A comparison of choline:urea and choline:oxalic acid deep eutectic solvents at 338 K

Rapid preparation of anti-retrogradation starch by choline chloride based deep eutectic solvents: A comparative study

Legumes are a widespread and cheap source of starch. However, legume starches are prone to retrogradation in applications. Therefore, enhancing the resistance to retrogradation is important to ensure the quality of starch products. In this study, the effects of amines, alcohols, and carboxylic acids deep eutectic solvents (DESs) on the retrogradation properties of pea starch (PS) were investigated. The results showed that the starches treated with carboxylic acid DESs for only 30 min exhibited low paste viscosity, high paste clarity and anti-retrogradation. The carboxylic acid DESs treatment resulted in holes or cracks appearing on the starch surface, a decrease in molecular ordering, relative crystallinity, and amylose content. The thermal enthalpy of starch as well as the pasting viscosity were substantially reduced after the treatment. The paste clarity and the resistance to retrogradation were significantly improved. Conversely, the structure, pasting and retrogradation properties of starch treated with amine and alcohol DESs exhibited only slight differences compared to PS. The carboxylic acid DESs took a short time for the starch modification, and the modified starch paste with low viscosity, high clarity, and resistance to retrogradation. The study could provide an environmentally-friendly and cost-effective method for the preparation of starch with well anti-retrogradation properties.

Voronoi Tessellation as a Tool for Predicting the Formation of Deep Eutectic Solvents

Deep eutectic solvents (DESs) have attracted increasing attention in recent years due to their broad applicability in different fields, but their computer-aided discovery, which avoids a time-consuming trial-and-error investigation, is still lagging. In this paper, a set of nine DESs, composed of choline chloride as a hydrogen-bond acceptor and nine functionalized phenols as hydrogen bond donors, is simulated by using classical molecular dynamics to investigate the possible formation of a DES. The tool of the Voronoi tessellation analysis is employed for producing an intuitive and straightforward representation of the degree of mixing between the different components of the solutions, therefore permitting the definition of a metric quantifying the propensity of the components to produce a uniform solution. The computational findings agree with the experimental results, thus confirming that the Voronoi tessellation analysis can act as a lightweight yet powerful approach for the high-throughput screening of mixtures in the optics of the new DES design.

Anion dynamics and motional decoupling in a glycerol-choline chloride deep eutectic solvent studied by one- and two-dimensional 35Cl NMR

Chlorine-35 is among the few nuclides that provide an experimental handle on the anion dynamics in choline based deep eutectic solvents. By combining several nuclear magnetic resonance (NMR) techniques, the present work examines the Cl- motions within glyceline, a glycerol : choline chloride 2 : 1 solution, in a large temperature range down to the glass transition temperature Tg. The applied methods include spin relaxometry, second-order line shape analysis, as well as two-dimensional central-transition exchange and stimulated-echo spectroscopy. The finding of unstructured central-transition NMR spectra characterized by a relatively small average quadrupolar coupling attests to a highly disordered, essentially nondirectional anionic coordination in glyceline. For temperatures larger than about 1.3Tg the chlorine motions are well coupled to those of the glycerol and the choline moieties. At lower temperatures the local translational anion dynamics become Arrhenian and increasingly faster than the motion of glyceline's matrix molecules. Upon further cooling, the overall ionic conductivity continues to display a super-Arrhenius behavior, implying that the choline cations rather than the Cl anions dominate the long-range charge transport also near Tg.

Everything You Wanted to Know about Deep Eutectic Solvents but Were Afraid to Be Told

Are deep eutectic solvents (DESs) a promising alternative to conventional solvents? Perhaps, but their development is hindered by a plethora of misconceptions. These are carefully analyzed here, beginning with the very meaning of DESs, which has strayed far beyond its original scope of eutectic mixtures of Lewis or Brønsted acids and bases. Instead, a definition that is grounded on thermodynamic principles and distinguishes between eutectic and deep eutectic is encouraged, and the types of precursors that can be used to prepare DESs are reviewed. Landmark works surrounding the sustainability, stability, toxicity, and biodegradability of these solvents are also discussed, revealing piling evidence that numerous DESs reported thus far, particularly those that are choline based, lack sufficient sustainability-related traits to be considered green solvents. Finally, emerging DES applications are reviewed, emphasizing their most remarkable feature: the ability to liquefy a solid compound with a target property, allowing its use as a liquid solvent.

Development of electrolytes for rechargeable zinc-air batteries: current progress, challenges, and future outlooks

This review presents the current developments of various electrolyte systems for secondary zinc air batteries (SZABs). The challenges and advancements in aqueous electrolytes (e.g., alkaline, acidic and neutral) and non-aqueous electrolytes (e.g., solid polymer electrolyte, ionic liquids, gel polymer electrolyte, and deep eutectic solvents) development have been reviewed. Moreover, chemical and physical characteristics of electrolytes such as power density, capacity, rate performance, cyclic ability, and safety that play a vital role in recital of the SZABs have been reviewed. Finally, the challenges and limitations that must be investigated and possible future research areas of SZABs electrolytes are discussed.

