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New Horizons in Deep Eutectic Solvents (DESs): Synthesis, Characterization and Applications

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Molecular Liquids".

Deadline for manuscript submissions: 28 February 2026 | Viewed by 6635

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Special Issue Editors

Dr. Dorota Warmińska

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Department of Physical Chemistry, Faculty of Chemistry, Gdańsk University of Technology, 80-233 Gdansk, Poland
Interests: green chemistry; ionic liquids; deep eutectic solvents; physicochemical properties; thermodynamic investigation; molecular interactions
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Dr. Iwona Cichowska-Kopczyńska

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Department of Process Engineering and Chemical Technology, Faculty of Chemistry, Gdańsk University of Technology, 80-233 Gdansk, Poland
Interests: ionic liquids; deep eutectic solvents; mass transfer; membrane processes; physicochemical properties; gas separation; sustainable technologies
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Dr. Maciej Smiechowski

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Department of Physical Chemistry, Gdańsk University of Technology, Narutowicza 11/12, 80-233 Gdańsk, Poland
Interests: molecular interactions; density functional theory; ab initio molecular dynamics; molecular simulations; hydration; infrared spectroscopy; computational spectroscopy; deep eutectic solvents

Special Issue Information

Dear Colleagues,

Deep eutectic solvents (DESs) have gained considerable attention in recent years due to their potential as environmentally friendly alternatives to conventional volatile organic solvents. The main objective of this Special Issue is to publish original research articles and reviews that focus on the current status and future trends in the field of these solvents. The aim of this issue is to provide comprehensive coverage of both the scientific foundations and the industrial applications of DESs. We invite submissions that explore various aspects, including the design, synthesis, characterization, and properties of deep eutectic solvents, as well as their potential in various fields of industry such as pharmaceuticals, energy storage, catalysis, extraction and separation, and environmental technologies.

Dr. Dorota Warmińska
Dr. Iwona Cichowska-Kopczyńska
Dr. Maciej Smiechowski
Guest Editors

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

deep eutectic solvent
methods of synthesis
physicochemical and thermal properties
extraction and separation
catalysis
pharmaceuticals
energy storage

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Published Papers (6 papers)

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Research

15 pages, 1808 KB

Open AccessArticle

Recovery of Lithium and Cobalt from Spent Lithium-Ion Batteries Using a Deep Eutectic Solvent Based on Choline Chloride and Oxalic Acid (Oxaline)

by Jessica M. Guamán-Gualancañay, Carlos F. Aragón-Tobar, Katherine Moreno, José-Luis Palacios and Diana Endara

Molecules 2025, 30(24), 4690; https://doi.org/10.3390/molecules30244690 - 7 Dec 2025

Viewed by 234

Abstract

The growing consumption of lithium-ion batteries (LIBs) in electronic devices and electric vehicles has led to a significant increase in waste containing valuable metals such as lithium and cobalt. Recovering these metals is essential to reducing dependence on primary sources and minimizing environmental impact. In this study, the leaching of the cathode active material from discarded LIBs was evaluated using oxaline, a deep eutectic solvent (DES) composed of oxalic acid and choline chloride in a 1:1 molar ratio. The process began with the collection, discharge, washing, drying, and dismantling of the LIBs, followed by the separation of their components. Subsequently, the cathode active material was characterized, revealing a primary composition of cobalt (54.5%) and lithium (6.5%), with the presence of LiCoO₂ confirmed by XRD analysis. Leaching experiments were conducted to evaluate the effects of temperature, time, and solid percentage, demonstrating that oxaline is effective for the selective leaching of lithium and cobalt. Under optimal conditions (90 °C, 1–2 wt.% cathode active material, 400 rpm), lithium underwent complete dissolution within the first hour, while cobalt achieved complete leaching by 4 h. Both metals were recovered as oxalates and separated based on differences in solubility. Oxaline proves to be an efficient and environmentally friendly alternative for the selective recovery of lithium and cobalt from LIB waste, supporting a circular economy in the management of critical metals. Full article

(This article belongs to the Special Issue New Horizons in Deep Eutectic Solvents (DESs): Synthesis, Characterization and Applications)

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28 pages, 3184 KB

Open AccessArticle

Data-Driven Classification of Solubility Space in Deep Eutectic Solvents: Deciphering Driving Forces Using PCA and K-Means Clustering

by Piotr Cysewski, Maciej Przybyłek and Tomasz Jeliński

Molecules 2025, 30(23), 4563; https://doi.org/10.3390/molecules30234563 - 26 Nov 2025

