Deep eutectic solvents Towards more sustainable processes

Solvents are chemical substances used to dissolve or extract other compounds. They are widely used in many industrial sectors (degreasing, cleaning, stripping, printing, paints, adhesives, pharmaceutical manufacturing). Traditional organic solvents are generally volatile organic compounds (VOCs). Their use is a major concern, since many of them are associated with harmful effects, both for the health and safety of workers and for the environment and health of the general population. This is why many environmental regulations are encouraging companies to use alternative solvents. In addition, more and more companies are looking to go green, i.e. to ensure that their activities and products are part of a sustainable development approach. It's against this backdrop that a number of solvents have emerged, described as green, ecological, biodegradable or sustainable, and presented as an alternative to conventional organic solvents. These include ionic liquids, supercritical fluids, biomass-based solvents and deep eutectic solvents.

Deep eutectic solvents (DES) have received considerable attention in recent years. Until 2010, little had been explored about these systems, both in terms of fundamental work and applications, but since then the number of scientific papers and patents has grown exponentially. Indeed, the ease with which they can be prepared, the ready availability of their components, their adaptability and the versatility of their physico-chemical properties have made deep eutectic solvents suitable for applications in a wide range of fields.

As the definition of a deep eutectic solvent is still a matter of debate, we present in this manuscript the evolution of the definitions proposed in the literature, from their discovery to the present day. In addition to a brief history of the evolution of the classification and nomenclature used for deep eutectic solvents, we outline the different methods that can be used to prepare these solvents, and describe some of their most remarkable physico-chemical properties. Due to the relatively recent discovery of these solvents, divergent values may be observed in the literature, depending in particular on the purity of these solvents (presence of water or other impurities). Recommendations are made to avoid parasitic reactions during preparation. Finally, we present some applications for these solvents. From electrochemistry to new applications such as cryopreservation, the field is evolving as new results on DES properties are unveiled. These solvents, presented as an alternative to the usual, generally volatile organic solvents, have even been used to combat the latter's atmospheric emissions.

At the end of the article, readers will find a glossary and a table of symbols used.