The Role of Polyfunctionality in the Formation of [Ch]Cl-Carboxylic Acid-Based Deep Eutectic Solvents

Deep eutectic solvents as an alternative for extraction of flavonoids from soybean (Glycine max (L) Merrill) and okara: An experimental and computational approach based on COSMO-SAC model

In this study, different Deep Eutectic Solvents based on choline chloride ([Ch]Cl) with carboxylic acids, sugars, and glycerol, were investigated as alternative solvents for the extraction of flavonoids from soybean and okara. Initially, the COSMO-SAC was investigated as a tool in solvent screening for the extraction of flavonoids. Experimental validation was performed using total flavonoid analysis with the solvents that showed greater interaction with the solutes. The extracts obtained from soybean and okara using the DES [Ch]Cl:acetic acid added with 30 % water showed the highest total flavonoid content, 1.05 mg eq. of catechin/g dry soybean and 0.94 mg eq. of catechin /g dry okara, respectively. For phenolic compound extraction, [Ch]Cl: acetic acid DES extracted approximately 1.16 mg GAE/g of soybean and 0.69 mg GAE/g of okara. For antioxidant activity, soybean and okara extracts obtained with [Ch]Cl: acetic acid showed FRAP results of 0.40 mg Trolox/mL of extract and 0.45 mg Trolox/mL of extract, respectively. In addition, the isoflavones daidzein, genistein, glycitein, daidzin, genistin, and glycitin were identified and quantified in the soybean and okara extracts obtained with DES [Ch]Cl: acetic acid with 30% water, totaling 1068.05 and 424.32 µg total isoflavones/g dry sample. Therefore, The COSMO-SAC model was a useful tool in solvent screening, saving time and costs. Also, DES can be an alternative solvent for extracting flavonoids to replace conventional organic solvents, respecting current environmental and human health concerns.

Interactions in plasticizer mixtures used for sugar replacement

In our quest for novel ingredients to be used in sugar replacement strategies, we have investigated the thermodynamics of polycarboxylic acids, such as citric acid. We have demonstrated the applicability of the Flory-Huggins (FH) theory to describe the thermodynamics of polycarboxylic acids solutions. Moreover, for citric acid we can describe the complete phase diagram with the theory. It shows that polycarboxylic acids have similar plasticizing and hygroscopic properties as sugars and polyols. Regarding mixtures of polycarboxylic acids and carbohydrates, the FH theory is able to describe a) the water activity of the mixtures, b) the solubility of ternary mixtures of acids and sugars, c) the lowering of the deliquescence point for binary mixtures of crystals, and d) the melting point depression in eutectic mixtures. Unexpectingly, our investigations show there is a strong non-zero FH interaction parameter between carboxylic acids and carbohydrates. In our prior sugar replacement strategy we have assumed zero interactions between plasticizers. Here, we will readdress this assumption. Carefull investigations of solid-liquid equilibrium of eutectic mixtures involving polycarboxylic acids and/or carbohydrates, shows nearly zero interaction in eutectic mixtures consisting only of two carbohydrates or two polycarboxylic acids. We now hold the hypothesis that there is strong non-zero interaction if the mixture contains plasticizers strongly differing in the amount of hydrogen bonding groups. This strong interaction explains why these mixtures, like polycarboxylic acids and carbohydrates, are excellent candidates as deep eutectic solvents. Furthermore, we conclude that polycarboxylic acids are useful additions to the toolbox of sugar replacers, albeit that there are some limitations to their amounts used.

Design of Deep Eutectic Systems: Plastic Crystalline Materials as Constituents

Deep eutectic solvents (DESs) are a class of green and tunable solvents that can be formed by mixing constituents having very low melting entropies and enthalpies. As types of materials that meet these requirements, plastic crystalline materials (PCs) with highly symmetrical and disordered crystal structures can be envisaged as promising DES constituents. In this work, three PCs, namely, neopentyl alcohol, pivalic acid, and neopentyl glycol, were studied as DES constituents. The solid–plastic transitions and melting properties of the pure PCs were studied using differential scanning calorimetry. The solid–liquid equilibrium phase diagrams of four eutectic systems containing the three PCs, i.e., L-menthol/neopentyl alcohol, L-menthol/pivalic acid, L-menthol/neopentyl glycol, and choline chloride/neopentyl glycol, were measured. Despite showing near-ideal behavior, the four studied eutectic systems exhibited depressions at the eutectic points, relative to the melting temperatures of the pure constituents, that were similar to or even larger than those of strongly nonideal eutectic systems. These findings highlight that a DES can be formed when PCs are used as constituents, even if the eutectic system is ideal.

