Resilience of Malic Acid Natural Deep Eutectic Solvent Nanostructure to Solidification and Hydration

Brillouin Spectroscopy as a Suitable Technique for the Determination of the Eutectic Composition in Mixtures of Choline Chloride and Water

Deep eutectic solvents (DESs) resulting from the right combination between a hydrogen-bond donor (HBD) and a hydrogen-bond acceptor (HBA) are becoming quite popular in number of applications. More recently, natural DESs (NADESs) containing sugars, natural organic acids, and amino acids as HBDs and ChCl as HBA have received great attention because of their further environmental sustainability as compared to regular DESs. Within this context, mixing water in controlled amounts has been widely accepted as a simple and practical way of altering DES chemical and thermodynamic properties, with viscosity and conductivity experiencing the most significant changes. However, the number of papers describing eutectic mixtures with water as the only HBD is scarce and basically none has been done in fundamental terms. Herein, we investigated mixtures composed of water as the only HBD and ChCl as the HBA using differential scanning calorimetry (DSC) as well as ¹H nuclear magnetic resonance (NMR) and Brillouin spectroscopies. We found the aqueous dilution of ChCl/2H₂O with a ChCl/2H₂O content of ca. 80 wt % was an eutectic. Interestingly, this mixture could be considered a NADES according to its eutectic distance (ΔT_me), in range to eutectics obtained in aqueous dilutions of salt hydrates.

Influence of Hydration on the Structure and Interactions of Ethaline Deep-Eutectic Solvent: A Spectroscopic and Computational Study

Deep-eutectic solvents (DESs) are regarded as alternative green solvents to ionic liquids. In this work we report the structural properties and hydrogen bonding (H-bonding) interactions of an aqueous DES system. The used DES, ethaline (ETH), is composed of choline chloride and ethylene glycol (EG) in 1 : 2 molar ratio. The investigations were carried out by FTIR spectroscopy combined with quantum chemical calculations. Excess spectroscopy and two-dimensional correlation spectroscopy (2D-COS) were used to explore the data in detail. The results showed that, upon mixing, ETH transforms to EG dimers and trimers and D₂ O clusters transform to various ETH-D₂ O complexes. Theoretical calculations show that water molecules insert between the anion and cation of ETH, break the strong doubly ionic Cl^-… H-O_Ch+ H-bond, share charges of the ions and form H-bond with them, thus modulate the interaction properties of ETH. This study deepens our molecular-level understanding of the system and would shed light on its applications.

Carbon and carbon composites obtained using deep eutectic solvents and aqueous dilutions thereof

Extending the “all-in-one” features of DESs to DES/H₂O binary mixtures.

Potential Dependence of Surfactant Adsorption at the Graphite Electrode/Deep Eutectic Solvent Interface

Atomic force microscopy and cyclic voltammetry are used to probe how ionic surfactant adsorbed layer structure affects redox processes at deep eutectic solvent (DES)/graphite interfaces. Unlike its behavior in water, sodium dodecyl sulfate (SDS) in DESs only adsorbs as a complete layer of hemicylindrical hemimicelles far above its critical micelle concentration (CMC). Near the CMC it forms a tail-to-tail monolayer at open-circuit potential (OCP) and positive potentials, and it desorbs at negative potentials. In contrast, cetyltrimethylammonium bromide (CTAB) adsorbs as hemimicelles at low concentrations and remains adsorbed at both positive and negative potentials. The SDS horizontal monolayer has little overall effect on redox processes at the graphite interface, but hemimicelles form an effective and stable barrier. The stronger solvophobic interactions between the C₁₆ versus C₁₂ alkyl chains in the DES allow CTAB to self-assemble into a robust coating at low concentrations and illustrate how the structure of the DES/electrode interface and electrochemical response can be engineered by controlling surfactant structure.

