Hydrophobic Eutectic Solvents Research Articles

Chemical Characterization of Terpene-Based Hydrophobic Eutectic Solvents and Their Application for Pb(II) Complexation during Solvent Extraction Procedure.

Solvents prepared from natural terpenes (menthol and thymol), as H-bond acceptors, and a series of organic acids (chain lengths of 8, 10, and 14 C atoms), as H-bond donors, were characterized and tested as reaction media for liquid-liquid extraction purposes. Due to their high hydrophobicity, they seem to be promising alternatives to conventional (nonpolar and toxic) solvents, since they possess relatively less toxic, less volatile, and consequently, more environmentally friendly characteristics. Assuming that the equilibrium is established between solvent and analyte during a ligandless procedure, it can be concluded that those nonpolar solvents can efficiently extract nonpolar analytes from the aqueous environment. Previous investigations showed a wide range of applications, including their use as solvents in extractions of metal cations, small molecules, and bioactive compounds for food and pharmaceutical applications. In this work, hydrophobic solvents based on natural terpenes, which showed chemical stability and desirable physicochemical and thermal properties, were chosen as potential reaction media in the liquid-liquid extraction (LLE) procedure for Pb(II) removal from aqueous solutions. Low viscosities and high hydrophobicities of prepared solvents were confirmed as desirable properties for their application. Extraction parameters were optimized, and chosen solvents were applied. The results showed satisfactory extraction efficiencies in simple and fast procedures, followed by low solvent consumption. The best results (98%) were obtained by the thymol-based solvent, thymol-decanoic acid (Thy-DecA) 1:1, followed by L-menthol-based solvents: menthol-octanoic acid (Men-OctA) 1:1 with 97% and menthol-decanoic acid (Men-DecA) 1:1 with 94.3% efficiency.

Extraction of ofloxacin from water using hydrophobic eutectic solvents

Wastewater treatment plants do not cope well with emerging micropollutants, such as pharmaceuticals, because they are not effective to remove them. Therefore, this work aims to develop a process for the extraction of ofloxacin (OFX) based on a liquid-liquid extraction using hydrophobic eutectic solvents (HES), coupled with the Conductor-like Screening Model for Real Solvent (COSMO-RS) for solvent screening. First, COSMO-RS was used to predict the partition coefficient of OFX for four HES families: L-menthol: fatty acid, L-menthol: fatty alcohol, fatty acid: fatty acid and fatty acid: fatty alcohols, at three different molar ratios (2:1, 1:1, and 1:2). These results showed that HES formed by fatty acids, especially those with a longer alkyl chain and a higher fatty acid molar ratio, were the most suitable solvents for OFX extraction. Then, the most promising HES to extract OFX was evaluated experimentally. The HES decanoic acid: dodecanoic acid (C10 acid: C12 acid) in a molar ratio of 2:1 being the system with best extraction results of OFX was selected to optimize the extraction conditions of OFX, namely pH, HES: water ratio (v/v), and OFX concentration, using a response surface methodology (RSM). The results obtained showed that at a pH of 5.2, HES-water ratio (v/v) of 1.3, and 2.5mg/ml of OFX that an extraction efficiency of OFX of (98.8 ± 0.9)% could be achieved. The results show that one-step OFX extraction is possible, but is significantly influenced by both the extraction parameters and the hydrophobic properties of the HES.

Tuning the hydrophobicity-hydrophilicity: D,L-Menthol:Dodecanoic acid (2:1) in glycols – n-Hexane biphasic systems

Hydrophobic molecular eutectic solvents have been largely explored in the last decade especially due to the high tunability of their properties, according to the nature of their components and their proportions. Their use together with conventional solvents, such as water to develop hydrophobic-hydrophilic biphasic system is growing at a fast pace. This work explores the use of glycols to separate binary mixtures composed of two hydrophobic miscible components: a conventional solvent and neutral hydrophobic eutectic solvent (ES). For that purpose, D,L-menthol:dodecanoic acid (2:1), and n-hexane were used, adding low-molecular weight glycols, ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and 200, 400 g/mol polyethylene glycol (PEG 200 and PEG 400 respectively) to promote phase splitting. As glycol molecular weight grows from EG to PEG 400, its hydrophobicity increases, and thus dodecanoic acid (C12) greatly changes its partition preferences, from n-hexane rich phase to an evener distribution between both phases, while D,L-menthol has the highest affinity for the hexane-rich phase. Finally, these two phases hydrophobic systems were used, as a proof of concept, in the separation of carotenoids from chlorophyll, where the easy tunability of the phases just by altering the ratio between the ES components is demonstrated, directing the components from one phase to the other.