Neutron Diffraction Study of Indole Solvation in Deep Eutectic Systems of Choline Chloride, Malic Acid, and Water

Deep eutectic systems are currently under intense investigation to replace traditional organic solvents in a range of syntheses. Here, indole in choline chloride‐malic acid deep eutectic solvent (DES) was studied as a function of water content, to identify solute interactions with the DES which affect heterocycle reactivity and selectivity, and as a proxy for biomolecule solvation. Empirical Potential Structure Refinement models of neutron diffraction data showed [Cholinium]⁺ cations associate strongly with the indole π‐system due to electrostatics, whereas malic acid is only weakly associated. Trace water is sequestered into the DES and does not interact strongly with indole. When water is added to the DES, it does not interact with the indole π‐system but is exclusively in‐plane with the heterocyclic rings, forming strong H‐bonds with the ‐NH group, and also weak H‐bonds and thus prominent hydrophobic hydration of the indole aromatic region, which could direct selectivity in reactions.

From a eutectic mixture to a deep eutectic system via anion selection: Glutaric acid + tetraethylammonium halides

In pursuit of understanding structure-property relationships for the melting point depression of binary eutectic mixtures, the influence of the anion on the solid-liquid (S-L) phase behavior was explored for mixtures of glutaric acid + tetraethylammonium chloride, bromide, and iodide. A detailed experimental evaluation of the S-L phase behavior revealed that the eutectic point is shifted toward lower temperatures and higher salt contents upon decreasing the ionic radius. The salt fusion properties were experimentally inaccessible owing to thermal decomposition. The data were inter- and extrapolated using various models for the Gibbs energy of mixing fitted to the glutaric-acid rich side only, which allowed for the assessment of the eutectic point. Fitting the experimental data to a two-parameter Redlich-Kister expansion with Flory entropy, the eutectic depth could be related to the ionic radius of the anion. The anion type, and in particular its size, can therefore be viewed as an important design parameter for the liquid window of other acid and salt-based deep eutectic solvents/systems.

Liquid structure and dynamics in the choline acetate:urea 1:2 deep eutectic solvent

We report on the thermodynamic, structural, and dynamic properties of a recently proposed deep eutectic solvent, formed by choline acetate (ChAc) and urea (U) at the stoichiometric ratio 1:2, hereinafter indicated as ChAc:U. Although the crystalline phase melts at 36-38 °C depending on the heating rate, ChAc:U can be easily supercooled at sub-ambient conditions, thus maintaining at the liquid state, with a glass-liquid transition at about -50 °C. Synchrotron high energy x-ray scattering experiments provide the experimental data for supporting a reverse Monte Carlo analysis to extract structural information at the atomistic level. This exploration of the liquid structure of ChAc:U reveals the major role played by hydrogen bonding in determining interspecies correlations: both acetate and urea are strong hydrogen bond acceptor sites, while both choline hydroxyl and urea act as HB donors. All ChAc:U moieties are involved in mutual interactions, with acetate and urea strongly interacting through hydrogen bonding, while choline being mostly involved in van der Waals mediated interactions. Such a structural situation is mirrored by the dynamic evidences obtained by means of ¹H nuclear magnetic resonance techniques, which show how urea and acetate species experience higher translational activation energy than choline, fingerprinting their stronger commitments into the extended hydrogen bonding network established in ChAc:U.

Deep Eutectic Solvents: Molecular Simulations with a First-Principles Polarizable Force Field

The unique properties of deep eutectic solvents make them useful in a variety of applications. In this work we develop a first-principles force field for reline, which is composed of choline chloride and urea in the molar ratio 1:2. We start with the symmetry adapted perturbation theory (SAPT) protocol and then make adjustments to better reproduce the structure and dynamics of the liquid when compared to first-principles molecular dynamics (FPMD) simulations. The resulting force field is in good agreement with experiments in addition to being consistent with the FPMD simulations. The simulations show that primitive molecular clusters are preferentially formed with choline-chloride ionic pairs bound with a hydrogen bond in the hydroxyl group and that urea molecules coordinate the chloride mainly via the trans-H chelating hydrogen bonds. Incorporating polarizability qualitatively influences the radial distributions and lifetimes of hydrogen bonds and affects long-range structural order and dynamics. The polarizable force field predicts a diffusion constant about an order of magnitude larger than the nonpolarizable force field and is therefore less computationally intensive. We hope this study paves the way for studying complex hydrogen-bonding liquids from a first-principles approach.