Viewed by 305

Abstract

This study presents a robust, data-driven framework for classifying and predicting drug solubility in deep eutectic solvents (DESs), moving beyond empirical approaches to enable rational formulation design. By analyzing 2010 solubility measurements of 21 diverse pharmaceutical compounds across numerous choline chloride, betaine, and menthol-based DESs, we employed Principal Component Analysis to reduce 16 COSMO-RS-derived descriptors into four chemically interpretable dimensions explaining 86.7% of the total variance. Persistence analysis confirmed component stability, revealing two key factors: PC1 (global solvation propensity, i.e., the overall capacity of the solvent to stabilize solutes through all interaction types) and PC2 (specific interaction complementarity, i.e., the degree of matching between solute and solvent hydrogen-bonding/polarity features). K-means clustering identified four distinct solubility regimes: high-solubility DES-optimized systems (Cluster 1), reliable moderate performers (Cluster 0), intermediate candidates for optimization (Cluster 3), and fundamentally challenging combinations (Cluster 2). Comparative analysis demonstrated choline chloride’s broad utility while revealing specialized roles for menthol and betaine in specific chemical spaces. Case studies of Sulfasalazine and Caffeine illustrated how multi-cluster distributions guide formulation strategies, distinguishing precision-requiring from forgiving compounds. This taxonomy provides formulation scientists with a rational framework for DES selection, emphasizing aqueous modification, HBD and HBA diversity, and balanced solvation-interaction optimization. The integrated PCA-clustering approach transforms DES development from trial-and-error screening to targeted design, offering fundamental insights into solubility mechanisms while accelerating sustainable pharmaceutical formulation. Full article

(This article belongs to the Special Issue New Horizons in Deep Eutectic Solvents (DESs): Synthesis, Characterization and Applications)

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15 pages, 1883 KB

Open AccessArticle

Duality of Simplicity and Accuracy in QSPR: A Machine Learning Framework for Predicting Solubility of Selected Pharmaceutical Acids in Deep Eutectic Solvents

by Piotr Cysewski, Tomasz Jeliński, Julia Giniewicz, Anna Kaźmierska and Maciej Przybyłek

Molecules 2025, 30(22), 4361; https://doi.org/10.3390/molecules30224361 - 11 Nov 2025

Cited by 1 | Viewed by 1217

Abstract

We present a systematic machine learning study of the solubility of diverse pharmaceutical acids in deep eutectic solvents (DESs). Using an automated Dual-Objective Optimization with Iterative feature pruning (DOO-IT) framework, we analyze a solubility dataset compiled from the literature for ten pharmaceutically important carboxylic acids and augment it with new measurements for mefenamic and niflumic acids in choline chloride- and menthol-based DESs, yielding N = 1020 data points. The data-driven multi-criterion measure is applied for final model selection among all collected accurate and parsimonious models. This three-step procedure enables extensive exploration of the model’s hyperspace and effective selection of models fulfilling notable accuracy, simplicity, and also persistency of the descriptors selected during model development. The dual-solution landscape clarifies the trade-off between complexity and cost in QSPR for DES systems and shows that physically meaningful energetic descriptors can replace or enhance explicit COSMO-RS predictions depending on the application. Full article

(This article belongs to the Special Issue New Horizons in Deep Eutectic Solvents (DESs): Synthesis, Characterization and Applications)

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18 pages, 1356 KB

Open AccessArticle

Sustainable Recovery of Cu, Fe, Ni, and Zn from Multilayer Ceramic Capacitors Using a Ternary Deep Eutectic Solvent

by Jordy Masache-Romero, Katherine Moreno, Fernando Sánchez and Carlos F. Aragón-Tobar

Molecules 2025, 30(21), 4254; https://doi.org/10.3390/molecules30214254 - 31 Oct 2025

Viewed by 594

Abstract

The rapid growth in electronic waste (e-waste) generation highlights the urgent need for efficient and environmentally sustainable methods for metal recovery. This study focuses on the selective recovery of valuable metals from multilayer ceramic capacitors (MLCCs), commonly found in printed circuit boards (PCBs) of post-consumer electronics. MLCCs were manually recovered from dismantled computer PCBs, thermally treated, pulverized, and characterized using X-ray fluorescence and X-ray diffraction techniques. To evaluate green alternatives to traditional acid leaching, three deep eutectic solvents (DESs) based on choline chloride (ChCl) were prepared: citric acid (CA), glycerol (GLY), and a ternary (GLY-CA) mixture of both (GLY-CA). Leaching experiments were conducted over a 24 h period and analyzed using atomic absorption spectroscopy. The results showed complete recovery (100%) of copper using both CA and the GLY-CA mixture, while nickel recovery reached 100% with CA and moderate levels with GLY-CA. Zinc recovery was also high (99%) with both CA and GLY-CA. Iron showed a maximum recovery of 60%, potentially due to its occurrence in various chemical forms. The ternary DES (GLY-CA) demonstrated lower viscosity, improving handling and operational efficiency. These findings highlight the potential of citric-acid-based and ternary (GLY-CA) DESs as effective, low-toxicity leaching agents for the recovery of critical metals from MLCCs. Full article

