Insights into the nature of eutectic and deep eutectic mixtures

A stricter definition of a deep eutectic solvent (DES) is urgent, so that it may become a sound basis for further developments in this field. This communication aims at contributing to deepening the understanding of eutectic and deep eutectic mixtures concerning their definition, thermodynamic nature and modelling. The glut of literature on DES applications should be followed by a similar effort to address the fundamental questions on their nature. This hopefully would contribute to correct some widespread misconceptions, and help to establish a stringent definition of what a DES is. DES are eutectic mixtures for which the eutectic point temperature should be lower to that of an ideal liquid mixture. To identify and characterize them, their phase diagrams should be known, in order to compare the real temperature depression to that predicted if ideality is assumed, and to define composition ranges for which they are in the liquid state at operating temperatures. It is also shown that hydrogen bonding between the DES components should not be used to define or characterize a DES, since this would describe many ideal mixtures. The future of deep eutectic solvents is quite promising, and we expect that this work will contribute to the efficient design and selection of the best DES for a given application, and to model properties and phase equilibria without which the process design is impractical.This work was developed in the scope of the Project CICECO – Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (Ref. FCT UID/CTM/50011/2013) and Associate Laboratory LSRE-LCM, POCI-01-0145-FEDER-006984 (Ref. FCT UID/EQU/50020/2013), both financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. This work is also a result of Project “AIProcMat@N2020 - Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020”, with the reference NORTE-01-0145- FEDER-000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). FCT is also acknowledged for funding the project DeepBinfo:eu-repo/semantics/publishedVersio

Solid-liquid equilibria for hexafluorophosphate-based ionic liquid quaternary mixtures and their corresponding subsystems

The present work describes an experimental study and the thermodynamic modeling for the solid−liquid phase diagram of an ionic liquid quaternary system constituted by hexafluorophosphate ([PF6]−) as the common anion and by 1-methyl-3-propylimidazolium ([C3mim]+), 1-methyl-1-propylpyrrolidinium ([C3mpyrr]+), 1-methyl-3-propylpyridinium ([C3mpy]+), or 1-methyl- 1-propylpiperidinium ([C3mpip]+) as the cations. The Modified Quasichemical Model was used to model the liquid solution, and the Compound Energy Formalism was used for the relevant solid solutions. The liquidus projections of the four ternary subsystems (1) [C3mim][PF6]−[C3mpip][PF6]−[C3mpyrr]- [PF6], (2) [C3mpy][PF6]−[C3mpip][PF6]−[C3mpyrr][PF6], (3) [C3mpip]- [PF6]−[C3mpy][PF6]−[C3mim][PF6], and (4) [C3mpyrr][PF6]−[C3mpy]- [PF6]−[C3mim][PF6] were predicted using a standard symmetric (for systems 3 and 4) or asymmetric (for systems 1 and 2) interpolation method. In order to test the accuracy of the thermodynamic model, two isoplethal sections were experimentally measured in each of the four ternary systems using differential scanning calorimetry. Overall, agreement was very satisfactory, not requiring fitting of any ternary interaction parameters for the liquid solution model. In each of the four calculated ternary liquidus projections, the region of composition corresponding to room temperature ionic liquid mixtures was determined. The global minimum of the liquidus temperature in the complete composition space was calculated to be about −16 °C, with a mole percentage composition of (33.8% [C3mpyrr][PF6] + 33.9% [C3mpy][PF6] + 32.3% [C3mim][PF6]).The modeling part of this project was supported by the Natural Sciences and Engineering Research Council of Canada (Discovery Grant RGPIN 435893-2013). The new DSC measurements in the four ternary subsystems were supported by the laboratories CICECOAveiro Institute of Materials, project POCI-01-0145-FEDER-007679 (ref FCT UID/CTM/ 50011/2019), and Associate Laboratory LSRE-LCM (ref FCT UID/EQU/50020/2019), both financed by national funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement, and the project “AIProcMat@N2020Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020” (ref NORTE-01-0145-FEDER-000006) supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). Constructive discussions with Dr. Aım̈ en Gheribi, Prof. Youn- Bae Kang, and Prof. Nick Virgilio were much appreciated.info:eu-repo/semantics/publishedVersio

Sustainable hydrophobic terpene-based eutectic solvents for the extraction and separation of metals

Sustainable hydrophobic eutectic solvents, composed of low-priced and biodegradable terpenes and fatty acids, were used for the extraction and separation of Cu(ii) from other transition metals in mildly acidic solutions. Multiple parameters were evaluated for metal extraction and the hydrophobic eutectic solvent was successfully recovered and reused.This work was part of BATRE-ARES project (ERA-MIN/0001/2015) funded by ADEME and FCT and partly developed in the scope of the project CICECO – Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013). C. M. S. S. Neves acknowledges FCT for the postdoctoral grant (SFRH/BPD/109057/2015). M. A. R. Martins acknowledges financial support from NORTE 2020 (NORTE-01-0145-FEDER- 000006) and DeepBiorefinery (PTDC/AGRTEC/1191/2014) projects.info:eu-repo/semantics/publishedVersio