Selective extraction of high-added value carboxylic acids from aqueous fermentative effluents with new hydrophobic eutectic solvents (HES)

SummaryRecovering these carboxylic acids from the fermentative streams in a sustainable, green, and economical way is a significant challenge. This work assessed hydrophobic eutectic solvents (HES) – water-immiscible – for the selective recovery of carboxylic acids via liquid–liquid extraction. Different trioctylphosphine oxide (TOPO) mixtures with menthol and thymol were studied and deeply characterized by 1H and 31P NMR to yield stable eutectic solvents, including a novel experiment of 31P NMR at variable temperatures for the first time. Those stable eutectic solvents were tested in the liquid extraction of complex aqueous mixtures containing C2–C6 carboxylic acids and simple sugars (glucose and xylose). The back-extraction of the carboxylic acids for the recovery of the HES was optimized, being necessary in three stages for the complete cleaning of the eutectic solvent using NaOH 0.1 M. The eutectic mixture of TOPO and thymol in a molar ratio of 1:2 exhibited an overall recovery of C5 and C6 carboxylic acids over 70 %, allowing its selective extraction from the rest of the compounds in the complex mixture. Likewise, this HES (after back extraction) was successfully reused in a second extraction cycle, keeping the performance of the fresh one. Therefore, this study demonstrated that HES can have a high extraction selectivity for carboxylic acids of ≥C5. Moreover, these solvents were stable and allowed reusability, reducing the environmental impact and process costs.

Eutectic solvents for the valorisation of the aqueous phase from hydrothermally liquefied black liquor

The potential valorisation of the aqueous phase obtained after the hydrothermal liquefaction of Kraft black liquor by means of liquid-liquid extraction with new generation solvents was analysed for the first time ever. For this purpose, hydrophobic eutectic solvents (ES), based on combinations of menthol or thymol with octanoic, decanoic or dodecanoic acid, were tested to recover phenolic compounds from this wastewater. All of them showed high affinity for phenolic compounds and ethanol, but low affinity for the rest of the compounds, leaving a more biodegradable raffinate. Regarding phenolic compounds, the average extraction yields ranged from 66 % to 91 % with menthol-based ES and from 34 % to 98 % with thymol-based ES. The best solvent in terms of recovery and selectivity for phenolic compounds was 1:1 Menthol:Octanoic acid, with separation factors of 104.2 and 29.2 for phenolic compounds to volatile fatty acids and alcohols, respectively. In this regard, the results obtained open the simultaneous valorisation of the extract as a source of phenolic compounds, regenerating the ES, and the raffinate as a sustainable feedstock for further fermentation or catalytic processes.

A comprehensive study of the water stability of eutectic solvents using COSMO-SAC

The water stability of deep eutectic solvents (DESs) is an important factor because the preferential interaction with water of one or both DESs precursors may change the nature of the solvent and impact its performance. In this work, Conductor-like Screening Model-Segment Activity Coefficient (COSMO-SAC) was applied to investigate the stability of both hydrophilic and hydrophobic eutectic solvents (ESs) in the presence of water. With this objective, an initial computational evaluation of all possible combinations of ES composed by twenty-two different hydrogen bond acceptors (HBA) and forty-four hydrogen bond donors (HBD) at three different molar ratios of HBA: HBD (2:1, 1:1, and 1:2) was carried. The results revealed that the hydrophobic ESs based on tetraalkyl -ammonium salts, terpenes and fatty acids as HBA and fatty alcohols and fatty acids as HBD are the DESs with higher water stability. Therefore, four different groups of eutectic solvents, namely menthol: fatty alcohol, menthol: fatty acid, fatty acid: fatty acid and fatty acid: fatty alcohols were selected to evaluate the COSMO-SAC predictions using water activity measurements. It is shown that a good agreement was obtained between the calculated and the experimental results and that the hydrophobicity of the eutectic solvents controls its water stability. A thermophysical characterization of the eutectic solvents studied is also reported.

