Environmental Toxicity of Deep Eutectic Solvents

Deep eutectic solvents (DESs) have emerged as versatile, low-cost media for chemical synthesis, extraction and formulation. Traditionally lauded for low volatility and presumed biodegradability, they are often promoted as greener alternatives to conventional organic solvents. However, growing evidence indicates that their environmental footprint cannot be generalised as benign. Toxicity varies widely with the nature of the hydrogen bond acceptor (HBA) and donor (HBD), molar ratio, pH, temperature and the presence of impurities or degradation products. Studies conducted on bacteria, aquatic organisms, fish and mammalian cell lines reveal that some DESs can induce oxidative stress, membrane disruption and acute toxicity in non-target species. Moreover, thermal or photochemical breakdown can generate esters, organic acids or other by-products with distinct bioactivities. Consequently, assessing ecotoxicological risk requires standardised assays across trophic levels, rigorous monitoring of stability and a deeper understanding of biodegradation pathways. This multifaceted perspective is essential to ensure real-world applications of DESs align with sustainability goals.

Research from Nature Portfolio

In a 2023 study, ammonium-based DESs composed of tetrabutylammonium bromide with glycerol, ethylene glycol or diethylene glycol were evaluated across bacterial cultures, fish embryos and human fibroblasts. Results demonstrated that the glycerol-based mixture exhibited the lowest toxicity in both in vivo and in vitro models, while ethylene and diethylene glycol variants showed moderate to high lethal concentrations in aquatic tests. This work underscores the importance of component selection in minimising environmental harm.

A 2019 investigation into skin-cell models assessed the cytotoxicity of DESs featuring various quaternary ammonium HBAs and organic-acid or polyol HBDs. Certain combinations proved highly biocompatible with keratinocytes and melanocytes, whereas longer-chain ammonium HBAs drove membrane destabilisation and cell death. Findings highlight how minor structural changes can pivot a DES from low toxicity to harmful, informing both cosmetic safety and environmental release considerations.

Environmental Toxicity of Deep Eutectic Solvents publication trend

The graph below shows the total number of articles in environmental toxicity of deep eutectic solvents across all publications each year (not limited to Nature Index journals).

Technical terms

Deep eutectic solvent (DES): A mixture of two or more components that, through hydrogen bonding, exhibits a melting point lower than that of its individual constituents.

Hydrogen bond donor (HBD): A molecule within a DES that provides a hydrogen atom for hydrogen-bond formation.

Hydrogen bond acceptor (HBA): A molecule within a DES that accepts a hydrogen bond from an HBD.

Ecotoxicity: The capacity of a substance to cause adverse effects in ecosystems, including impacts on flora, fauna and microbial communities.

Biodegradation: The breakdown of a chemical substance by biological organisms, indicating its potential persistence or removal in the environment.

References

1. Stability of Cholinium Chloride-Based Deep Eutectic Solvents with Carboxylic Acids: A Study on Ester Formation. ACS Sustainable Chemistry & Engineering (2024).
2. In vivo and in vitro toxicity profile of tetrabutylammonium bromide and alcohol-based deep eutectic solvents. Scientific Reports (2023).
3. Cytotoxicity profiling of deep eutectic solvents to human skin cells. Scientific Reports (2019).
4. Thermal Instability of Choline Chloride-Based Deep Eutectic Solvents and Its Influence on Their ToxicityImportant Limitations of DESs as Sustainable Materials. Industrial & Engineering Chemistry Research (2022).
5. Controversy on the toxic nature of deep eutectic solvents and their potential contribution to environmental pollution. Heliyon (2022).