Solid-liquid phase behavior of eutectic solvents containing sugar alcohols

Mixtures of carbohydrates are often reported in the literature as deep eutectic solvents yet, in most cases, their solid–liquid phase diagrams are poorly characterized and no evidence is available to validate this classification. In this work, the phase diagrams of the binary systems composed of the sugar alcohols mannitol or maltitol and meso-erythritol, xylitol, or sorbitol, were experimentally determined. The results obtained reveal that these systems have a thermodynamic ideal behavior, questioning their classification as deep eutectic solvents and showing that intermolecular hydrogen bonding between the components of a mixture is not a sufficient condition to prepare deep eutectic solvents. The phase diagrams of the systems composed of mannitol or maltitol and cholinium chloride were also measured in this work. In sharp contrast to the mixtures composed solely by sugar alcohols, and unlike numerous other choline-based eutectic systems reported in the literature, these systems revealed significant deviations to thermodynamic ideality, leading to significant melting temperature depressions. The Cl-OH interaction between cholinium chloride and the sugar alcohols is identified as the main reason for these deviations to ideality, paving the way for the rational choice of hydrogen bond acceptors to prepare deep eutectic solvents.This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, and CIMO-Mountain Research Center, UIDB/00690/2020, both financed by national funds through the Portuguese Foundation for Science and Technology(FCT)/MCTES. L.P.S. acknowledges FCT for her PhD grant (SFRH/BD/135976/2018).info:eu-repo/semantics/publishedVersio

The impact of oligomeric anions on the speciation of protic ionic liquids

The synthesis of protic ionic liquids based in carboxylate anions, too often admitted as being straightforward, is actually challenging and must be carefully addressed. This review discusses the importance of oligomeric anions, in particular those based on carboxylates, in the behavior and nature of protic ionic liquids. There is strong evidence in the literature that the synthesis, and subsequent purification, of protic ionic liquids involving carboxylic acids, leads to structures in the liquid phase with an acid:base proportion different from the expected 1:1 stoichiometry. The formation of the oligomeric anions, mostly dependent on the proton transfer from the Brønsted acid to the Brønsted base, may lead to a higher ionicity that suggests the formation of true ionic liquids. It is here stressed the relevance of deepening the understanding of the interactions between the species and the speciation of the liquid phase, combining experimental and theoretical approaches to establish foundations for insightful advances in the area.This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020 , CIMO-Mountain Research Center, UIDB/00690/2020 , and CIQUP, University of Porto (Projects: PEstUID/QUI/00081/2013 ) financed by national funds through the Portuguese Foundation for Science and Technology (FCT) /MCTES. This work was also developed within the scope of the Indo- Portuguese Program for Cooperation in Science & Technology DST/INT/Portugal/P-01/2017 , financed by FCT and the Government of India. P.J.C. acknowledge FCT for his contract under the Investigator FCT 2015 contract number IF/00758/2015 .info:eu-repo/semantics/publishedVersio

Liquefying Compounds by Forming Deep Eutectic Solvents: A Case Study for Organic Acids and Alcohols

The criterion to distinguish a simple eutectic mixture from a deep eutectic solvent (DES) lies in the deviations to thermodynamic ideality presented by the components in the system. In this work, the current knowledge of the molecular interactions in types III and V DES is explored to liquefy a set of three fatty acids and three fatty alcohols, here used as model compounds for carboxyl and hydroxyl containing solid compounds. This work shows that thymol, a stronger than usual hydrogen bond donor, is able to form deep eutectic solvents of type V with the fatty alcohols studied. This is particularly interesting, since these DES formed are hydrophobic. Regarding type III DES, the results suggest that the prototypical DES hydrogen bond acceptor, cholinium chloride, is unable to induce negative deviations to ideality in the model molecules studied. By substituting choline with tetramethylammonium chloride, it is shown that the choline hydroxyl group is responsible for the difficulty in forming choline-based deep eutectic solvents and that its absence induces strong negative deviations to ideality in the alkylammonium side. Finally, it is demonstrated that tetrabutylammonium chloride acts as a chloride donning agent, causing significant negative deviations to ideality in both fatty acids and alcohols and leading to the formation of deep eutectic solvents of type III.This work was developed within the scope of the projects CICECO-Aveiro Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/MCTES, and CIMO-Mountain Research Center, UIDB/00690/2020, financed by national funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement. L.P.S. acknowledges FCT for her PhD grant SFRH/BD/135976/2018.info:eu-repo/semantics/publishedVersio

