Fate of a Deep Eutectic Solvent upon Cosolvent Addition: Choline Chloride-Sesamol 1:3 Mixtures with Methanol

Deep Eutectic Solvents Meet Non-Aqueous Cosolvents: A Modeling and Simulation Perspective-A Tutorial Review

Deep eutectic solvents (DESs) have recently gained attention due to their tailorable properties and versatile applications in several fields, including green chemistry, pharmaceuticals, and energy storage. Their tunable properties can be enhanced by mixing DESs with cosolvents such as ethanol, acetonitrile, and water. DESs are structurally complex, and molecular modeling techniques, including quantum mechanical calculations and molecular dynamics simulations, play a crucial role in understanding their intricate behavior when mixed with cosolvents. While the most studied cosolvent is water, in some applications, even a small content of water is considered a contaminant, for example, when the processes of interest require dry conditions. Only quite recently have modeling studies begun to focus on DES mixed with cosolvents other than water. This tutorial provides the first comprehensive overview of these studies. It highlights how modern molecular modeling increases our understanding of their structural organization, transport properties, phase behavior, and thermodynamic properties. Additionally, case studies and recent developments in the field are discussed along with the challenges and future directions in molecular modeling of DES in cosolvent mixtures. Overall, this review offers valuable insights into the molecular-level understanding of DES-cosolvent systems and their implications for designing novel solvent mixtures with tailored properties for various applications.

Efficient phosphorus recovery from waste activated sludge: Pretreatment with natural deep eutectic solvent and recovery as vivianite

Recycling phosphorus from waste activated sludge (WAS) is an effective method to address the nonrenewable nature of phosphorus and mitigate environmental pollution. To overcome the challenge of low phosphorus recovery from WAS due to insufficient disintegration, a method using a citric acid-based natural deep eutectic solvent (CA-NADES) assisted with low-temperature pretreatment was proposed to efficiently release and recover phosphorus. The results of 31P nuclear magnetic resonance (NMR) confirmed that low-temperature pretreatment promoted the conversion of organic phosphorus (OP) to inorganic phosphorus (IP) and enhanced the effect of CA-NADES. Changes in the three-dimensional excitation-emission matrix (3D-EEM) and flow cytometry (FCM) indicated that the method of CA-NADES with low-temperature thermal simultaneously release IP and OP by disintegrating sludge flocs, dissolving extracellular polymeric substances (EPS) structure, and cracking cells. When 5 % (v/v) of CA-NADES was added and thermally treated at 60 °C for 30 min, 43 % of total phosphorus (TP) was released from the sludge. The concentrations of proteins and polysaccharides reached 826 and 331 mg/L, respectively, which were 6.30 and 14.43 times higher than those of raw sludge. The dewatering and settling of the sludge were also improved. Metals were either enriched in the solid phase or released into the liquid phase in small quantities (most efficiencies of less than 10 %) for subsequent clean recovery. The released phosphorus was successfully recovered as vivianite with a rate of 90 %. This study develops an efficient, green, and sustainable method for phosphorus recovery from sludge using NADES and provides new insights into the high-value conversion of sludge.

On the composition and isomerism effect in the thermal and structural properties of choline chloride/hydroxyphenol deep eutectic solvents