Molecular-Level Insights into the Microstructure of a Hydrated and Nanoconfined Deep Eutectic Solvent

Despite the recent advancements in the field of deep eutectic solvents (DESs), their high viscosity often prevents practical applications. A versatile strategy to overcome this problem is either to add a co-solvent or to confine the DES inside a nanoscaled self-organized system. This work assesses the microstructures of a hydrated and nanoconfined DES comprising benzyltripropylammonium chloride [BTPA]Cl and ethylene glycol (EG). They act as a hydrogen-bond acceptor and a donor, respectively. The hydrogen bonding between [BTPA]Cl and EG in the DES (i.e., BTEG) and the molecular states of water in the hydrated BTEG were studied by Raman spectroscopy. The results show different hydrogen-bonding associations between water-water and water-BTEG or EG molecules. In addition, we investigated the confinement effects of BTEG in a Polysorbate 80 (Tween-80)/cyclohexane reverse micellar (RM) system. The results are compared with those of an ionic liquid-encapsulated RM system. The formation, bonding characteristics, and thermal stability of the RM droplets were studied by solubilization, dynamic light scattering, rheology, and Raman spectroscopy experiments. Furthermore, it is shown that hydrogen bonding between the DES and the surfactant leads to a stable RM system. Interestingly, the viscosity of the RM system is significantly lower than that of the neat DES suggesting that DESs have a much wider practical applicability in the form of RMs.

Formation mechanisms of Caprolactam-tetraalkyl ammonium halide deep eutectic and its hydrate

Deep eutectic solvents (DES) are generally composed of two or three cheap and safe components that are associated with each other via hydrogen bonds. The formation Caprolactam (CPL)-tetraalkyl ammonium halide (TAAX) DES and its hydrate were investigated using React IR. TAAX was introduced to CPL in the molten state at 353.15 K, the intermolecular H-N-C=O⋯H-N-C=O hydrogen bonds (CPL dimer) transfer to X^-⋯H-N-C=O (CPL-TAAX DES). When water was added to CPL-TAAX DES, intermolecular X^-⋯H-N-C=O⋯H-O-H hydrogen bonds appeared, and CPL-TAAX DES hydrate was formed. However, if the ratio of water exceeded the moral fraction of 0.61, CPL-TAAX DES hydrate turned into a CPL-TAAX aqueous solution. The CO (1660 cm^-1) bonds were linearly correlated with hydrogen bonds number and TBAB amount in CPL-TBAB DES. When the molar fraction of CPL-TBAB DES was 0.39 to 0.78, the CO peak redshift has a better linear correlation with water. Halide anions and the alkyl chain length of TAAX have little influence on the redshifts and peak intensity of the CO peak.

Nanostructure of the deep eutectic solvent/platinum electrode interface as a function of potential and water content

The interfacial nanostructure of the three most widely-studied Deep Eutectic Solvents (DESs), choline chloride:urea (ChCl:Urea), choline chloride:ethylene glycol (ChCl:EG), and choline chloride:glycerol (ChCl:Gly) at a Pt(111) electrode has been studied as a function of applied potential and water content up to 50 wt%. Contact mode atomic force microscope (AFM) force-distance curves reveal that for all three DESs, addition of water increases the interfacial nanostructure up to ∼40 wt%, after which it decreases. This differs starkly from ionic liquids, where addition of small amounts of water rapidly decreases the interfacial nanostructure. For the pure DESs, only one interfacial layer is measured at OCP at 0.5 nm, which increases to 3 to 6 layers extending ∼5 nm from the surface at 40 or 50 wt% water. Application of a potential of ±0.25 V to the Pt electrode for the pure DESs increases the number of near surface layers to 3. However, when water is present the applied potential attenuates the steps in the force curve, which are replaced by a short-range exponential decay. This change was most pronounced for ChCl:EG with 30 wt% or 50 wt% water, so this system was probed using cyclic voltammetry, which confirms the interfacial nanostructure is akin to a salt solution.

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.

Water absorption by deep eutectic solvents

Deep eutectic solvents are found to be highly hygroscopic when exposed to the atmosphere.

Bayesian determination of the effect of a deep eutectic solvent on the structure of lipid monolayers

A novel reflectometry analysis method reveals the structure of lipid monolayers at the air-DES interface.