Furfural and 5-Hydroxymethylfurfural Production from Sugar Mixture Using Deep Eutectic Solvent/MIBK System

Choline chloride (ChCl) / glycolic acid (GA) deep eutectic solvent (DES) media with high water content but without any additional catalyst are introduced in furfural and 5-hydroxymethylfurfural (HMF) production. The effects of water content, reaction time, and reaction temperature are investigated with two feedstocks: a glucose/xylose mixture and birch sawdust. Based on the results, 10 equivalent quantities of water (32.9 wt.%) were revealed to be beneficial for conversions without rupturing the DES structure. The optimal reaction conditions were 160 °C and 10 minutes for the sugar mixture and 170 °C and 10 minutes for birch sawdust in a microwave reactor. High furfural yields were achieved, namely 62 % from the sugar mixture and 37.5 % from birch sawdust. HMF yields were low, but since the characterization of the solid residue of sawdust, after DES treatment, was revealed to contain only cellulose (49 %) and lignin (52 %), the treatment could be potentially utilized in a biorefinery concept where the main products are obtained from the cellulose fraction. Extraction of products into the organic phase (methyl isobutyl ketone, MIBK) during the reaction enabled the recycling of the DES phase, and yields remained high for three runs of recycling.

Enabling formulations of aprepitant: in vitro and in vivo comparison of nanocrystalline, amorphous and deep eutectic solvent based formulations

A deep eutectic solvent (DES) is a eutectic system consisting of hydrogen bond donor and acceptor has been suggested as a promising formulation strategy for poorly soluble drugs. A DES consisting of choline chloride and levulinic acid in a 1:2  molar ratio was used to formulate a liquid solution of the model drug aprepitant. This formulation was tested in vitro (drug release and permeability) and in vivo (rat model) and compared with the performance of amorphous aprepitant and the commercial aprepitant nanocrystalline formulation. In this study a DES formulation is compared for the first time directly to other established enabling formulations. The in vitro drug release study demonstrated that the DES formulation and the amorphous form both were able to induce an apparent supersaturation followed by subsequent drug precipitation. To mitigate the risk of precipitation, HPMC was predissolved in the dissolution medium, which successfully reduced the degree of precipitation. In line with the results from the release study, an in vitro permeation study showed superior permeation of the drug from the DES formulation and from the amorphous form compared to the nanocrystalline formulation. However, the promising in vitro findings could not be directly translated into an increased in vivo performance in rats compared to the nanocrystalline formulation. Whilst the DES formulation (34 ± 4%) showed a higher oral bioavailability compared to amorphous aprepitant (20 ± 4%), it was on par with the oral bioavailability obtained from the nanocrystalline formulation (36 ± 2%).

Predicting Deliquescence Relative Humidities of Crystals and Crystal Mixtures

The presence of water in the form of relative humidity (RH) may lead to deliquescence of crystalline components above a certain RH, the deliquescence RH (DRH). Knowing the DRH values is essential, e.g., for the agrochemical industry, food industry, and pharmaceutical industry to identify stability windows for their crystalline products. This work applies the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) to purely predict the DRH of single components (organic acids, sugars, artificial sweeteners, and amides) and multicomponent crystal mixtures thereof only based on aqueous solubility data of the pure components. The predicted DRH values very well agree with the experimental ones. In addition, the temperature influence on the DRH value could be successfully predicted with PC-SAFT. The DRH prediction also differentiates between formation of hydrates and anhydrates. PC-SAFT-predicted phase diagrams of hydrate-forming components illustrate the influence of additional components on the hydrate formation as a function of RH. The DRH prediction via PC-SAFT allows for the determining of the stability of crystals and crystal mixtures without the need for time-consuming experiments.