We have carried out a comparative study on three sets of eutectic mixtures based on choline chloride (ChCl) and hydroxyphenol isomers having two hydroxyl groups in the ortho-, meta-, and para-positions of the aromatic ring, namely catechol (Cate), resorcinol (Reso), and hydroquinone (Hydro), respectively. Differential scanning calorimetry highlighted a different thermal behavior of the mixtures depending on the composition and precursor isomerism. These systems behave as deep eutectic solvents (DESs) with the exception of the ChCl/Cate mixture at a 1 : 0.75 molar ratio, the ChCl/Reso mixtures at 1 : 2 and 1 : 3 molar ratios, and the ChCl/Hydro mixture at a 1 : 3 molar ratio. Infrared spectroscopy measurements and molecular dynamics simulations show that the stronger hydrogen-bonding (H-bonding) in the mixed states compared to the pure precursors is key for the formation of a DES. This interaction is mostly reliant on interconnected chloride anion coordination shells thanks to the two hydroxyl groups of the hydroxyphenol molecules, which can bridge between different anions to form an extended H-bond network. This structural arrangement maximizes the interactions and is enhanced by the 1 : 0.75 and 1 : 1 molar ratios, while increasing the hydroxyphenol concentration translates into a lowering of the total number of H-bonds formed in the mixture. This is the basis for the different thermal behavior and points to nearly equimolar compositions between the components, as ideally suited to achieve a DES from these precursors. The obtained insights are able to explain the structure-property relationships for the studied systems and are deemed useful for more conscious development of these inherently tuneable solvents.

60 Years of Betaine 30─From Solvatochromic Discovery to Future Frontiers

Betaine-30 (B30) was reported by Karl Dimroth and Christian Reichardt et al. in 1963 as a solvatochromic probe that can be easily synthesized, shows good solubility, and remains stable in various organic solvents and solutions. Its strongly negatively solvatochromic behavior arises from differential solvation between its electronic ground and excited states, making it a valuable tool for assessing solvent polarity using the ET(30) polarity scale, also devised by Dimroth and Reichardt. In addition, advancements in femtosecond laser spectroscopy in the 1990s greatly improved the understanding of B30's relaxation dynamics following photoexcitation. In solvents capable of hydrogen bonding, such as alcohols, intermolecular hydrogen-bond rearrangement contributes to the multiple relaxation components observed. Since the 1990s, the applications of B30 have expanded beyond simple organic solvents to include complex solvent mixtures, such as electrolyte solutions for battery technologies and eutectic solvent mixtures. Given the growing importance of these complex solvent mixtures, B30 is becoming an increasingly valuable tool for studying previously unexplored solvation properties.

Enhancing polyethylene terephthalate conversion through efficient microwave-assisted deep eutectic solvent-catalyzed glycolysis

Chemical recycling of plastics is a promising approach for effectively depolymerizing plastic waste into its constituent monomers, thereby contributing to the realization of a sustainable circular economy. Glycolysis, which converts polyethylene terephthalate (PET) into the monomer bis(2-hydroxyethyl) terephthalate (BHET), has emerged as a cost-effective and commercially viable chemical recycling process. However, glycolysis requires long reaction times and high energy consumption, limiting its industrialization. In this study, we develop an energy-efficient microwave-assisted deep eutectic solvent-catalyzed glycolysis method to degrade PET effectively and rapidly, resulting in a high BHET yield. This combined approach enables the quantitative degradation of PET within 9 min, achieving a high BHET yield of approximately 99% under optimal reaction conditions. Furthermore, the proposed approach has a low specific energy consumption (45 kJ/g) and minimizes waste generation. The thermal behavior of PET and its degradation mechanism are systematically investigated using scanning electron microscopy and density functional theory-based calculations. The results obtained suggest that the proposed straightforward, swift, and energy-efficient strategy has the potential to offer a sustainable solution to plastic waste management challenges and expedite the industrialization of chemical recycling.

Liquid Structure Scenario of the Archetypal Supramolecular Deep Eutectic Solvent: Heptakis(2,6-di-O-methyl)-β-cyclodextrin/levulinic Acid