Tuning the solvation of indigo in aqueous deep eutectics

The solubility of synthetic indigo dye was measured at room temperature in three deep eutectic solvents (DESs)-1:3 choline chloride:1,4-butanediol, 1:3 tetrabutylammonium bromide:1,4-butanediol, and 1:2 choline chloride:p-cresol-to test the hypothesis that the structure of DESs can be systematically altered, to induce specific DES-solute interactions, and, thus, tune solubility. DESs were designed starting from the well-known cholinium chloride salt mixed with the partially amphiphilic 1,4-butanediol hydrogen bond donor (HBD), and then, the effect of increasing salt hydrophobicity (tetrabutylammonium bromide) and HBD hydrophobicity (p-cresol) was explored. Measurements were made between 2.5 and 25 wt. % H₂O, as a reasonable range representing atmospherically absorbed water, and molecular dynamics simulations were used for structural analysis. The choline chloride:1,4-butanediol DES had the lowest indigo solubility, with only the hydrophobic character of the alcohol alkyl spacers. Solubility was highest for indigo in the tetrabutylammonium bromide:1,4-butanediol DES with 2.5 wt. % H₂O due to interactions of indigo with the hydrophobic cation, but further addition of water caused this to reduce in line with the added water mole fraction, as water solvated the cation and reduced the extent of the hydrophobic region. The ChCl:p-cresol DES did not have the highest solubility at 2.5 wt. % H₂O, but did at 25 wt. % H₂O. Radial distribution functions, coordination numbers, and spatial distribution functions demonstrate that this is due to strong indigo-HBD interactions, which allow this system to resist the higher mole fraction of water molecules and retain its solubility. The DES is, therefore, a host to local-composition effects in solvation, where its hydrophobic moieties concentrate around the hydrophobic solute, illustrating the versatility of DES as solvents.

Structural evolution of iron forming iron oxide in a deep eutectic-solvothermal reaction

Deep eutectic solvents (DES) and their hydrated mixtures are used for solvothermal routes towards greener functional nanomaterials. Here we present the first static structural and in situ studies of the formation of iron oxide (hematite) nanoparticles in a DES of choline chloride : urea where xurea = 0.67 (aka. reline) as an exemplar solvothermal reaction, and observe the effects of water on the reaction. The initial speciation of Fe3+ in DES solutions was measured using extended X-ray absorption fine structure (EXAFS), while the atomistic structure of the mixture was resolved from neutron and X-ray diffraction and empirical potential structure refinement (EPSR) modelling. The reaction was monitored using in situ small-angle neutron scattering (SANS), to determine mesoscale changes, and EXAFS, to determine local rearrangements of order around iron ions. It is shown that iron salts form an octahedral [Fe(L)3(Cl)3] complex where (L) represents various O-containing ligands. Solubilised Fe3+ induced subtle structural rearrangements in the DES due to abstraction of chloride into complexes and distortion of H-bonding around complexes. EXAFS suggests the complex forms [-O-Fe-O-] oligomers by reaction with the products of thermal hydrolysis of urea, and is thus pseudo-zero-order in iron. In the hydrated DES, the reaction, nucleation and growth proceeds rapidly, whereas in the pure DES, the reaction initially proceeds quickly, but suddenly slows after 5000 s. In situ SANS and static small-angle X-ray scattering (SAXS) experiments reveal that nanoparticles spontaneously nucleate after 5000 s of reaction time in the pure DES before slow growth. Contrast effects observed in SANS measurements suggest that hydrated DES preferentially form 1D particle morphologies because of choline selectively capping surface crystal facets to direct growth along certain axes, whereas capping is restricted by the solvent structure in the pure DES.

Connecting chloride solvation with hydration in deep eutectic systems

The Deep Eutectic Solvents (DESs) choline chloride:urea (x_ChCl = 0.33) and choline chloride:glycolic acid (x_ChCl = 0.5) were studied using viscosity-corrected ³⁵Cl NMR and MD simulations to probe the role of chloride as a function of water content.

Hydrophobic functional liquids based on trioctylphosphine oxide (TOPO) and carboxylic acids

Trioctylphosphine oxide (TOPO) is a hydrophobic extracting agent used in a number of commercially important separations of valuable solutes from aqueous streams (with examples ranging from lanthanides, through gallium, to carboxylic acids). TOPO is traditionally used as a solute in kerosene, its extraction efficiency limited by its solubility in the organic diluents. In this work, eighteen hydrogen bond donors (HBDs) were screened for their capacity to liquefy TOPO, employing strategies used to design deep eutectic solvents (DES). The selected HBDs were all useful in separations and were designed to formulate solvent-free, hydrophobic, bi-functional liquid extracting agents. Some TOPO:HBD mixtures yielded hydrophobic liquids that offer potential to be extremely efficient extractants, incorporating high intrinsic concentrations of TOPO. Following this initial screening, two systems: TOPO:malonic acid and TOPO:levulinic acid, were selected for detailed physico-chemical characterisation across their complete compositional ranges. Phase diagrams, thermal stabilities and the mechanism of thermal decomposition are reported, along with densities and insights from ³¹P NMR spectroscopic studies. The work was concluded with a proof-of-concept demonstration of the use of the TOPO:malonic acid (2 : 1 mol ratio) mixture for the extraction of gallium from acidic chloride feedstock (simulated pre-digestate of zinc leach residue). The loading capacity of the TOPO:malonic acid extractant was three orders of magnitude greater than that of the literature benchmark, encouraging further application-oriented studies.