Hydrophobic eutectic solvents for the selective solvent extraction of molybdenum (VI) and rhenium (VII) from a synthetic pregnant leach solution

In this work, a study on the selective solvent extraction (SX) of molybdenum (Mo) and rhenium (Re) from a synthetic pregnant leach solution (PLS) has been carried out using nine different hydrophobic eutectic solvents (HES). Experiments were carried out using pure HES to evaluate the selective extraction and the subsequent stripping of these metal ions; then, the best HES was dissolved in kerosene to study the SX stoichiometry. Results indicated that only TOPO-based and N8881-Cl-based HES did not form emulsions, third phases or precipitates. These HES achieved a selective extraction because these obtained almost a 100% extraction towards Mo and Re from the PLS and very low extractions for Copper (Cu) and Iron (Fe) in one equilibrium stage. The stripping experiments showed that 95% of Mo and 22% of Re were stripped out from the loaded TOPO-based HDES, respectively, opening the possibility for the selective stripping of the metal species for further purification. Finally, the extraction stoichiometry was proposed based on an experimental slope analysis and the measurement of cations and anions transferred into the aqueous phase in which HES acts as a neutral extractant in the complexation of the Mo and Re salts. These promising results suggest that HES could be attractive for more sustainable mining industry.

PVT behaviour of hydrophilic and hydrophobic eutectic solvents

Among the basic principles of green chemistry is the search for less harmful alternative solvents than conventional solvents. Knowing the thermophysical properties of fluids under different pressure and temperature conditions is essential to propose them. Herein, we present data on the densities at several pressures (from 0.1 to 65 MPa) and temperatures (from 283.15 to 338.15 K) of two deep eutectic solvents with hydrophilic characteristics (choline chloride + ethylene glycol or glycerol) and two eutectic solvents with hydrophobic characteristics (camphor + thymol or menthol). We used the Tait equation of state to correlate and calculate derived properties. Moreover, we modelled the mixtures with the PC-SAFT equation of state. The results showed that the hydrophilic solvents were more compact than the hydrophobic ones. The former exhibited an abnormal thermal behaviour of the isobaric thermal expansibility. The deviations in the correlation of densities with the thermodynamic model were between 0.5 and 3%. They were lower for the mixtures with weaker interactions.

Solvent extraction in non-ideal eutectic solvents – Application towards lanthanide separation

Hydrophobic eutectic solvents (HES) emerged as promising substitutes to the conventional organic phase used in the solvent extraction (SX) of metal ions due to the avoidance of diluents and the suppression of third-phase manifestation. However, HES present a highly structured liquid phase defined by hydrogen-bonded interactions such that intercomponent interactions are expected to play a greater role in SX using HES compared to a diluted extractant solution. In this work, the extraction of lanthanides from nitrate media by the non-ideal solvent composed of decanoic acid (C10OOH) and trioctylphosphine oxide (TOPO) was determined as a function of the lanthanide cation selection, HES molar fraction, HNO3 concentration, and temperature. Through the systematic variation of these parameters, the delicate balance between complexation and solvent reorganisation and its influence on the extraction and metal selectivity is determined by metal partition experiments, time resolved fluorescence and Raman spectroscopy, and small-angle X-ray scattering (SAXS). A compromise between maximum Ln3+ loading and selectivity was observed, driven by the variation in the quantity of weakly H-bonded TOPO and the change in the configurational entropy of the mixture. All results point towards the need to explicitly consider the HES phase structuration and its compositional changes with solute co-extraction on the Ln3+ extraction as no change in the extracted complex speciation was observed across all tested conditions. The C10OOH + TOPO was used as a model system due as lanthanide extraction using TOPO and carboxylic extractants is well described whilst the phase diagram and properties of the eutectic are reported. Nevertheless, the derived conclusions are translatable to other systems and can help guide the design and application of possible HES combinations for SX.

