Interesting and very timely discussion. However, I also think 'absolutely green solvent' is an exaggerated term. It is true, that in recent years we have witnessed huge number of published research on ionic liquid (IL) containing systems and applications. When compared to well established and conventional processes, by far it is not either economically feasible or practically applicable (toxicity, biodegradability issues) to use ILs. But we have to keep in mind one property of ILs - designer nature/tunability - that's where the strength of IL-based research lies. Of course in order to implement them in a certain process we have to start testing simpler systems, i.e. first generation ILs (may contain a whole lot of drawbacks). In the last decade the learning curve regarding suitability of a particular ILs for a certain application has considerably improved. As Boyan and Michal indicated, although DESs are analogous to ILs, but for certain conditions like working under vacuum, ILs are by far the best choice. I think we have still a lot to see. The last decade was just a warm-up !!

Every solvent, even DES, has significant environmental impacts during its manufacture, during use, and during recycling or disposal. Therefore there's no such thing as an "absolutely green solvent". It's all relative. A green solvent is one which makes the application or process less damaging than would be the case if another solvent were used. That comparison, preferably by LCA, has to include the manufacture of the solvent. The other thing to consider is what you mean by "replacement". Except for a few very niche applications, ILs aren't used in industry, so there's little need to replace them. I wonder, therefore, if you're asking instead what the best green solvents will be?

As in most cases, it depends on what you want to dissolve and then what properties do you need for your solution. As for just dissolution power and low toxicity, with normal viscosity they can be of course many applications. But when you go to thermal or electrochemical stability, or working under vacuum you go back to all the disadvantages of molecular solvents which are not well suited for such things. There are a few applications, where the lack of vapor pressure and high thermal/electrochemical stability of ILs is hard to beat.

To overcome the high price and toxicity of ILs, attempting to combine sustainability with the promising high-values of ILs, a third generation is currently under development: a new designer solvents emerged as a promising green media for multiple applications The ease of synthesis, availability and biodegradability of components make DESs versatile alternative green solvents to LIs. For instance, the answer is DESs are the next LIs generation and promising green solvents.

in current time, DESs are most attractive green solvents, but it would be not surprising if the inquisitiveness of the scientific communities would discover some new DEEP green solvents which will be able to overcome the drawbacks which are not yet resolved even by the DESs. Nutural Molecules, especially those which can be derived from the plants and Marine natural products (because these are available in huge amount) have great potentials, which might open a new horizon in Solvent Chemistry.

Deep Eutectic Solvents may be think as the special ionic liuqids or ionic liquids mixtures. Chemists are interested in the DES.

Thank you for your discussion. Deep eutectic solvents (DESs) are regarded as ionic liquids analogues.

From an industrial point of view, the main questions with ionic liquids as well as with DES are the recyclability, toxicity, ecotoxicity. In addition, for larger scale uses, the solvent systems have to be robust enough to maintain the ability to dissolve and stay in liquid form.-And of course a reasonable price.

Biodegradability, low toxicity, recyclability makes DES attaractive solvent. Till date many applications are known for them so by application point of view DES is the alternative to the ILs.

Interesting and very timely discussion. However, I also think 'absolutely green solvent' is an exaggerated term. It is true, that in recent years we have witnessed huge number of published research on ionic liquid (IL) containing systems and applications. When compared to well established and conventional processes, by far it is not either economically feasible or practically applicable (toxicity, biodegradability issues) to use ILs. But we have to keep in mind one property of ILs - designer nature/tunability - that's where the strength of IL-based research lies. Of course in order to implement them in a certain process we have to start testing simpler systems, i.e. first generation ILs (may contain a whole lot of drawbacks). In the last decade the learning curve regarding suitability of a particular ILs for a certain application has considerably improved. As Boyan and Michal indicated, although DESs are analogous to ILs, but for certain conditions like working under vacuum, ILs are by far the best choice. I think we have still a lot to see. The last decade was just a warm-up !!

I think it depends on how you use them! I dont think there is such a thing as "absolutely green" - as soon as you use energy to make something, it is not absolutely green, and that's before you've even used it. The only genuine way is to fully assess the process in which you use the solvent in comparison with alternatives (to the solvent and also alternative routes). this has to include product isolation (and purification if required, as well as solvent recovery and reusability, and energy costs..Not easy in many cases, and almost never seen in the literature! this will give you a feel for whether one process is greener than another, and where there are areas for improvement.

According to the definition proposed by Anastas and Warner (Green Chemistry: Theory and Practice, P. T. Anastas and J. C. Warner, Oxford University Press, Oxford, 1998): Green chemistry is the utilization of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products. This definition is also adopted by Journal of Green Chemistry, RSC.

Ionic liquids and DESs are both needed. The main criterion to decide which is better is the type of application that targeted.

DESa are liquids when the compounds are mixing in a given composition. Are they stables along an industrial process? Can they be pumped, heated, cooled or mixed with other compounds without changes in the eutectic composition?

DES is the future. Although there are some limitation but lot of work is in process.

See this paper publihsed

https://www.researchgate.net/publication/308164132_PHYTOCHEMICAL_AND_PHARMACOGNOSTIC_STUDY_OF_CATHARNTHUS_ROSEUS_USING_DEEP_EUTECTIC_SOLVENTS

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