DESs are performed to elucidate the interaction among them. The above analyses have indicated that DES is a better VOC-capturing medium compared to a room-temperature ionic liquid, which is more extensively studied in the literature.

Computer Simulations of Deep Eutectic Solvents: Challenges, Solutions, and Perspectives

Deep eutectic solvents (DESs) are one of the most rapidly evolving types of solvents, appearing in a broad range of applications, such as nanotechnology, electrochemistry, biomass transformation, pharmaceuticals, membrane technology, biocomposite development, modern 3D-printing, and many others. The range of their applicability continues to expand, which demands the development of new DESs with improved properties. To do so requires an understanding of the fundamental relationship between the structure and properties of DESs. Computer simulation and machine learning techniques provide a fruitful approach as they can predict and reveal physical mechanisms and readily be linked to experiments. This review is devoted to the computational research of DESs and describes technical features of DES simulations and the corresponding perspectives on various DES applications. The aim is to demonstrate the current frontiers of computational research of DESs and discuss future perspectives.

A study to discover novel pharmaceutical cocrystals of pelubiprofen with a machine learning approach compared

Pelubiprofen (PF), a biopharmaceutical classification system (BCS) class II non-steroidal anti-inflammatory drug, has been on the market only in its crystalline form. To discover the first cocrystal form(s) of the...

Effect of Lithium-Ion on the Structural Organization of Monocationic and Dicationic Ionic Liquids

In recent times, ionic liquid-based (ILs) electrolytic system has emerged as suitable alternative to the conventional organic solvent-based electrolytic system. However, since, anion of ILs is known to form aggregates in the presence of lithium-ions (Li⁺), and this can influence the transport properties of Li⁺ ion in a significant manner, it is, therefore, important to understand how lithium-ions influence the structure and dynamics of ILs. With this objective, in the present study, intermolecular interaction, structural organization, and dynamics of monocationic ILs (MILs) and dicationic IL (DIL) have been studied in the absence and presence of lithium salt. Specifically, for this purpose, two MILs, 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₃C₁im][NTf₂]), 1-hexyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C₆C₁im][NTf₂]), and a DIL, 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C₆(mim)₂][NTf₂]₂) have been chosen in such a way that either the alkyl chain of MILs becomes equal or half of the spacer chain length of DIL. To understand the effect of the addition of lithium-ion on the structural organization of MILs and DIL, steady-state absorption and fluorescence spectroscopies, time-resolved fluorescence anisotropy and nuclear magnetic resonance (NMR) techniques have been used. Structural organization in the apolar and polar domains of ILs has been probed by following the rotational diffusion of suitably chosen solute in the concerned media through time-resolved fluorescence anisotropy (TRFA) measurements. TRFA studies have revealed that with the addition of Li⁺ ion, coordination between the Li⁺ ions and anions of MILs and DILs takes place in the ionic region leading to a change in the structural organization of the apolar regions of the respective medium. In fact, upon adding lithium-ions, a reduction in the packing of alkyl chains has also been observed for the MILs. However, not much change in the structural organization of the apolar region of the DIL has been observed when Li⁺ ion is added to it. In the presence of Li⁺ ions, a similar trend in the change of structural organization of polar regions for both MILs and DIL has been observed. Further, measurements of the self-diffusion coefficient through NMR have also supported the observation that Li⁺ ion also perturbs the nanostructural organization of the MIL in a significant manner than that it does for the DIL. The behavior of DIL in the presence of Li⁺ ion, as revealed by the present study, has been rationalized by considering the folded arrangement of DIL in the fluid-structure. Essentially, all of these investigations have suggested that the addition of lithium-ion significantly alters the microscopic behavior of MILs in comparison to that of DIL. The outcome of this study is expected to be helpful in realizing the potentials of these media as electrolytes in battery applications.

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

We report here temperature-dependent (293 ≤ T (K) ≤ 336) dielectric relaxation (DR) measurements of (acetamide + LiBr/NO₃^-/ClO₄^-) deep eutectic solvents (DESs) in the frequency window of 0.2 ≤ ν (GHz) ≤ 50 and explore, via molecular dynamics simulations, the relative roles for the collective single-particle reorientational relaxations and the H-bond dynamics of acetamide in the measured DR response. In addition, DR measurements of neat molten acetamide were performed. Recorded DR spectra of these DESs require multi-Debye fits and produce well-separated DR time scales that are spread over several picoseconds to ∼1 ns. Simulations suggest DR time scales derive contributions from both the collective reorientational (Cl(t)) relaxation and structural H-bond (C_HB(t)) dynamics of acetamide. A good correlation between the measured and simulated activation energies further reveals a strong connection between the measured DR and the simulated Cl(t) and C_HB(t). Average DR times exhibit a strong fractional viscosity dependence, suggesting substantial microheterogeneity in these media. Simulations of Cl(t) and C_HB(t) reveal strong stretched exponential relaxations with a stretching exponent, 0.4 ≤ β ≤ 0.7. The ratio between the average reorientational correlation times of first and second ranks, ⟨τ⟩l=1/⟨τ⟩l=2, deviates appreciably from Debye's l(l+1) law for homogeneous media. Importantly, a pronounced translation-rotation decoupling between the simulated reorientation and center-of-mass diffusion times was observed.