Stability Mechanism of Menthol and Fatty Acid Based Hydrophobic Eutectic Solvents: Insights from Nonbonded Interactions

Deep eutectic solvents (DESs), being ionic liquid (IL) analogues, are easy to prepare, cost-effective, recyclable, and biocompatible and have low toxicity. Hydrophobic deep eutectic solvents (HDESs) composed of natural compounds are highly effective in the extraction of micropollutants (metal ions, pharmaceuticals, and so on) from an aqueous stream. In this work, the formation mechanism of dl-menthol and carboxylic acid (acetic, butanoic, hexanoic, octanoic, nonanoic, decanoic, and dodecanoic acid) based DESs in a molar ratio of 1:1 has been presented along with their structural composition, charge transfer analysis, chemical stability, and aqueous phase solvation analysis using density functional theory (DFT) calculations. The intermolecular hydrogen bonding and dispersion interactions between menthol (−OH group) and carboxylic acids (−COOH) at a short to medium range are responsible for the stable DES formation. CHELPG and natural bonding orbital (NBO) analyses suggest that the acetic acid based DES is more closely structured and has lesser resistance to charge transfer as compared to higher chain length acid (C8 to C12) based DESs. Frontier molecular orbital (FMO) analysis confirms higher chemical stability and lesser reactivity of long-chain fatty acid based DESs as compared to their short-chain counterparts. The atom-in-molecules (AIM) and noncovalent interaction (NCI) analyses suggest that a strong network of hydrogen bonding and dispersion interactions are solely responsible for the formation and strength of the DESs. The DFT-based solvation study supported by spatial distribution function (SDF) from molecular dynamics (MD) indicates that the lower fatty acid (C1 to C6) based DESs are disrupted by water penetration to a higher extent as compared to the higher fatty acid (C8 to C12) based DESs. According to the solvation-based NCI analysis, van der Waals and dispersion connections are the interactions that are largely responsible for the chemical and physical stability of the DESs in the aqueous phase. The overall findings from the computational study have a remarkable degree of coherence with the findings of the experiments that were previously reported.

Structural evolution of the butylated hydroxytoluene/menthol hydrophobic eutectic solvent upon methanol and ethanol cosolvent addition

The changes upon methanol (MeOH) and ethanol (EtOH) addition in the structural arrangement of the hydrophobic eutectic solvent formed by butylated hydroxytoluene (BHT) and L-menthol (MEN) in 1:3 M ratio have been studied using molecular dynamics (MD) simulations integrated by small- and wide-angle X-ray scattering (SWAXS) measurements. Different mixtures containing the eutectic solvent and a variable amount of MeOH and EtOH have been investigated and the cosolvent introduction at any concentration has been found to break the hydrogen-bonds (H-bonds) among the MEN molecules, that are the most relevant interactions in the pristine eutectic, while the BHT component remains substantially non-interacting through all the explored composition range. This H-bond network is replaced by interactions between the MEN component and the cosolvent molecules, where the MEN species acts preferentially as H-bond donor towards the MeOH and EtOH molecules that behave mostly as H-bond acceptors. An increasing interplay between the excess cosolvent molecules is also observed upon increasing their concentration. SWAXS data show a contraction of the electron-dense regions corresponding to the hydroxyl groups upon cosolvent addition. This evidence is compatible with the intercalation of the MeOH and EtOH molecules in the MEN-MEN H-bond network to promote MEN-cosolvent and cosolvent-cosolvent interactions, in agreement with the MD results.

