Executive Summary
- Deep eutectic solvents (DESs) are a group of environmentally-friendly solvents made of readily available, biodegradable materials like ammonium salts, and they can be prepared with relatively low heat and simple mixing procedures.
- Deep eutectic solvents are used in biomedical applications and extracting nutrients from foods, and they can play an important role in battery electrolytes, environmental clean-up, and greener coatings for metals.
- There are five types of deep eutectic solvents, with Type III as the most studied thanks to its simplicity, cost-effectiveness, and biodegradability.
- Choline chloride is the most common substance in the pairs of substances that make up deep eutectic solvents.
Deep eutectic solvents are a rapidly emerging class of designer solvents that have transformed green chemistry, materials synthesis, and electrochemical systems. First conceptually formalized in the early 2000s, these solvents are formed by mixing two or more components, typically a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), in specific molar ratios to generate a eutectic mixture whose melting point is dramatically lower than that of any individual component.
This melting point depression occurs because strong hydrogen bonding interactions disrupt the crystalline structure of the pure substances, resulting in a liquid phase at relatively low temperatures. As a result, deep eutectic solvents can often be prepared via simple heating and stirring without the need for complex purification or drying steps.
A critical factor in the growth of deep eutectic solvent research is their sustainability. Unlike conventional organic solvents or some ionic liquids, they can be synthesized from inexpensive, readily available, and often biodegradable components. Common examples of HBAs include quaternary ammonium salts such as choline chloride (ChCl), while HBDs often include compounds like urea, glycerol, or carboxylic acids, organic acids, and natural metabolites.
Their low vapor pressure decreases flammability concerns and limits harmful emissions, while their tunable polarity, viscosity, acidity, and solvation capability give researchers enormous flexibility to tailor solvents to specific applications. Because deep eutectic solvents can incorporate biocompatible molecules, also known as natural deep eutectic solvents (NADES), they are uniquely suited for applications involving biological systems, natural product extraction, and pharmaceutical formulation.
Why deep eutectic solvents are garnering research attention
We leveraged the CAS Content CollectionTM, the largest human-curated repository of scientific information, to extract actionable insights about deep eutectic solvents from the vast body of published information on this subject. We formed a specific query in CAS IP Finder, powered by STN™, to analyze over 20,000 relevant journal and patent publications (see Figure 1).
Figure 1 presents temporal trends for documents related to deep eutectic solvents, obtained using the “Analyze” function offered by CAS IP Finder™. Researchers exploring similar trends can also access AI-enabled search capabilities in CAS SciFinder® to identify patterns in their specific research areas.
The publication trends for deep eutectic solvents demonstrate the steep growth characteristic of an emerging field transitioning toward maturity. This pattern of dormancy followed by a dramatic acceleration indicates the topic has evolved from a niche area to a mainstream research focus within the scientific community.
A critical observation, however, is the pronounced disparity between publication types, with journal articles dominating the landscape while patent families represent a substantially smaller fraction. This imbalance suggests the field remains predominantly in the fundamental research phase with limited translation to commercial applications. The gap between academic interest and intellectual property development indicates either commercialization barriers or that foundational knowledge is still being established before practical implementation can accelerate.
Why are deep eutectic solvents so promising, even if they haven’t reached commercialization? These solvents share several similarities with ionic liquids, as both are considered low-volatility, tunable solvents with ionic character. However, there are notable differences between the two. While ionic liquids are composed entirely of ions and exhibit full ionic character, deep eutectic solvents are only partially ionic because they consist of molecular species interacting through hydrogen bonding.
Deep eutectic solvents are also significantly cheaper because they are synthesized from readily available, often bio-based components, whereas ionic liquids require multi-step synthesis and expensive salts. Furthermore, deep eutectic solvents are easier to synthesize, which contrasts sharply with the complex preparation and purification processes associated with ionic liquids.