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.

Modeling of Solid-Liquid Equilibria in Deep Eutectic Solvents: A Parameter Study

Deep eutectic solvents (DESs) are potential alternatives to many conventional solvents in process applications. Knowledge and understanding of solid-liquid equilibria (SLE) are essential to characterize, design, and select a DES for a specific application. The present study highlights the main aspects that should be taken into account to yield better modeling, prediction, and understanding of SLE in DESs. The work is a comprehensive study of the parameters required for thermodynamic modeling of SLE-i.e., the melting properties of pure DES constituents and their activity coefficients in the liquid phase. The study is carried out for a hypothetical binary mixture as well as for selected real DESs. It was found that the deepest eutectic temperature is possible for components with low melting enthalpies and strong negative deviations from ideality in the liquid phase. In fact, changing the melting enthalpy value of a component means a change in the difference between solid and liquid reference state chemical potentials which results in different values of activity coefficients, leading to different interpretations and even misinterpretations of interactions in the liquid phase. Therefore, along with reliable modeling of liquid phase non-ideality in DESs, accurate estimation of the melting properties of their pure constituents is of clear significance in understanding their SLE behavior and for designing new DES systems.

On the hydration of DOPE in solution

The atomic-scale hydration structure around the 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) headgroup in a chloroform/water solution has been investigated using neutron diffraction enhanced by isotopic substitution and NMR, coupled with empirical potential structure refinement and molecular dynamics simulations. The results obtained show the preferential binding sites for water molecules on the DOPE headgroups, with the most predominant interactions being with the ammonium and phosphate groups. Interestingly, the level of hydration, as well as the association of DOPE molecules, varies according to the simulation method used. The results here suggest the presence of a tight water network around these lipid headgroups that could affect the permeability of the membrane for lipid-mediated diffusion.

Investigation of glycerol hydrogen-bonding networks in choline chloride/glycerol eutectic-forming liquids using neutron diffraction

Neutron scattering reveals the persistent three-dimensional hydrogen-bonding network between glycerol molecules in the 1 : 2 choline chloride/glycerol eutectic.

What a difference a methyl group makes - probing choline-urea molecular interactions through urea structure modification

There is a lack of fundamental knowledge on deep eutectic solvents, even for the most extensively studied mixtures, such as the mixture of cholinium chloride and urea, which prevents a judicious choice of components to prepare new solvents. The objective of this work is to study and understand the fundamental interactions between cholinium chloride and urea that lead to the experimentally observed melting temperature depression. To do so, the structure of urea was strategically and progressively modified, in order to block certain interaction centres, and the solid-liquid equilibrium data of each new binary system was experimentally measured. Using this approach, it was concluded that the most important interaction between cholinium chloride and urea occurs through hydrogen bonding between the chloride anion and the amine groups. Any blockage of these groups severely hampers the melting point depression effect. Raman spectroscopy and DFT calculations were utilized to study in more detail this hydrogen bonding and its nuances.

Frustrated Lewis pairs in ionic liquids and molecular solvents - a neutron scattering and NMR study of encounter complexes

The presence of the weakly-associated encounter complex in the model frustrated Lewis pair solution (FLP): tris(tert-butyl)phosphine (P(^tBu)₃) and tris(pentafluorophenyl)borane (BCF) in benzene, was confirmed via PB correlation analysis from neutron scattering data. On average, ca. 5% of dissolved FLP components were in the associated state. NMR spectra of the FLP in benzene gave no evidence of such association, in agreement with earlier reports and the transient nature of the encounter complex. In contrast, the corresponding FLP solution in the ionic liquid, 1-decyl-3-methylimidazolium bistriflamide, [C₁₀mim][NTf₂], generated NMR signals that can be attributed to formation of encounter complexes involving over 20% of the dissolved species. The low diffusivity characteristics of ionic liquids is suggested to enhance high populations of encounter complex. The FLP in the ionic liquid solution retained its ability to split hydrogen.

X-Ray structure and ionic conductivity studies of anhydrous and hydrated choline chloride and oxalic acid deep eutectic solvents

X-Ray, conductivity and molecular dynamics studies shed light on the effect of water of crystallization on choline chloride–oxalic acid DESs