(This article belongs to the Special Issue New Horizons in Deep Eutectic Solvents (DESs): Synthesis, Characterization and Applications)

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15 pages, 1520 KB

Open AccessArticle

Insights from Femtosecond Transient Absorption Spectroscopy into the Structure–Function Relationship of Glyceline Deep Eutectic Solvents

by Rathiesh Pandian and Clemens Burda

Molecules 2025, 30(5), 1059; https://doi.org/10.3390/molecules30051059 - 26 Feb 2025

Viewed by 1424

Abstract

This study aimed to determine the structure–function relationship (SFR) for ChCl–glycerol mixtures, a deep eutectic solvent (DES), by investigating their microscopic solvation dynamics and how it relates to their macroscopic properties across varying concentrations of ChCl. Femtosecond transient absorption (fs-TA) spectroscopy revealed two distinct solvation dynamics time constants: τ₁, governed by glycerol–glycerol interactions, and τ₂, dominated by the choline response. The τ₂ minimum at 25–30 mol % ChCl closely aligned with the eutectic composition (~33.33 mol % ChCl), where the glycerol network was the most organized and the choline ions exhibited the fastest relaxation. The viscosity decreased sharply up to ~25 mol % ChCl and then plateaued, while the conductivity increased monotonically with ChCl concentration, reflecting enhanced ionic mobility. The density decreased with both increasing ChCl concentration and temperature, indicating disrupted hydrogen bonding and reduced molecular packing. The polarity, measured using betaine-30 (B30) and the E_T(30) polarity scale, increased steeply up to approximately 25 mol % ChCl before reaching a plateau. These findings identified the eutectic composition as the optimal concentration range for balancing stability, fluidity, conductivity, and enhanced dynamics within the glycerol system. Full article

(This article belongs to the Special Issue New Horizons in Deep Eutectic Solvents (DESs): Synthesis, Characterization and Applications)

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23 pages, 5174 KB

Open AccessArticle

Designed Reactive Natural Deep Eutectic Solvents for Lipase-Catalyzed Esterification

by Alina Ramona Buzatu, Anamaria Todea, Raluca Pop, Diana Maria Dreavă, Cristina Paul, Ioan Bîtcan, Marilena Motoc, Francisc Peter and Carmen Gabriela Boeriu

Molecules 2025, 30(4), 778; https://doi.org/10.3390/molecules30040778 - 7 Feb 2025

Cited by 6 | Viewed by 2311

Abstract

Natural deep eutectic solvents (NADESs) are a sustainable, green option for extraction and reaction media in biorefineries and various chemical and biotechnological applications. Particularly, enzymatic reactions profit from NADES applications, as these solvents help to maintain high substrate solubility while improving both enzyme stability and efficiency. Recent studies confirmed that NADESs can perform multiple functions simultaneously, as reaction media for biocatalytic conversions, but also as substrates and catalysts for reactions, fulfilling the role of a reactive solvent. This study reports the beneficial effect of designed reactive natural deep eutectic solvents (R-NADESs) on the esterification activity and thermal stability of free and immobilized lipases in the synthesis of polyol- and carbohydrate-based biosurfactants. We manufactured and characterized 16 binary and ternary R-NADES systems with choline chloride (ChCl) as the hydrogen bond acceptor (HBA) and carbohydrate polyols; mono-, di-, and oligosaccharides; urea (U); N-methyl urea (MU); and water as the hydrogen bond donors (HBDs), in different combinations and molar ratios, most of which are reported for the first time in this paper. We determined their physicochemical, thermal, and molecular properties, including among others viscosity, polarizability, and the number of hydrogen bonds, and we showed that these properties are controlled by composition, molar ratio, molecular properties, temperature, and water content. Many lipases, both native and immobilized, showed high stability and remarkable catalytic performance in R-NADESs during esterification reactions. Full article

(This article belongs to the Special Issue New Horizons in Deep Eutectic Solvents (DESs): Synthesis, Characterization and Applications)

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