Sustainable recovery of phenolic antioxidants from real olive vegetation water with natural hydrophobic eutectic solvents and terpenoids

Olive oil production leads to the generation of olive mill wastewater (OMWW). Due to the presence of phenolic compounds, they are difficult to process, but they represent a source of high-added value chemicals since they have antioxidant and therapeutic properties. This work has studied the extraction of phenolic compounds from a type of OMWW, olive vegetation water, which presents these compounds in a more diluted dosage than in other studied to date, to revalue this waste stream. A real olive vegetation water from a Spanish olive oil producer was used, and liquid-liquid extraction was applied. Terpenoids and terpene-based hydrophobic eutectic solvents were systematically used to extract phenolic compounds following the concentrations of tyrosol, catechol, caffeic acid, and total phenolic content. By molecular simulation with the COSMO-RS method, 4 terpenoids, and 2 eutectic solvents were selected and compared with 2 conventional solvents. The Solvent/Feed ratio in the extraction of phenolic compounds was studied, showing that the solvents with the highest extraction results were geraniol, eucalyptol, and eutectic solvent menthol + camphor, which outperformed conventional solvents methyl isobutyl ketone and diisopropyl ether. Menthol + camphor gave total phenol extraction yields of 88.73% at a Solvent/Feed ratio in volume of 0.50, surpassing all solvents tested. A solvent reuse and regeneration process was applied by back-extraction of the 4 solvents: FTIR results showed the stability of the solvents while maintaining yields in the solvent reuse process. The phenolic compounds could be concentrated in the alkaline phase to factors up to 49.3 to the initial concentration in olive vegetation water. The alkaline phases were neutralized to obtain a precipitate with a caffeic acid content of up to 26 % wt%, and a tyrosol-rich supernatant with a concentration of up to 6.54 g/L. This work proposes a process using natural solvents to extract phenolic compounds from olive vegetation water.

A feasibility study on the use of hydrophobic eutectic solvents as pseudo-stationary phases in capillary electrophoresis for chiral separations

A critical challenge in using deep eutectic solvents (DESs) in capillary electrophoresis (CE) is to develop separation systems in which a DES can really work as a single entity. To achieve this, the authors recently demonstrated a novel strategy that takes advantage of the aqueous dispersibility of hydrophobic DESs (or more accurately hydrophobic eutectic solvents (HESs)). However, the previous work was limited only to the separation of achiral analytes, e.g., analogues, homologues, and isomers. The present study was designed as a follow-up study in order to explore the feasibility of employing HES-type pseudo-stationary phases (PSPs) in CE for chiral separations. By using carboxymethyl-β-cyclodextrin (CM-β-CD) as a model chiral selector, we provide the first evidence that there is a potential synergistic effect between HESs and traditional chiral selectors. Specifically, the combined use of HES (−)-menthol:octanoic acid and CM-β-CD allowed excellent enantioseparations of several basic drugs which were not able to be resolved in the single CM-β-CD system. The enantioresolutions were significantly improved while the migration times of the enantiomers were also shortened due to the hydrophobic mechanism of the HES-type PSP. Critical factors influencing the novel chiral CE system were systematically investigated. Since HESs are considered as “designer” solvents with highly tunable properties, this study demonstrates the potential of employing HESs (or HDES)-type PSPs in CE for chiral separations.

Eutectic solvents formed by (+)-DTTA and L-menthol as novel chiral recognition and separation media for enantioselective extraction of valsartan enantiomers

The enantioseparation of valsartan enantiomers using chiral eutectic solvents (ESs) as extractants was initially studied. The hydrophobic ESs (LM-DTTA) formed by L-menthol and (+)-Di-p-toluoyl-D-tartaric acid is a suitable chiral medium for the enantioselective extraction of R- and S-valsartan. Four principal parameters affecting chiral extraction, such as phase volume ratio, initial rac-valsartan concentration, extraction temperature, and system pH, were evaluated. The results showed that LM-DTTA has better and higher enantioselectivity levels, and one-step chiral extraction achieved an ee value of S-valsartan up to 91% under the optimized conditions. In addition, this chiral medium can be recycled through simple back-extraction with sodium bicarbonate solution and reused at least five times, giving similar results. This work provides a clear example of enantioseparation of the rac-valsartan via liquid–liquid enantioselective extraction using the LM-DTTA as a novel chiral recognition and separation medium. As a result, the established enantioselective extraction method can efficiently separate the R- and S-enantiomer and promote sustainable development for the production of S-valsartan.