Terpenes solubility in water and their environmental distribution

Terpenes and terpenoids belong to the largest and most diverse class of natural products. Due to the increasing importance of their applications and the emerging perception of their impact on the environment, the available physico-chemical characterization is insufficient. In this work the water solubility of geraniol, linalool, DL-citronellol, thymol, eugenol, carvacrol and p-cymene, in the temperature range from (298.15 to 323.15) K, and at atmospheric pressure, is studied. Due to the low solubility of these compounds a novel technique was adopted for their measurements and validated using the aqueous solubility data for sparingly soluble aromatic compounds. The thermodynamic properties of solution were derived from the experimental data at infinite dilution. It is shown that the solubility of terpenes in water is an endothermic process confirming the existence of UCST phase diagrams, and only for carvacrol and eugenol is entropically driven. The experimental information is shown in a two-dimensional chemical space diagram providing indications to their probable distribution in the environment once released.Thisworkwas developed in the scope of the project CICECO - Aveiro Institute ofMaterials, POCI-01-0145-FEDER-007679, and Associate Laboratory LSRE-LCM, POCI-01-0145-FEDER-006984, both funded by FEDER under the PT2020 Partnership Agreement - Programa Operacional Competitividade e Internacionalização (POCI) - and by national funds through FCT - Fundação para a Ciência e a Tecnologia. M.A.R.M acknowledges FCT for her PhD grant (SFRH/BD/87084/2012).info:eu-repo/semantics/publishedVersio

The role of the anion in imidazolium-based ionic liquids for fuel and terpenes processing

Abstract: The potentialities of methylimidazolium-based ionic liquids (ILs) as solvents were evaluated for some relevant separation problems—terpene fractionation and fuel processing—studying selectivities, capacities, and solvent performance indices. The activity coefficients at infinite dilution of the solute (1) in the IL (3), g¥ 13, of 52 organic solutes were measured by inverse gas chromatography over a temperature range of 333.2–453.2 K. The selected ILs are 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6], and the equimolar mixture of [C4mim][PF6] and 1-butyl-3-methylimidazolium chloride, [C4mim]Cl. Generally, low polar solutes follow g¥ 1,[C4mim]Cl > g¥ 1,[C4mim][PF6]+[C4mim]Cl > g¥ 1,[C4mim][PF6] while the opposite behavior is observed for alcohols and water. For citrus essential oil deterpenation, the results suggest that cations with long alkyl chains, such as [C12mim]+, promote capacity, while selectivity depends on the solute polarity. Promising results were obtained for the separation of several model mixtures relevant to fuel industries using the equimolar mixture of [C4mim][PF6] and [C4mim]Cl. This work demonstrates the importance of tailoring the polarity of the solvents, suggesting the use of ILs with mixed anions as alternative solvents for the removal of aliphatic hydrocarbons and contaminants from fuels.This research was funded by the European Regional Development Fund (ERDF) through the Regional Operational Program North 2020, within the scope of Project GreenHealth—Digital strategies in biological assets to improve well-being and promote green health, Norte-01-0145-FEDER- 000042, to which A. Zambom is thankful for her grant. S. M. Vilas-Boas thanks FCT and the European Social Fund (ESF) for his Ph.D. grant (SFRH/BD/138149/2018 and COVID/BD/152936/2022). L.P.Silva acknowledges FCT for her Ph.D. grant (SFRH/BD/135976/2018).info:eu-repo/semantics/publishedVersio

Selection and characterization of non-ideal ionic liquids mixtures to be used in CO

Due to the costs involved, the capture of CO2 in post-combustion is not currently seen as economically viable. Aiming at changing the perception of post-combustion CO2 from a costly and non-profitable process to a valuable commodity and fostering the development of the next-generation of technologies, novel solvents and their mixtures have been investigated. In this work, mixtures of non-volatile ionic liquids were screened by COSMO-RS aiming to find mixtures with positive excess volumes that could present an increased CO2 capture by physical sorption. The most promising mixtures identified by COSMO-RS, [C4C1im][DMP] or [C4C1im][NTf2] þ carboxylate-based protic ILs were characterized through the measurement of their thermophysical properties, namely density and viscosity. Both properties were measured for pure ILs and their binary mixtures at different temperatures and compositions. The temperature dependence of density of pure ILs was described using the Gardas and Coutinho model while viscosity was accurately described using the Vogel Tammann Fulcher equation. The Redlich-Kister equation was used to predict the excess molar volumes and the non-ideality of the mixtures’ viscosity was assessed using the Grunberg and Nissan mixing law. The excess molar volumes for mixtures containing [C4C1im][DMP] show large positive values all over the range of compositions and temperatures, making them good candidates for CO2 capture. To the best of our knowledge, the excess molar volumes obtained in this work were the highest reported so far. COSMO-RS was able to correctly predict the trend of the experimental excess molar volumes for these mixtures. Regarding viscosity, mixtures of [C4C1im] [DMP] with the carboxylate-based protic ILs led to the desired viscosity decrease compared to the pure aprotic IL, and large deviations from ideality were observed. The mixing of ILs is thus an efficient way to fine-tune the properties, in this case decreasing the viscosity while increasing the sorption capacity.This work is a result of the Indo-Portuguese Program for Cooperation in Science & Technology DST/INT/Portugal/P-01/2017, financed by FCT and the Government of India. This work was also developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, and CIMOMountain Research Center, UIDB/00690/2020, both financed by national funds through the FCT, MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement. MARM acknowledges the project “AIProcMat@N2020 - Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020”, with the reference NORTE-01-0145-FEDER- 000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020). P.J.C. acknowledges FCT for his contract under the Investigator FCT 2015 contract number IF/00758/2015.info:eu-repo/semantics/publishedVersio