Deep Eutectic Solvents: Physicochemical Properties and Applications

Deep eutectic solvents (DESs) are a class of designer liquids formed by combining a hydrogen‐bond acceptor such as choline chloride with hydrogen‐bond donors including urea, glycerol or organic acids. The strong intermolecular hydrogen bonding and favourable mixing enthalpies produce a pronounced melting‐point depression, yielding liquids with tunable viscosity, conductivity and solvation properties over a broad temperature range. Key physicochemical parameters—density, viscosity, refractive index, thermal stability and enthalpy of fusion—govern both fundamental understanding and practical implementation of DESs. Recent work has revealed that certain ammonium salts exhibit ionic plastic crystal behaviour, characterised by low fusion enthalpies and enhanced resilience to melting. Such insights enable the rational design of task‐specific solvents, with applications spanning biomass pretreatment, extraction of bioactive compounds, CO₂ capture, catalysis and advanced materials synthesis. Natural deep eutectic solvents (NADESs), derived from renewable biomolecules, offer additional benefits in terms of biodegradability and low toxicity, aligning with green chemistry principles. Advances in computational modelling, rapid calorimetric techniques and spectroscopic analysis continue to deepen our understanding of structure–property relationships and accelerate the development of sustainable solvent systems for industrial and laboratory processes.

Research from Nature Portfolio

Recent studies have succeeded in directly measuring the enthalpy of fusion and melting point of choline chloride using fast‐scanning calorimetry combined with micro‐XRD and high‐speed optical microscopy. These measurements identify choline chloride as an ionic plastic crystal, exhibiting a fusion enthalpy of approximately 14 kJ mol⁻¹ and a melting point near 687 K, and reveal significant negative deviations from ideal mixing. The discovery of ionic plastic crystal behaviour overturns previous assumptions about eutectic depression mechanisms and highlights a new platform for designing DESs with low melting points and tailored thermal properties. This work opens avenues for the creation of ionic plastic crystal–based solvents optimised for catalysis, separations and materials processing across diverse temperature regimes.

Research from all publishers

A recent review of supramolecular deep eutectic solvents for liquid–liquid extraction demonstrates their superior environmental and performance profiles compared with conventional organic solvents. By fine‐tuning component choice and molar ratios, these systems achieve high capacity and enrichment factors for applications in bioactive compound recovery, lignin valorisation and fuel desulphurisation, while maintaining low toxicity and facile recyclability.

A systematic investigation of choline chloride‐ and betaine‐based DESs combined with ethylene glycol, glycerol and levulinic acid across varied molar ratios has elucidated the influence of hydrogen‐bond donor and acceptor identity on physicochemical performance. Ethylene glycol–based systems display the lowest viscosity, glycerol‐based mixtures offer the greatest thermal stability, and variations in donor concentration modulate density and refractive index in predictable ways. These findings provide a practical guide for solvent selection according to specific process requirements.

Phase‐diagram mapping and spectroscopic analysis of tetrabutylammonium bromide and nonanoic acid mixtures have identified a true deep eutectic at a 1:2 molar ratio. Detailed investigation of hydrogen‐bonding interactions clarifies the molecular origins of eutectic formation, extending the DES toolkit to hydrophobic systems and informing the design of non‐aqueous eutectic solvents for separations and extraction technologies.

Deep Eutectic Solvents: Physicochemical Properties and Applications publication trend

The graph below shows the total number of articles in deep eutectic solvents: physicochemical properties and applications across all publications each year (not limited to Nature Index journals).

Technical terms

Deep eutectic solvent (DES): A multi‐component mixture whose melting point is substantially lower than that of any individual constituent, due to strong intermolecular interactions and non‐ideal mixing.

Eutectic point: The precise composition and temperature at which a multi‐component system transitions between solid and liquid phases as a single entity.

Enthalpy of fusion: The heat energy absorbed when a solid changes to a liquid at its melting point, reflecting the energy required to disrupt the crystalline lattice.

Ionic plastic crystal: A solid phase in which ions exhibit significant orientational freedom within a crystalline framework, resulting in low fusion enthalpies and rapid phase transitions.

Hydrogen‐bond donor and acceptor: Molecules or ions that respectively supply or accept hydrogen bonds, the primary interactions responsible for DES formation and properties.

References

1. Supramolecular deep eutectic solvents in extraction processes: a review. Environmental Chemistry Letters (2024).
2. Defying decomposition: the curious case of choline chloride. Nature Communications (2023).
3. Everything You Wanted to Know about Deep Eutectic Solvents but Were Afraid to Be Told. Annual Review of Chemical and Biomolecular Engineering (2023).
4. Deep Eutectic Solvents for Pretreatment, Extraction, and Catalysis of Biomass and Food Waste. Molecules (2019).
5. Comparison of physicochemical and thermal properties of choline chloride and betaine-based deep eutectic solvents: The influence of hydrogen bond acceptor and hydrogen bond donor nature and their molar ratios. Journal of Molecular Liquids (2023).
6. Deep Eutectic Solvents or Eutectic Mixtures? Characterization of Tetrabutylammonium Bromide and Nonanoic Acid Mixtures. The Journal of Physical Chemistry B (2022).