From a functional perspective, deep eutectic solvents exhibit unique solvation environments that differ significantly from traditional aqueous or organic solvents. They can dissolve numerous inorganic salts, metal oxides, polymers, and biomolecules, often enabling reactions or processes that are difficult in other media. Their wide electrochemical windows, high ionic conductivity (in some formulations), and stability make them attractive candidates for next-generation batteries, metal electrodeposition, and supercapacitors.
In materials chemistry, these solvents have emerged as powerful media for synthesizing nanomaterials, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), porous carbons, and catalysts, offering precise control over nucleation and growth while avoiding harsh conditions. They are increasingly employed in processes that support resource recovery and recycling, such as metal extraction from electronic waste, biomass processing, and carbon dioxide capture. These applications contribute to the circular economy by enabling sustainable material recovery and reuse.
Overall, deep eutectic solvents represent a promising alternative to conventional organic solvents and expensive ionic liquids, offering a versatile, cost-effective, and eco-friendly solution for applications in catalysis, electrochemistry, pharmaceuticals, and materials science.
Types of deep eutectic solvents
Deep eutectic solvents are categorized into several types based on the nature of HBD and HBA components. Abbott et. al originally proposed a classification system comprising Types I-IV, which was later expanded to include Type V to accommodate systems beyond conventional quaternary ammonium salts. This classification provides a systematic framework for understanding the formation, physicochemical behavior, and potential applications of deep eutectic solvents.
- Type I: These are formed by combining a quaternary ammonium salt, typically a halide such as choline chloride (ChCl), with a metal salt containing a metal chloride such as ZnCl₂ or SnCl₂. The metal salt acts as a Lewis acid, and its interaction with the halide anion leads to a significant melting point depression.
- Type II: These solvents also rely on a quaternary ammonium halide salt, but the metal component is present in its hydrated form. The presence of water molecules coordinated to the metal ions influences hydrogen bonding and ionic interactions, further lowering the melting point and increasing the ionic conductivity.
- Type III: These are the most studied due to their simplicity, cost-effectiveness, and biodegradability. In these systems, a quaternary ammonium salt acts as the HBA, while a molecular compound — commonly urea, glycerol, ethylene glycol (EG), or organic acids — as the HBD. The strong hydrogen bonding between the components results in deep melting point depression, forming liquids at or near room temperature. Their tunable polarity and low toxicity make them attractive for extraction processes, biomass conversion, and pharmaceutical applications.
- Type IV: These deep eutectic solvents differ from the earlier categories in that the metal salt serves as the HBA, while the HBD is a molecular compound such as urea or EG. These systems can incorporate high concentrations of metal ions, and the coordination between metal centers and molecular donors contributes to their unique structural and electrochemical properties.
- Type V: Type V represents a broader, newer category that does not rely on quaternary ammonium salts or metal halides. Instead, both components are molecular species capable of forming strong hydrogen bonds, such as organic acids, amides, polyols, sugars, and other neutral molecules. This category overlaps with the concept of NADES, which use naturally derived and biocompatible components. Type V solvents offer exceptional flexibility in designing environmentally benign solvents tailored for food, pharmaceutical, and biological applications.
The documents that we analyzed included associated indexed substances. From these records, we identified the top fifteen substance pairs known to form deep eutectic solvents (see Figure 2).
The distribution of research attention reveals a highly concentrated landscape dominated by ChCl-based systems. The overwhelming presence of choline chloride in most top-ranked pairs indicates it has become the predominant HBA in the field. This is mainly due to its accessibility, straightforward preparation, and extensive characterization, which facilitates reuse across diverse methods and applications.
Importantly, the analysis shows that the top 15 pairs constitute solely Type III and Type V systems. The predominance of Type III solvents indicates that ionic component‑based systems have received the greatest research emphasis due to their simplicity and cost-effectiveness.
