Deep eutectic solvents: Teaching nature lessons that it knew already

When Brønsted and Lewis acids and bases are mixed strong hydrogen bonds develop and these deviate from ideal mixtures resulting in a significant depression of freezing point. Many of these mixtures are liquid at ambient temperatures. These so-called deep eutectic solvents have become an area of intense study since their discovery. Mixtures of Brønsted and Lewis acids and bases are commonly found in nature and the properties of these mixtures are similar to ionic liquids. This article explains the types of mixtures which form and how bioinspired formulations can be used to form materials which mimic natural structures and can be used for applications such as drug delivery. The ability of sugars to form concentrated complex liquid mixtures is discussed and compared to aqueous solutions. DESs have complex phase behavior when solutes of lower polarity are included and it is shown that microemulsions are formed when water is introduced into DESs. This may explain the behavior of proteins and bio-macromolecules in DES-water mixtures. The thermodynamics of species partitioning into DESs is discussed together with methods to calculate enthalpies of interaction.