New family of Type V eutectic solvents based on 1,10-phenanthroline and their application in metal extraction

Although a theoretically extensive number of hydrophobic eutectic mixtures suitable for solvent extraction are obtainable, to date reported mixtures rely on a limited number of chelating groups, namely phosphine oxide, carboxylic acid, and ketones. Herein, a new family of Type V Deep Eutectic Solvents (DES) based on the polypyridyl ligand 1,10-phenanthroline (phen) is presented with the aim of facilitating the extrapolation of the disclosed findings to other potential systems based on phen or bipyridine derivatives. The hydrophobic eutectic solvent composed of the natural phenolic extract thymol with phen exhibits a solid-liquid phase diagram with a useable liquidus compositional range on the thymol-rich side. The physical properties of the mixture are dependent on the presence of solutes liable to change the nature of the existing hydrogen-bonded network, most notably through the protonation of phen. The metal chelating ability of phen are retained when included as a DES component. Quantitative extraction of various transition metals cations (Cd(II), Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)) as well as the platinum group metal Pd(II) was obtained across a range of solvent compositions. The extraction of Li(I), Mn(II), Co(II) and Ni(II) was further studied as a function of the aqueous phase pH, with the extent of metal partition decreasing with the protonation of phen at pH values below 1.0. Building upon the luminescent properties of phen, the eutectic solvent was further applied as a simultaneous pre-concentration and quantification medium for Fe(II/III), allowing for the facile UV–vis detection of Fe up to 12 μg L−1.

Hydrophobic Natural Eutectic Solvents for the Gas Chromatographic Determination of Suspected Allergens in Fragrances by Dispersive Liquid-Liquid Microextraction

The fragrance industry plays a key role in the global economy, producing a wide range of personal care and household products. However, some fragrance ingredients have been linked to allergic reactions in sensitive people, and their concentrations are regulated at the European level. For this reason, reliable, rapid, and sustainable analytical methods are needed to rapidly detect and quantify these compounds. Recently, a new class of hydrophobic eutectic solvents (HES) has been introduced; they consist of natural terpenoids or phenolic compounds that can be used as hydrogen bond donors (HBD) and acceptors (HBA), and they are more suitable for GC applications due to their higher volatility. In this study, a dispersive liquid–liquid microextraction (DLLME) approach is proposed for the analysis and quantification of a range of allergens in hydroalcoholic perfumes. The optimized method requires only 50 µL of a natural HES (thymol–eugenol), which is readily dispersed by vortexing in 2 mL of sample. After centrifugation, the HES rich phase is diluted in 400 µL EtOH and directly injected into the GC-FID system. The proposed method has been successfully applied in the analysis and quantification of commercial fragrances, demonstrating good enrichment of target allergens and suitability for aqueous matrices analysis.

Hydrophobic eutectic solvents for endocrine disruptors purification from water: Natural and synthetic estrogens study

Effective elimination of natural and artificially synthesized estrogens from water sources and wastewater is very important because even minimal amounts of these substances, considered as endocrine disruptors, are dangerous to humans and wildlife. For the extraction of five estrogens namely estrone, 17α-estradiol, 17β-estradiol, 17α-ethynylestradiol and estriol, hydrophobic eutectic solvents (ES) are newly tested as a green alternative to conventional organic solvents. For this purpose, a series of ES based on a combination of menthol with natural organic acids (octanoic, decanoic and dodecanoic) was prepared in the laboratory. The extraction of estrogens was studied in terms of ES composition, molar ratio of hydrogen bond acceptor and donor, extraction time, extraction rate and reuse. Estrogens were eliminated from the waters at low levels of 10 and 20 μg/mL and their concentrations were monitored using a sensitive HPLC-MS/MS technique. The ES of menthol and octanoic acid in a 1:1 ratio showed the highest extraction efficiency for all estrogens studied, reaching values up to 99.9 % for artificially prepared samples and 97.9 % for real natural and waste waters. Comparable results can be achieved with conventional solvents such as tert-butyl methyl ether, but not with hexane and chloroform. Nevertheless, ES solvents are environmentally sustainable, non-toxic and biodegradable and therefore suitable for industrial wastewater treatment processes.