In contrast, the comparatively limited representation of Type V solvents suggests that these all‑molecular systems are valued for their biocompatibility and environmental advantages. This pattern demonstrates a clear shift in research toward metal‑free formulations, consistent with broader movement toward designing safer, more sustainable solvents.
Choline chloride and newer substance pairs
We further examined substance pairs to identify those that are emerging or exhibiting elevated year‑over‑year (YoY) growth during 2021–2025 (see Figure 3).
The landscape of emerging deep eutectic solvent pairs exhibits a pronounced inverse relationship between established research presence and growth momentum, wherein systems with lower cumulative publication outputs demonstrate substantially elevated growth rates. This pattern suggests the field is diversifying beyond historically dominant formulations, with traditional benchmark systems occupying the lower-growth, high-volume quadrant, indicating saturation in fundamental characterization, while novel combinations demonstrate high-growth trajectories typical of expanding investigative interest and potential discovery of previously unexplored applications.
A noteworthy observation is the conspicuous dominance of betaine-based formulations within the high-growth regime. The clustering of betaine paired with various hydrogen bond donors at elevated growth rates signals a paradigmatic shift away from choline chloride as the universal hydrogen bond acceptor, potentially driven by betaine's zwitterionic nature, enhanced biocompatibility, natural origin, or favorable regulatory profile.
This compositional diversification represents a second generation of deep eutectic solvent research. It points to methodological maturation and systematic exploration of alternative ionic frameworks that may address limitations of traditional choline-based systems.
The percentage of patent data highlights a divergence between publication maturity and commercial translation. Emerging betaine‑based formulations are attracting simultaneous academic and IP interest, indicating strong perceived commercial potential. In contrast, established choline‑chloride pairs, despite extensive research coverage, show comparatively lower patent engagement, suggesting limited novelty for protection or lingering barriers to market uptake.
Overall, the pattern implies a shift in innovation toward alternative, betaine‑centered systems that promise differentiated performance, regulatory advantages, or solutions to unmet industrial needs beyond conventional choline‑based deep eutectic solvents.
Applications from pharmaceutical manufacturing to wastewater treatment and beyond
We identified major applications for deep eutectic solvents and analyzed the journal and patent activity across these domains (see Figure 4). The distribution of research activity across application areas reflects the versatility of these solvents in enabling concrete technological advances.
In biomedical science, deep eutectic solvents, particularly NADES, support drug delivery through enhanced solubilization of hydrophobic pharmaceuticals, stabilization of macromolecules such as proteins and enzymes, and facilitation of transdermal and mucosal transport. Their ability to maintain biological compatibility while improving molecular stability has also made them attractive media for antimicrobial formulations and cryopreservation strategies.
Biomass valorization represents another major focus area, where these solvents are used for selective delignification, dissolution of hemicellulose, and pretreatment of lignocellulosic feedstocks to increase sugar yields for bioconversion. Their capacity to extract platform chemicals such as furfural or 5-(hydroxymethyl)furfural under relatively mild conditions further underscores their relevance to sustainable biorefinery operations.
In the food and nutraceutical domain, deep eutectic solvents are widely used for extracting polyphenols, flavonoids, carotenoids, and other bioactive compounds from plant matrices, offering cleaner alternatives to organic solvents while preserving antioxidant functionality.
Catalysis research leverages these substances as reaction media for acid-base catalysis, metal-catalyzed transformations, and biocatalytic processes. Their structured hydrogen bond networks often enhance catalyst stability, facilitate substrate activation, and support improved product selectivity.
Environmental remediation, meanwhile, uses deep eutectic solvents for tasks such as dye removal, heavy metal sequestration, and extraction of persistent organic pollutants from wastewater. Some also act as co-media for enzymatic or photocatalytic degradation, supporting greener and more energy efficient water treatment pathways.
Energy-related applications of deep eutectic solvents are similarly diverse, encompassing their role as electrolytes in lithium-ion and sodium-ion batteries, proton-conducting media in fuel cells, and ionic environments for high-performance supercapacitors. Their thermal stability, low flammability, and broad electrochemical windows make them compelling alternatives to conventional electrolytes.