Bulk and interfacial nanostructure and properties in deep eutectic solvents: Current perspectives and future directions

Deep eutectic solvents (DESs) are a tailorable class of solvents that are rapidly gaining scientific and industrial interest. This is because they are distinct from conventional molecular solvents, inherently tuneable via careful selection of constituents, and possess many attractive properties for applications, including catalysis, chemical extraction, reaction media, novel lubricants, materials chemistry, and electrochemistry. DESs are a class of solvents composed solely of hydrogen bond donors and acceptors with a melting point lower than the individual components and are often fluidic at room temperature. A unique feature of DESs is that they possess distinct bulk liquid and interfacial nanostructure, which results from intra- and inter-molecular interactions, including coulomb forces, hydrogen bonding, van der Waals interactions, electrostatics, dispersion forces, and apolar-polar segregation. This nanostructure manifests as preferential spatial arrangements of the different species, and exists over several length scales, from molecular- to nano- and meso-scales. The physicochemical properties of DESs are dictated by structure-property relationships; however, there is a significant gap in our understanding of the underlying factors which govern their solvent properties. This is a major limitation of DES-based technologies, as nanostructure can significantly influence physical properties and thus potential applications. This perspective provides an overview of the current state of knowledge of DES nanostructure, both in the bulk liquid and at solid interfaces. We provide definitions which clearly distinguish DESs as a unique solvent class, rather than a subset of ILs. An appraisal of recent work provides hints towards trends in structure-property relationships, while also highlighting inconsistencies within the literature suggesting new research directions for the field. It is hoped that this review will provide insight into DES nanostructure, their potential applications, and development of a robust framework for systematic investigation moving forward.

Can the microscopic and macroscopic transport phenomena in deep eutectic solvents be reconciled?

Deep eutectic solvents (DESs) have become ubiquitous in a variety of industrial and pharmaceutical applications since their discovery. However, the fundamental understanding of their physicochemical properties and their emergence from the microscopic features is still being explored fervently. Particularly, the knowledge of transport mechanisms in DESs is essential to tune their properties, which shall aid in expanding the territory of their applications. This perspective presents the current state of understanding of the bulk/macroscopic transport properties and microscopic relaxation processes in DESs. The dependence of these properties on the components and composition of the DES is explored, highlighting the role of hydrogen bonding (H-bonding) interactions. Modulation of these interactions by water and other additives, and their subsequent effect on the transport mechanisms, is also discussed. Various models (e.g. hole theory, free volume theory, etc.) have been proposed to explain the macroscopic transport phenomena from a microscopic origin. But the formation of H-bond networks and clusters in the DES reveals the insufficiency of these models, and establishes an antecedent for dynamic heterogeneity. Even significantly above the glass transition, the microscopic relaxation processes in DESs are rife with temporal and spatial heterogeneity, which causes a substantial decoupling between the viscosity and microscopic diffusion processes. However, we propose that a thorough understanding of the structural relaxation associated to the H-bond dynamics in DESs will provide the necessary framework to interpret the emergence of bulk transport properties from their microscopic counterparts.

Multiple evidences of dynamic heterogeneity in hydrophobic deep eutectic solvents

Hydrophobic deep eutectic solvents (HDESs) have gained immense popularity because of their promising applications in extraction processes. Herein, we employ atomistic molecular dynamics simulations to unveil the dynamics of DL-menthol (DLM) based HDESs with hexanoic (C6), octanoic (C8), and decanoic (C10) acids as hydrogen bond donors. The particular focus is on understanding the nature of dynamics with changing acid tail length. For all three HDESs, two modes of hydrogen bond relaxations are observed. We observe longer hydrogen bond lifetimes of the inter-molecular hydrogen bonding interactions between the carbonyl oxygen of the acid and hydroxyl oxygen of menthol with hydroxyl hydrogen of both acids and menthol. We infer strong hydrogen bonding between them compared to that between hydroxyl oxygen of acids and hydroxyl hydrogens of menthol and acids, marked by a faster decay rate and shorter hydrogen bond lifetime. The translational dynamics of the species in the HDES becomes slower with increasing tail length of the organic acid. Slightly enhanced caging is also observed for the HDES with a longer tail length of the acids. The evidence of dynamic heterogeneity in the displacements of the component molecules is observed in all the HDESs. From the values of the α-relaxation time scale, we observe that the molecular displacements become random in a shorter time scale for DLM-C6. The analysis of the self-van Hove function reveals that the overall distance covered by DLM and acid molecules in the respective HDES is more than what is expected from ideal diffusion. As marked by the shorter time scale associated with hole filling, the diffusion of the oxygen atom of menthol and the carbonyl oxygen of acid from one site to the other is fastest for hexanoic acid containing HDES.