Separation of Short‐Chain Diols via Formation of Hydrophobic Eutectic Solvents

Separation of Short‐Chain Diols via Formation of Hydrophobic Eutectic Solvents

A pathway to improve detoxification processes by selective extraction of phenols and sugars from aqueous media using sustainable solvents

Integrated biorefineries are the current trend to produce energy, biochemicals and biofuels. A variety of sugar-rich biorefinery streams can be further valorized by fermentation but the presence of inhibitory compounds hinders the fermentation yields. This paper proposes a detoxification method by selective liquid extraction of phenolic compounds using sustainable solvents to improve the fermentation of sugar-rich aqueous streams. Specifically, three hydrophobic eutectic solvents constituted by binary mixtures of octanoic, decanoic or dodecanoic acid (octanoic: decanoic acid 1:1, octanoic: dodecanoic acid 3:1, decanoic: dodecanoic acid 2:1) and three bio-based solvents (2-methyltetrahydrofuran, cyclopentyl methyl ether and limonene) were selected to analyze their capacity to extract representative phenolic compounds (phenol, o-cresol, guaiacol, syringol and vanillin) from aqueous solutions. Extraction efficiencies of individual compounds and mixtures of them were evaluated at different concentrations and temperatures using the proposed solvents. Finally, the liquid extraction of a solution containing 500 mg/L of phenolic compounds and 50 g/L of glucose, as a representative concentration in biorefinery streams, was performed. The distribution of the target compounds in each phase after the extraction process was analyzed by HPLC-DAD in the case of phenolic compounds and HPLC-RI for glucose, and discussed in terms of selectivity. The results obtained are very promising, as the studied solvents can easily extract phenolic compounds but not glucose, which could improve the subsequent sugar fermentation process. In particular, the bio-based solvent cyclopentyl methyl ether provided extraction efficiencies of phenolic compounds above 93%, while that of glucose remained lower than 4%. Finally, all the experimental results were analyzed by means of COSMO-RS methodology, obtaining reliable trends and concluding that the ability of the solvents to form hydrogen bonds with phenolic compounds determines the extraction efficiencies.

Structural Study of a Eutectic Solvent Reveals Hydrophobic Segregation and Lack of Hydrogen Bonding between the Components

An in-depth study of the hydrophobic eutectic solvent formed by butylated hydroxytoluene (BHT) and l-menthol (MEN) in a 1:3 molar ratio has been carried out using an integrated approach that combines differential scanning calorimetry (DSC), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, small- and wide-angle X-ray scattering (SWAXS), and molecular dynamics (MD) simulations. The obtained data have been step-by-step compared to those collected on the eutectic formed by 3,5-di-tert-butyltoluene (TBT) and MEN in the same molar ratio, where TBT is analogous to the BHT without the hydroxyl group. The DSC characterization showed comparable results between the two systems, evidencing that the hydroxyl group of the BHT has little or no impact on the thermal behavior of the BHT:MEN eutectic. Both the FTIR and MD results agree in finding that no hydrogen bond (H-bond) interactions are played by the BHT because of the high steric hindrance suffered by its hydroxyl group so that the only established H-bonds are those between MEN molecules. The incompatibility between the components in terms of H-bonds formation results in hydrophobic segregation promoting the MEN–MEN interactions, which are even more intense than in the pure compound. The three-dimensional arrangement between the components showed a remarkable degree of structural order among the alkyl functional groups, suggesting that the apolar–apolar attraction might be the driving force of the eutectic formation. This picture is translated into the establishment of an intermediate-range organization in solution, as evidenced by the SWAXS data. The overall impact of this study is that of pushing a little bit further the definition of these eutectics, indicated until now as extensively H-bonded systems.