Stability of Pharmaceutical Co-Crystals at Humid Conditions Can Be Predicted

Knowledge of the stability of pharmaceutical formulations against relative humidity (RH) is essential if they are to become pharmaceutical products. The increasing interest in formulating active pharmaceutical ingredients as stable co-crystals (CCs) triggers the need for fast and reliable in-silico predictions of CC stability as a function of RH. CC storage at elevated RH can lead to deliquescence, which leads to CC dissolution and possible transformation to less soluble solid-state forms. In this work, the deliquescence RHs of the CCs succinic acid/nicotinamide, carbamazepine/nicotinamide, theophylline/citric acid, and urea/glutaric acid were predicted using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). These deliquescence RH values together with predicted phase diagrams of CCs in water were used to determine critical storage conditions, that could lead to CC instability, that is, CC dissolution and precipitation of its components. The importance of CC phase purity on RH conditions for CC stability is demonstrated, where trace levels of a separate phase of active pharmaceutical ingredient or of coformer can significantly decrease the deliquescence RH. The use of additional excipients such as fructose or xylitol was predicted to decrease the deliquescence RH even further. All predictions were successfully validated by stability measurements at 58%, 76%, 86%, 93%, and 98% RH and 25 °C.

Choline chloride-based deep eutectic solvents as potential solvent for extraction of phenolic compounds from olive leaves: Extraction optimization and solvent characterization

This study evaluates the use of deep eutectic solvents (DES) prepared with choline chloride ([Ch]Cl) and carboxylic acids for phenolic compound extraction from olive leaves. These extracts were then compared to those obtained using ethanol. The effects of temperature and water addition during DES- and ethanol-based extractions were analyzed using response surface methodology. Due to the lack of solid-liquid equilibrium (SLE) data for [Ch]Cl + acetic acid, SLE, and DES density and viscosity with and without water addition were measured and analyzed. [Ch]Cl:acetic acid (54.1 °C, 50.0% water addition) extracted 15% more phenolic compounds than ethanol (54.1 °C, 0.5% water addition), according to UHPLC-MS based analyses. SLE analyses showed that [Ch]Cl + acetic acid presented a eutectic region at close to a 1:2 molar ratio. DES precursors and water addition influenced solvent physical properties and phenolic compound yield. DES was confirmed to be an innovative, strong solvent for phenolic compound extraction from olive leaves.

Unravelling the nature of citric acid:L-arginine:water mixtures: the bifunctional role of water

The use of water as a component of deep eutectic systems (DES) has raised some questions regarding its influence on the nature of the mixture. Does it form a DES or an aqueous solution and what is the role of water? In this work, the nature of citric acid:l-arginine:water mixtures was explored through phase equilibria studies and spectroscopic analysis. In a first step, PC-SAFT was validated as a predictive tool to model the water influence on the solid liquid equilibria (SLE) of the DES reline using the individual-component approach. Hence, activity coefficients in the ternary systems citric acid:l-arginine:water and respective binary combinations were studied and compared using ePC-SAFT. It was observed that the water-free mixtures citric acid:l-arginine showed positive deviation from Raoult's law, while upon addition of water strong negative deviation from Raoult's law was found, yielding melting depressions around 100 K. Besides these strong interactions, pH was found to become acidic (pH = 3.5) upon water addition, which yields the formation of charged species ([H2Cit]- and [l-arg]+). Thus, the increased interactions between the molecules upon water addition might be caused by several mechanisms such as hydrogen bonding or ionic forces, both being induced by water. For further investigation, the liquid mixtures citric acid:l-arginine:water were studied by FTIR and NMR spectroscopy. FTIR spectra disproved a possible solubility enhancement caused by salt formation between citric acid and l-arginine, while NMR spectra supported the formation of a hydrogen bonding network different from the binary systems citric acid:water and l-arginine:water. Either being a DES or other type of non-ideal solution, the liquefaction of the studied systems is certainly caused by a water-mediator effect based on the formation of charged species and cross interactions between the mixture constituents.

The role of ionic vs. non-ionic excipients in APIs-based eutectic systems

Aiming to contribute to drug pre-formulation, new eutectic mixtures were developed. Thymol, coumarin, or quaternary ammonium chlorides as excipients, were combined with the active pharmaceutical ingredients (APIs) acetylsalicylic acid, acetaminophen, ibuprofen, ketoprofen, or lidocaine. Their solid-liquid equilibrium (SLE) binary phase diagrams were measured to study eventual phase separation between the compounds, preventing manufacturing problems, and to study the molecular interactions between the APIs and ionic or non-ionic excipients. The Conductor-like Screening Model for Real Solvents (COSMO-RS) capability to predict the SLE of mixtures containing non-ionic excipients was further evaluated. COSMO-RS gives a good quantitative description of the experimental SLE being a tool with great potential in the screening of eutectic systems containing APIs and non-ionic excipients. While thymol presents strong interactions with the APIs, and consequently negative deviations to thermodynamic ideality, systems containing coumarin follow a quasi-ideal behavior. Regarding the ionic excipients, both choline chloride and the tetraalkylammonium chlorides are unable to establish relevant interactions with the APIs, and no significant negative deviations to ideality are observed. The liquefaction of the APIs here studied is favored by using non-ionic excipients, such as thymol, due to the strong interactions it can establish with the APIs.