Aggregation Behavior of Sodium Dioctyl Sulfosuccinate in Deep Eutectic Solvents and Their Mixtures with Water: An Account of Solvent's Polarity, Cohesiveness, and Solvent Structure

An anionic surfactant sodium dioctyl sulfosuccinate (AOT) aggregates in deep eutectic solvents (DESs) and their mixtures with water (up to 50% w/w) in a contrasting manner. Two DESs, a mixture of choline chloride + urea and choline chloride + ethylene glycol, commonly known as Reline and Ethaline, respectively, are used as solvents. Behavior of AOT at air-solution interface and aggregation in bulk is investigated using surface tension, conductivity, fluorescence, and dynamic light scattering measurements. The obtained results are correlated with structural aspects of solvent systems as well as with inherent properties of solvent such as Kamlet-Taft polarity parameters, degree of cohesiveness derived from Gordon parameter (G), and cohesive energy density. It is observed that the spontaneity of aggregation in neat DESs or DES-water mixtures follows a trend reflected by various solvent parameters. However, characteristic properties of aggregation in water does not fit into this trend, where critical aggregation concentration of AOT is found in between 30 and 50% (w/w) of respective DES-water mixtures. 1H NMR and 1H-1H 2D NOESY spectroscopy is employed to get insights into reason behind this anomalous behavior. It is observed that AOT forms self-assembled structures similar to that of other surfactants in neat DESs, whereas it undergoes nanosegregation in DESs-water mixtures. The present results are expected to be useful for colloidal aspects of DESs and their mixtures with water.

Deep eutectic-water binary solvent associations investigated by vibrational spectroscopy and chemometrics

Investigation of the behaviour of deep eutectic solvents (DESs) as novel green solvents in the presence of other solvents is of great interest. In this study the behaviour of a common natural DES, namely choline chloride-glycerol deep eutectic solvent (GDES), was studied in the presence of water. A detailed study of the association of the two solvents was performed by integration of two vibrational spectroscopic methods (FTIR and Raman spectroscopy) followed by multivariate analysis. Moreover, a binary mixture of glycerol (Gly) as one of the liquid constituents of GDES and water was explored under the same conditions. A quintuplet and ternary systems were resolved for GDES-water and Gly-water probes, respectively, using multivariate analysis of global data (multi-technique and multi-experiment data arrangements). The results confirmed that in the presence of water the GDES showed different behaviour from its components. Therefore, a DES can be introduced as an independent solvent with its unique properties. Also, different H-bond interaction energies of GDES and its pure components in the presence of water were shown by theoretical calculations based on a density functional theory framework. To investigate the effects of water on the structure of GDES, molecular dynamics (MD) simulations of GDES-water liquid mixtures were performed at 0.9 mole fraction of water.

Glycerol Hydrogen-Bonding Network Dominates Structure and Collective Dynamics in a Deep Eutectic Solvent

The deep eutectic solvent glyceline formed by choline chloride and glycerol in 1:2 molar ratio is much less viscous compared to glycerol, which facilitates its use in many applications where high viscosity is undesirable. Despite the large difference in viscosity, we have found that the structural network of glyceline is completely defined by its glycerol constituent, which exhibits complex microscopic dynamic behavior, as expected from a highly correlated hydrogen-bonding network. Choline ions occupy interstitial voids in the glycerol network and show little structural or dynamic correlations with glycerol molecules. Despite the known higher long-range diffusivity of the smaller glycerol species in glyceline, in applications where localized dynamics is essential (e.g., in microporous media), the local transport and dynamic properties must be dominated by the relatively loosely bound choline ions.

Janus-faced role of water in defining nanostructure of choline chloride/glycerol deep eutectic solvent

Effect of hydration on deep eutectic solvent properties.

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

Counterion binding alters surfactant self-assembly in deep eutectic solvents

Counterion adsorption unexpectedly changes self-assembly behaviour in deep eutectic solvents.