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.

Deep Eutectic Solvents: A Review of Fundamentals and Applications.

Fluorescence up-conversion (∼250 fs instrumental response) coupled with time correlated single photon counting measurements was performed to explore the complete Stokes shift dynamics of a dipolar ...

Subpicosecond Solvation Response and Partial Viscosity Decoupling of Solute Diffusion in Ionic Acetamide Deep Eutectic Solvents: Fluorescence Up-Conversion and Fluorescence Correlation Spectroscopic Measurements.

Recently, the introduction of novel deep eutectic solvents having a hydrophobic character with greener starting materials has motivated researchers to explore these water-immiscible solvents, owing to their new potential applications. In this regard, herein atomistic molecular dynamics simulations have been performed to understand the bulk phase morphology present in DL-menthol based hydrophobic deep eutectic solvents (HDESs) with organic acids of different chain lengths used as hydrogen bond donors. From the appearance of a prepeak in the simulated total X-ray scattering structure function (S(q)), we found an evidence of intermediate-range structural organization in these HDESs. We show that the prepeak originates from the self-segregation of DL-menthol and the polar hydroxyl groups of the acids, as witnessed from partitioning of the total S(q). Surprisingly, even for a very long tail containing HDES, the apolar–apolar component shows only nominal contribution to the prepeak. We show that the structure of these HDESs is dominated by a set of very strong intermolecular hydrogen bonding between menthol–menthol, acid–acid, and menthol–acid molecules. Our results show that there is competition between the hydroxyl hydrogen of the acids and menthol to form an intermolecular hydrogen bond with the carbonyl oxygen of the acids and hydroxyl oxygen of menthol.

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

Molecular dynamics simulations and dielectric relaxation (DR) measurements in the frequency window, 02 ≤ ν/GHz ≤ 50, have been performed to explore the heterogeneous reorientation dynamics in [f choline chloride + (1 - f) urea] deep eutectic solvents (DESs) at f = 033 and 040 in the temperature range 293 ≤ T/K ≤ 333 The solution viscosity varies by more than an order of magnitude DR measurements in these DESs reveal multiple relaxation timescales-τ1 ∼ 500 ps, τ2 ∼ 100 ps, τ3 ∼ 30 ps, and τ4 ∼ 5 ps Simulated rank-dependent collective single-particle reorientational (Cl(t), l being the rank) and structural H-bond [CHB(t)] relaxations can explain the microscopic origin of all these DR timescales The average DR times, ⟨τDR⟩, exhibit a pronounced fractional viscosity dependence, ⟨τDR⟩ ∝ (η/T)p, with p = 01 This experimental evidence of pronounced heterogeneous reorientation dynamics in these DESs is supported by a strong translation-rotation decoupling and a significant deviation of the average reorientational correlation times (⟨τl⟩) from Debye's l(l + 1) law The simulated ratios between the average rotation and translation timescales for both urea and choline correctly reduce to the appropriate hydrodynamic limit at high temperatures The stretched exponential relaxations of the simulated self-dynamic structure factors and the non-Gaussian single-particle displacement distributions further support strong temporal heterogeneity in these DESs Dynamic susceptibilities from the simulated four-point correlations exhibit long correlated timescales Moreover, simulated activation energies estimated from the temperature-dependent C1(t) decays and the translational diffusion coefficients from the velocity autocorrelation functions agree favorably with those from the corresponding DR and the pulsed field gradient nuclear magnetic resonance measurements

Heterogeneous Orientational Relaxations and Translation-Rotation Decoupling in (Choline Chloride + Urea) Deep Eutectic Solvents: Investigation through Molecular Dynamics Simulations and Dielectric Relaxation Measurements.

Deep eutectic solvents (DESs) are novel environment-friendly media for a variety of applications. In order to obtain insight into the structure and dynamics of some less-explored DESs comprising ethylene glycol and tetraalkylammonium bromide salts with variable alkyl chain length, we have captured complete dynamics occurring in these solvents in a timescale of few femtoseconds to several nanoseconds by monitoring the time-dependent fluorescence Stokes shift of coumarin 153 employing a combination of time-correlated single-photon counting and fluorescence upconversion techniques. The solvent response function constructed from the measured data reveals a sub-picosecond component (∼0.8 ps, 20-35%) in addition to a slow component (180-475 ps) with a distribution of relaxation time. The slow time component is found to be strongly dependent on the viscosity of the medium, indicating that it arises from the diffusive motions of the solvent constituents into and out of the solvation shell, whereas the ultrafast time component, which is nearly independent of the solvent viscosity, arises from fast local motions of the constituents in the immediate vicinity of the solute molecule.

Complete Solvation Dynamics of Coumarin 153 in Tetraalkylammonium Bromide-Based Deep Eutectic Solvents

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 m...

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

How frictional response during solute solvation controls solute rotation in naturally abundant deep eutectic solvent (NADES)? A case study with amino acid derivative containing DES

Jayanta Mondal

Papers

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

How frictional response during solute solvation controls solute rotation in naturally abundant deep eutectic solvent (NADES)? A case study with amino acid derivative containing DES