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.

Ester-mediated peptide formation promoted by deep eutectic solvents: a facile pathway to proto-peptides

The ester-amide exchange reaction enables spontaneous formation of prebiotic proto-peptides under mild conditions. However, this reaction also leads to oligomers with a vast sequence diversity of ester and amide linkages. Here, we demonstrate using deep eutectic solvents as a universal strategy to regulate the reaction pathways and promote the formation of amino acid-enriched oligomers with peptide backbones.

Liquid Polymer Eutectic Mixture for Integrated Extractive-Oxidative Desulfurization of Fuel Oil: An Optimization Study via Response Surface Methodology

Hydrodesulfurization (HDS) has been commercially employed for the production of ultra-low sulfur fuel oil. However, HDS is unable to remove sterically hindered sulfur-containing compounds such as dibenzothiophene (DBT) and benzothiophene (BT). An alternative way to remove sulfur is via extractive desulfurization system (EDS) using deep eutectic solvents (DES) as sustainable extractant. In this work, liquid polymer DES was synthesized using tetrabutylammonium chloride (TBAC) and poly(ethylene glycol) 400 (PEG) with different molar ratios. Response surface methodology (RSM) was applied to study the effect of independent variables toward extraction efficiency (EE). Three significant operating parameters, temperature (25–70 °C), DES molar ratio (1–3), and DES volume ratio (0.2–2.0), were varied to study the EE of sulfur from model oil. A quadratic model was selected based on the fit summary test, revealing that the extraction efficiency was greatly influenced by the amount of DES used, followed by the extraction temperature and PEG ratio. Although molar ratio of DES was less sensitive towards EDS performance, the EE was much higher at lower PEG ratio. For the realization of an energy-efficient EDS system, optimization of EDS parameters and EE was carried out via a desirability tool. At 25 °C, 1:1 molar ratio of TBAC to PEG, and DES-to-model-oil-volume ratio of 1, removal of DBT reached as high as 79.01%. The present findings could provide valuable insight into the development of practicable EDS technology as a substitute to previous HDS process.

Design of Deep Eutectic Systems: A Simple Approach for Preselecting Eutectic Mixture Constituents

Eutectic systems offer a wide range of new (green) designer solvents for diverse applications. However, due to the large pool of possible compounds, selecting compounds that form eutectic systems is not straightforward. In this study, a simple approach for preselecting possible candidates from a pool of substances sharing the same chemical functionality was presented. First, the melting entropy of single compounds was correlated with their molecular structure to calculate their melting enthalpy. Subsequently, the eutectic temperature of the screened binary systems was qualitatively predicted, and the systems were ordered according to the depth of the eutectic temperature. The approach was demonstrated for six hydrophobic eutectic systems composed of L-menthol and monocarboxylic acids with linear and cyclic structures. It was found that the melting entropy of compounds sharing the same functionality could be well correlated with their molecular structures. As a result, when the two acids had a similar melting temperature, the melting enthalpy of a rigid acid was found to be lower than that of a flexible acid. It was demonstrated that compounds with more rigid molecular structures could form deeper eutectics. The proposed approach could decrease the experimental efforts required to design deep eutectic solvents, particularly when the melting enthalpy of pure components is not available.