Assessing rotation and solvation dynamics in ethaline deep eutectic solvent and its solutions with methanol

Steady-state and time-resolved fluorescence were used to investigate the solvation of coumarin 153 (C153) and coumarin 343 (C343) in methanol + ethaline binary solutions, a deep eutectic solvent composed of a 1:2 molar ratio choline chloride + ethylene glycol. In addition, time-resolved anisotropy decays were used to determine the solute's rotational reorientation time as a function of viscosity. Measurements were made in solutions covering the entire range of mole fraction. Viscosity measurements were used to characterize the bulk solvent properties, and as expected, addition of methanol resulted in an decreased viscosity, showing an exponential decrease with mole fraction, up to ∼50-fold at x_MeOH = 1.0. Probe rotational reorientation times were found to be biexponential at x_MeOH < 0.3 for C153 and x_MeOH < 0.5 for C343 and monoexponential at richer methanol content. In proportion to viscosity, C153 and C343 average rotation times decreased ∼30-fold from x_MeOH = 0 to 0.9 and showed a power law dependence of ∼η^0.85. Rotation times approached the stick boundary limit on dilution with methanol. Time-resolved Stokes shifts quantified the solvation dynamics and were nearly single exponential for C153 but were clearly biexponential for C343. Solvation times also tracked with viscosity according to a power law dependence, with exponents of 0.3 and 0.4 for C153 and C343, respectively. The dilution effect of methanol was not linear in proportion to the viscosity change and alone cannot account for the change in solvation. Dilution also showed a different correlation to solvation than did temperature variations to govern the viscosity change.

Water accelerates the hydrogen-bond dynamics and abates heterogeneity in deep eutectic solvent based on acetamide and lithium perchlorate

Deep eutectic solvents (DESs) have become a prevalent and promising medium in various industrial applications. The addition of water to DESs has attracted a lot of attention as a scheme to modulate their functionalities and improve their physicochemical properties. In this work, we study the effects of water on an acetamide based DES by probing its microscopic structure and dynamics using classical molecular dynamics simulation. It is observed that, at low water content, acetamide still remains the dominant solvate in the first solvation shell of lithium ions, however, beyond 10 wt. %, it is replaced by water. The increase in the water content in the solvent accelerates the H-bond dynamics by drastically decreasing the lifetimes of acetamide-lithium H-bond complexes. Additionally, water-lithium H-bond complexes are also found to form, with systematically longer lifetimes in comparison to acetamide-lithium complexes. Consequently, the diffusivity and ionic conductivity of all the species in the DES are found to increase substantially. Non-Gaussianity parameters for translational motions of acetamide and water in the DES show a conspicuous decrease with addition of water in the system. The signature of jump-like reorientation of acetamide is observed in the DES by quantifying the deviation from rotational Brownian motion. However, a notable decrease in the deviation is observed with an increase in the water content in the DES. This study demonstrates the intricate connection between H-bond dynamics and various microscopic dynamical parameters in the DES, by investigating the modulation of the former with addition of water.

Dynamic heterogeneity and viscosity decoupling: origin and analytical prediction

The molecular-level structure and dynamics decide the functionality of solvent media. Therefore, a significant amount of effort is being dedicated continually over time in understanding their structural and dynamical features. One intriguing aspect of solvent structure and dynamics is heterogeneity. In these systems, the dynamics follow , where p is the measure of viscosity decoupling. We analytically predicted that in such cases, the Stokes-Einstein relationship is modified to due to microdomain formation, and the second term on the right-hand side leads to viscosity decoupling. We validated our prediction by estimating the p values of a few solvents, and they matched well with the literature. Overall, we believe that our approach gives a simple yet unique physical picture to help us understand the heterogeneity of solvent media.

Heterogeneity in hydrophobic deep eutectic solvents: SAXS prepeak and local environments

The observation of the prepeak in the simulated total X-ray scattering structure function (S(q)) reveals the presence of intermediate-range structural heterogeneity in hydrophobic deep eutectic solvents.