The concept of supramolecular solvents has been recently introduced, and the extended liquid-state window accessible for mixtures of functionalized cyclodextrins (CDs) with hydrogen bond (HB) donor species, e.g., levulinic acid, led to the debut of supramolecular deep eutectic solvents (SUPRA-DES). These solvents retain CD's inclusion ability and complement it with enhanced solvation effectiveness due to an extended HB network. However, so far, these promising features were not rationalized in terms of a microscopic description, thus hindering a more complete capitalization. This is the first joint experimental and computational study on the archetypal SUPRA-DES: heptakis(2,6-di-O-methyl)-β-CD/levulinic acid (1:27). We used X-ray scattering to probe CD's aggregation level and molecular dynamics simulation to determine the nature of interactions between SUPRA-DES components. We discover that CDs are homogeneously distributed in bulk and that HB interactions, together with the electrostatic ones, play a major role in determining mutual interaction between components. However, dispersive forces act in synergy with HB to accomplish a fundamental task in hindering hydrophobic interactions between neighbor CDs and maintaining the system homogeneity. The mechanism of mutual solvation of CD and levulinic acid is fully described, providing fundamental indications on how to extend the spectrum of SUPRA-DES combinations. Overall, this study provides the key to interpreting structural organization and solvation tunability in SUPRA-DES to extend the range of sustainable applications for these new, unique solvents.

Solubilization and coordination of the HgCl2 molecule in water, methanol, acetone, and acetonitrile: an X-ray absorption investigation

X-ray absorption spectroscopy (XAS) has been employed to carry out structural characterization of the local environment around mercury after the dissolution of the HgCl₂ molecule. A combined EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) data analysis has been performed on the Hg L₃-edge absorption spectra recorded on 0.1 M HgCl₂ solutions in water, methanol (MeOH), acetone and acetonitrile. The Hg-Cl distance determined by EXAFS (2.29(2)-2.31(2) Å) is always comparable to that found in the HgCl₂ crystal (2.31(2) Å), demonstrating that the HgCl₂ molecule dissolves in these solvents without dissociating. A small sensitivity of EXAFS to the solvent molecules interacting with HgCl₂ has been detected and indicates a high degree of configurational disorder associated with this contribution. XANES data analysis, which is less affected by the disorder, was therefore carried out for the first time on these systems to shed light into the still elusive structural arrangement of the solvent molecules around HgCl₂. The obtained results show that, in aqueous and MeOH solutions, the XANES data are compatible with three solvent molecules arranged around the HgCl₂ unit to form a trigonal bipyramidal structure. The determination of the three-body Cl-Hg-Cl distribution shows a certain degree of uncertainty around the average 180° bond angle value, suggesting that the HgCl₂ molecule probably vibrates in the solution around a linear configuration.

On the Role of Water in the Formation of a Deep Eutectic Solvent Based on NiCl2·6H2O and Urea

The metal-based deep eutectic solvent (MDES) formed by NiCl₂·6H₂O and urea in 1:3.5 molar ratio has been prepared for the first time and characterized from a structural point of view. Particular accent has been put on the role of water in the MDES formation, since the eutectic could not be obtained with the anhydrous form of the metal salt. To this end, mixtures at different water/MDES molar ratios (W) have been studied with a combined approach exploiting molecular dynamics and ab initio simulations, UV–vis and near-infra-red spectroscopies, small- and wide-angle X-ray scattering, and X-ray absorption spectroscopy measurements. In the pure MDES, a close packing of Ni²⁺ ion clusters forming oligomeric agglomerates is present thanks to the mediation of bridging chloride anions and water molecules. Conversely, urea poorly coordinates the metal ion and is mostly found in the interstitial regions among the Ni²⁺ ion oligomers. This nanostructure is disrupted upon the introduction of additional water, which enlarges the Ni–Ni distances and dilutes the system up to an aqueous solution of the MDES constituents. In the NiCl₂·6H₂O 1:3.5 MDES, the Ni²⁺ ion is coordinated on average by one chloride anion and five water molecules, while water easily saturates the metal solvation sphere to provide a hexa-aquo coordination for increasing W values. This multidisciplinary study allowed us to reconstruct the structural arrangement of the MDES and its aqueous mixtures on both short- and intermediate-scale levels, clarifying the fundamental role of water in the eutectic formation and challenging the definition at the base of these complex systems.

The metal-based deep eutectic solvent formed by NiCl₂·6H₂O and urea in 1:3.5 a molar ratio was prepared for the first time, and its aqueous mixtures were characterized from a structural point of view, highlighting the fundamental role of water in the eutectic formation.