Electrodeposition research involves using deep eutectic solvents for forming uniform coatings of metals such as nickel, copper, cobalt, and their alloys, enabling controlled deposition without the corrosive additives required in aqueous plating baths. In metal‑extraction applications, they are used for selective leaching and recovering valuable metals from ores, spent catalysts, and electronic waste. For instance, choline‑based deep eutectic solvents can dissolve precious metals through ligand‑assisted complexation, providing a significantly safer and more environmentally benign alternative to conventional strong mineral acids.
Gas capture and separation studies explore the use of these solvents for CO₂ absorption, SO₂ removal, and biogas purification, capitalizing on their tunable hydrogen bonding affinity for specific gases.
Finally, in cosmetics, deep eutectic solvents function as natural solubilizers, humectants, and carriers for botanical extracts, enabling the development of milder, plant derived formulations aligned with clean label consumer trends.
The research shown in Figure 4 underscores that deep eutectic solvents are not merely theoretical green solvents but practical enablers of targeted technological solutions across biomedicine, sustainable chemistry, energy systems, environmental management, and consumer products.
Substance pairs and applications in green manufacturing
We further explored the most used deep eutectic solvent substance pairs across these applications (see Figure 5). This analysis revealed complex relationships between formulations of deep eutectic solvents and application domains.
Traditional chloride-based Type III systems (paired with polyols, urea, and organic acids) demonstrated remarkable versatility through substantial flow distributions across all application categories. This broad applicability indicates these established formulations function as general-purpose platforms capable of addressing diverse functional requirements, reflecting their historical prominence and potentially genuine multifunctional capabilities.
In parallel, a few Type V formulations appear as more targeted streams, linking preferentially to bio‑centric and remediation‑oriented contexts where attributes such as biocompatibility, tunable acidity, and viscosity control are advantageous.
Overall, this indicates that application selection is governed less by a single optimal mixture than by compositional families (e.g., polyols, carboxylic acids, carbohydrates) that permit systematic tuning of polarity, acidity, hydrogen‑bonding capacity, and ionic character. This underscores how the structural and chemical diversity of deep eutectic solvents enables their tailored deployment across advanced technologies. This diversity also motivates property‑driven selection frameworks and standardized reporting to minimize over‑reliance on legacy systems and to surface high‑potential, under‑explored formulations.
Future outlook: Consistency will drive adoption of deep eutectic solvents
Despite their growing prominence as versatile and sustainable alternatives to conventional solvents, deep eutectic solvents continue to face several scientific and technological challenges that hinder broader industrial adoption. The principal barrier is their acute sensitivity to composition, particularly water content, which reorganizes hydrogen‑bond networks and shifts viscosity, polarity, and electrochemical behavior. These changes in composition underscore the need for standardized preparation, qualification, and routine in situ water quantification to ensure reproducibility.
Equally important is high viscosity, one of the most recognized practical constraints. Elevated viscosities impair mass and heat transfer, limit separation efficiency, and constrain electrochemical or catalytic applications. In parallel, the toxicological and environmental profiles of deep eutectic solvents remain incompletely characterized. Although they are often labeled “green,” especially NADES, recent studies reported contradictory toxicological outcomes, emphasizing the need for standardized testing protocols and broader organism‐level assessments.
Looking forward, the advancement of DES research will depend on establishing rigorous standards for composition reporting, purity control, and water management, supported by rapid quality control tools capable of ensuring consistency from laboratory preparation to industrial deployment. Ongoing advances in molecular modeling, spectroscopy, and high‑throughput screening are beginning to close existing knowledge gaps, enabling a more rational and predictive design of application‑specific solvent systems.
Together, these developments will position deep eutectic solvents as a continually evolving platform whose full potential will only be realized through coordinated advances in design, characterization, and sustainable implementation.