Deep Eutectic Solvents: A Review of Fundamentals and Applications

Deep eutectic solvents (DESs) are an emerging class of mixtures characterized by significant depressions in melting points compared to those of the neat constituent components. These materials are promising for applications as inexpensive "designer" solvents exhibiting a host of tunable physicochemical properties. A detailed review of the current literature reveals the lack of predictive understanding of the microscopic mechanisms that govern the structure-property relationships in this class of solvents. Complex hydrogen bonding is postulated as the root cause of their melting point depressions and physicochemical properties; to understand these hydrogen bonded networks, it is imperative to study these systems as dynamic entities using both simulations and experiments. This review emphasizes recent research efforts in order to elucidate the next steps needed to develop a fundamental framework needed for a deeper understanding of DESs. It covers recent developments in DES research, frames outstanding scientific questions, and identifies promising research thrusts aligned with the advancement of the field toward predictive models and fundamental understanding of these solvents.

Metal-Free Deep Eutectic Solvents: Preparation, Physical Properties, and Significance

In the past decade, reports detailing the preparation, characterization, and application of deep eutectic solvents (DESs) have grown in number significantly, showing signs of increased interest and attention. Indeed, these systems provide tunable polar environments attractive for their ease of synthesis and lack of need for purification. DESs are homogeneous systems composed of two or more components having a significantly depressed melting point compared to either constituent material. As interest and application of DESs grow, the need for a common understanding of their preparation and characterization is required. In this Perspective, we discuss metal-free DESs, focusing on their preparation, characterization of physical properties, and considerations for their application. We highlight inconsistencies or omissions in literature reports as well as factors for researchers to consider when investigating these systems.

Interaction and Dynamics in a Fully Biodegradable Glucose-Containing Naturally Abundant Deep Eutectic Solvent: Temperature-Dependent Time-Resolved Fluorescence Measurements

A new room-temperature deep eutectic solvent (DES) composed of glucose, urea, and water has been prepared and its relaxation dynamics explored via temperature-dependent time-resolved fluorescence measurements employing hydrophilic and hydrophobic solute probes. Differential scanning calorimetry measurements indicate a glass transition temperature (T_g) of ∼236 K. Measured viscosity coefficients (η) vary from ∼600 to ∼100 cP in the temperature range 318 ≤ T/K ≤ 343 and exhibit Arrhenius-type temperature dependence with an activation energy of ∼65 kJ mol^-1. Interestingly, this DES forms a stable liquid at ∼300 K but is too viscous to be accurately measured by us below 318 K. Temperature-dependent dynamic fluorescence anisotropy measurements using hydrophobic and hydrophilic solutes of similar sizes reveal bi-exponential kinetics and Arrhenius-type temperature dependence for solute rotation times (⟨τ_r⟩) but with significantly decreased activation energies, ∼31 kJ mol^-1 (hydrophobic) and ∼21 kJ mol^-1 (hydrophilic). Deviation from hydrodynamics is further reflected in the strong fractional viscosity dependence of ⟨τ_r⟩: ⟨τ_r⟩ ∝ (η/T)^p with p ≈ 0.3-0.5, indicating pronounced temporal heterogeneity in the relaxation dynamics. Dynamic fluorescence Stokes shift measurements (temporal resolution ∼85 ps) produce dynamic shifts of ∼500-700 cm^-1, bi-exponential solvation energy relaxation with time constants in the range ∼0.2 ns and ∼4 ns, and estimated missing amplitudes of ∼65-75%. Impact of the density difference between a nonpolar solvent and this DES on the estimated missing amplitudes is explored via measuring the temperature-dependent densities and refractive indices of this DES. Lifetime measurements suggest considerable temperature dependence for the hydrophobic solute but no such dependence for the hydrophilic one. Excitation energy dependence of fluorescence emission of various solutes with widely different lifetimes indicates mild spatial heterogeneity for this DES.

Influence of Hydration on the Structure of Reline Deep Eutectic Solvent: A Molecular Dynamics Study

In this article, we have performed an all-atom molecular dynamics simulation study to investigate the influence of water on the molecular level arrangement of reline deep eutectic solvent for different hydration levels ranging from 3.4 to 58.1 wt % of water and complemented the observations of recently measured neutron scattering experimental data. This study is particularly important because water is being introduced as a second hydrogen bond donor/acceptor in reline, wherein the structure is primarily governed by hydrogen bonding and electrostatic interactions. We have analyzed the simulated X-ray scattering structure functions, their partial components, and hydrogen bonding interactions to understand the effects of water on various intermolecular interactions in reline-water mixtures. It is observed that at lower hydration level, reline structure is qualitatively retained. At higher hydration level, most water molecules preferentially solvate chloride anions and ammonium group of choline cations mostly impacting choline-choline, choline-chloride, and chloride-chloride interactions. The present study reveals that at and above 41 wt % of water, the molecular arrangement of reline drastically changes and set to transition from reline to an aqueous solution of reline components with further increase in the hydration level. Hydrogen bond analysis reveals the presence of strong chloride-water H-bonding interaction, which gradually replaces choline-chloride and urea-chloride hydrogen bondings as the hydration level in the mixture increases.