Enhanced Conversion of Xylan into Furfural using Acidic Deep Eutectic Solvents with Dual Solvent and Catalyst Behavior

An efficient process for the production of furfural from xylan by using acidic deep eutectic solvents (DESs), which act both as solvents and catalysts, is developed. DESs composed of cholinium chloride ([Ch]Cl) and malic acid or glycolic acid at different molar ratios, and the effects of water and γ-valerolactone (GVL) contents, solid/liquid (S/L) ratio, and microwave heating are investigated. The best furfural yields are obtained with the DES [Ch]Cl:malic acid (1:3 molar ratio)+5 wt % water, under microwave heating for 2.5 min at 150 °C, a S/L ratio of 0.050, and GVL at a weight ratio of 2:1. Under these conditions, a remarkable furfural yield (75 %) is obtained. Direct distillation of furfural from the DES/GVL solvent and distillation from 2-methyltetrahydrofuran (2-MeTHF) after a back-extraction step enable 89 % furfural recovery from 2-MeTHF. This strategy allows recycling of the DES/GVL for at least three times with only small losses in furfural yield (>69 %). This is the fastest and highest-yielding process reported for furfural production using bio-based DESs as solvents and catalysts, paving the way for scale-up of the process.

Catalytic Low-Temperature Dehydration of Fructose to 5-Hydroxymethylfurfural Using Acidic Deep Eutectic Solvents and Polyoxometalate Catalysts

HMF synthesis typically requires high temperature and is carried out in aqueous solutions. In this work, the low-temperature dehydration of fructose to HMF in different deep eutectic solvents (DES) was investigated. We found a very active and selective reaction system consisting of the DES tetraethyl ammonium chloride as hydrogen bond acceptor (HBA) and levulinic acid as hydrogen bond donor (HBD) in a molar ratio of 1:2 leading to a maximum HMF yield of 68% after 120 h at 323 K. The DES still contained a low amount of water at the initial reaction, and water was also produced during the reaction. Considering the DES properties, neither the molar ratio in the DES nor the reaction temperature had a significant influence on the overall performance of the reaction system. However, the nature of the HBA as well as the acidity of the HBD play an important role for the maximum achievable HMF yield. This was validated by measured yields in a DES with different combinations of HBD (levulinic acid and lactic acid) and HBA (choline chloride and tetra-n-alkyl ammonium chlorides). Moreover, addition of vanadium containing catalysts, especially the polyoxometalate HPA-5 (H8PV5Mo7O40) leads to drastically increased reaction kinetics. Using HPA-5 and the DES tetraethyl ammonium chloride-levulinic acid we could reach a maximum HMF yield of 57% after only 5 h reaction time without decreasing the very high product selectivity.

Characterization of xylitol or citric acid:choline chloride:water mixtures: Structure, thermophysical properties, and quercetin solubility

The industrial implementation of new eco-friendly solvents has highlighted the need to analyse both the structures and thermophysical properties of these solvents. Here, two deep eutectic solvents (DESs) used in the agro-food field were studied: xylitol:choline chloride:water (1:2:3 M ratio), XoCH, and citric acid:choline chloride:water (1:1:6 M ratio), CiCH. The H-bond network between the components of each DES was evaluated and the diffusion coefficients at 298.15 K were calculated using NMR spectroscopy. In addition, seven thermophysical properties were determined from 278.15 to 338.15 K. Also, the solubility of quercetin in water and in the two eutectic mixtures was measured and the interactions between components were studied. NMR experiments revealed the presence of water within the supramolecular structure of XoCH, but CiCH is a "DES-in-water" solution. Based on the results, XoCH is the most compact mixture. Finally, quercetin was remarkably more soluble in the studied DESs than in pure water.

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.

Activity modelling of the solid–liquid equilibrium of deep eutectic solvents

Compared to conventional solvents used in the chemical industry, deep eutectic solvents (DESs) are considered as promising potentially sustainable solvents. DESs are binary mixtures and the resulting liquid mixture is characterized by a large melting point depression with respect to the melting temperatures of its constituents. The relative melting point depression becomes larger as the two components have stronger attractive interactions, resulting in non-ideal behavior. The compositional range over which such binary mixtures are liquids is set by the location of the solid–liquid phase boundary. Here we present experimental phase diagrams of various recent and new DESs that vary in the degree of non-ideality. We investigate whether thermodynamic models are able to describe the solid–liquid equilibria and focus on relating the parameters of these models to the non-ideal behavior, including asymmetric behavior of the activity coefficients. It is shown that the orthogonal Redlich–Kister-like polynomial (OP) expansion, including an additional first order term, provides an accurate description. This theory can be considered as an extension of regular solution theory and enables physical interpretation of the fit parameters.

A methodology to parameterize SAFT-type equations of state for solid precursors of deep eutectic solvents: the example of cholinium chloride

Novel methodology for the development of coarse-grained models applicable to DES – a more realistic association scheme and model parameters regression from experimental data.