Remarks on use of the term "deep eutectic solvent" in analytical chemistry

About 20 years ago, Abbott and co-workers researched new solvents that were based on mixtures of choline chloride with urea and carboxylic acids and that were liquid at ambient temperature. The term "deep eutectic solvent " (DES) was later adopted for similar mixtures. As DESs have a number of interesting features, they quickly attracted the attention of researchers and found application in various branches of chemical and materials research. To date, various definitions of DES have been proposed. Nevertheless, there are still differing views among researchers on what a DES really is and what exact properties a mixture must have in order to be included into this group. The prefix "deep " should only be used for deep eutectic solvents with melting points significantly below ideal predictions. Otherwise, the simpler term "eutectic solvent " (ES) should be used. It therefore follows that in order to correctly assign a solvent to the DES or ES group, a phase diagram must be available to evaluate the deviation from the ideality, as well as to quantify the DES's liquid window. The term DES is now commonly used in analytical chemistry, but authors rarely go into a detailed examination of a DES from the viewpoint of whether it actually is a DES or an ES. Instead, they logically focus mainly on the analytical part, that is, on taking advantage of the benefits that the DES provides. Therefore, it may happen that the solvent used is designated as a DES, even if it does not meet all the necessary requirements. Of course, we cannot or should not expect analytical chemists to do a detailed study of the physical properties of DESs in addition to solving analytical tasks, although we should encourage analysts to pay due attention to the correct use of the terms (D)ES in their publications, as it would certainly help further development in the utilization of DESs in various areas, not just in analytics.