Preface

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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

Hydrodynamics of cholinium chloride-based aqueous biphasic systems (ABS): a key study for their industrial implementation

Aqueous biphasic systems (ABS) have been widely studied for extraction and purification processes. Although they are considered biocompatible, amenable, and sustainable separation platforms, their application on an industrial scale remains impractical without proper scaling and integration into existing processes. To better understand the hydrodynamics of ABS formation, three cholinium chloride ([Ch]Cl)-based ABS composed of polypropylene glycol with a molecular weight of 400 g.mol−1 (PPG-400), tripotassium phosphate (K3PO4), and dipotassium hydrogen-phosphate (K2HPO4) were studied. The hydrodynamics of phase separation of ABS composed of PPG-400/[Ch]Cl, [Ch]Cl/K3PO4 and [Ch]Cl/K2HPO4 was studied by analysing the relationship between the mixing time (Tm) and the phase settling time (Ts), at 25 °C and 50 °C. The results showed that Ts is independent of Tm, which is very long for the polymer/salt systems (Ts > 6 h), while for salt/salt ABS, a very fast phase settling was observed (Ts < 150 s). The hydrodynamics of each salt/salt system was then correlated with the physicochemical properties of the coexisting phases and the nature of the phase-forming compounds. The increase in the salting-out effect of the inorganic salts, and the consequent larger differences between the compositions of the coexisting phases, improved the hydrodynamics of the [Ch]Cl-based ABS. With the increase of the tie-line lengths, the composition of the phases stabilized, resulting also in more stable physicochemical properties in each phase and constant Ts. The correlations obtained in this work allow the definition of the best operating region within the biphasic (liquid-liquid) region of [Ch]Cl/salt-based ABS as being the largest TLLs within the LLE region. The definition of these criteria and region of operation is crucial for the design and industrial implementation of these types of LLE processes using conventional mixer-settler units.publishe

Aqueous two-phase systems: Towards novel and more disruptive applications

Aqueous two-phase systems (ATPS) have been mainly proposed as powerful platforms for the separation and purification of high-value biomolecules. However, after more than seven decades of research, ATPS are still a major academic curiosity, without their wide acceptance and implementation by industry, leading to the question whether ATPS should be mainly considered in downstream bioprocessing. Recently, due to their versatility and expansion of the Biotechnology and Material Science fields, these systems have been investigated in novel applications, such as in cellular micropatterning and bioprinting, microencapsulation, to mimic cells conditions, among others. This perspective aims to be a reflection on the current status of ATPS as separation platforms, while overviewing their applications, strengths and limitations. Novel applications, advantages and bottlenecks of ATPS are further discussed, indicating some directions on their use to create innovative industrial processes and commercial products.publishe

Eutectic mixtures based on polyalcohols as sustainable solvents: screening and characterization

Despite some promising potential applications of eutectic systems containing choline chloride ([Ch]Cl) and a polyalcohol, a detailed investigation of the thermodynamic behavior of these systems is still missing. In this work, the solid-liquid equilibria phase diagrams of binary systems containing [Ch]Cl and one from six different polyalcohols (ethylene glycol (EG), 1,3-propanediol, glycerol (Gly), meso-erythritol, xylitol, and sorbitol) were measured in the full composition. Except for the mixtures with Gly or EG, a quasi-ideal behavior in the [Ch]Cl solubility curve is observed. In the polyalcohol solubility curve, the mixtures present small negative deviations from ideality, except for [Ch]Cl + EG, which is slightly positive. The solid-liquid phase diagrams show a large liquid composition window, and not a fixed stoichiometry for the eutectic points, where the mixtures can be used as solvents close to, or below, room temperature. Aiming at their application, viscosities and densities were measured at the eutectic point in the temperature range from 278.15 to 373.15 K. All systems present densities and viscosities higher than water, which is directly related to the strong interactions between the components. Solvatochromic parameters were measured to characterize the solvents, and they show that solvent tunability can be achieved by varying the polyalcohol mole fraction.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/MCTES. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project Nº 022161 (cofinanced by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). L.P.S. acknowledges FCT for her PhD grant (SFRH/BD/135976/2018).info:eu-repo/semantics/publishedVersio

Ion speciation: A key for the understanding of the solution properties of ionic liquid mixtures

Recently, combinations of two (or more) ionic liquids, known as ionic liquid mixtures, have become popular and have a broad range of applications. However, the fundamental knowledge on the molecular interactions that exist in ionic liquid mixtures is far from being understood. In this work, the experimental measurement of the water activity coefficient and computational modelling using Conductor-like Screening Model for Real Solvent (COSMO-RS) were carried out to get an insight into the molecular interactions that are present in ionic liquid mixtures in aqueous solution. The results show that the combination of two ionic liquids of different basicity in aqueous solution allows fine tuning of the water activities, covering a wide range of values that could replace several pure fluids. This is an important feature resulting from the unexpected ion speciation of the ionic liquid mixtures in aqueous solution.We thank the CICECO – Aveiro Institute of Materials, POCI-01- 0145-FEDER-007679 (Ref. FCT UID/CTM/50011/2019) and Associate Laboratory LSRE-LCM, POCI-01-0145-FEDER-006984 (Ref. FCT UID/ EQU/50020/2019), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDDER under the PT2020 Partnership Agreement.info: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

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

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