Deep eutectic solvents (DES) have emerged in the past few years as a new class of solvents with promising applications in several fields. In the present work, the application of DES (formed by binary mixtures of cholinium chloride and carboxylic acids or urea) as phase forming components of aqueous biphasic systems (ABS) is inves tigated. The mechanisms associated with the phases demixing of ABS composed of DES, as well as the DES stability in aqueous solutions, are investigated to address the critical question whether DES based ABS are in fact ternary or quaternary mixtures. It is shown that the DES integrity is destroyed in ABS by the disruption of the hydrogen bonding interactions of the complex (a result of the isolated components preferential solvation by water), and as confirmed by a nonstoichiometric partition of the DES components between the coexisting phases. As a result, there are no "real" DES based ABS; instead, there is the formation of ABS composed of four components, where the carboxylic acid used as the hydrogen bond donor species seems to act as an additive. Finally, it is shown that these ABS have an outstanding potential to be used in extraction processes, as it is here demonstrated with the complete separation of two dyes. However, the volatile nature of short chain carboxylic acids and the nonstoichiometric partition of the DES components in ABS make the development of recovery and recycling steps more difficult to accomplish. The formation of DES results from the formation of strong hydrogen bonds between the two starting materials, namely, a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA). The formation of ion−HBD complexes leads to a lower entropic difference of the phase transition and to the further depression of the freezing temperature.5−7 The first reported DES (composed of [N 111(2OH) ]Cl and urea) 1 remains the most widely investigated. However, DES can be formed by mixing other substituted tetralkylammonium or phosphonium salts with a HBD, such as amines, carboxylic acids, and carbohydrates, among others.
8−10Many DES share some of the unique characteristics of ionic liquids (ILs), namely, a low volatility, high conductivity, wide liquid temperature range, and high solvation ability for a large number of compounds.3,4 DES are also able to overcome some of the disadvantages related to ILs. They are easier to prepare, by a simple mixture of the starting materials at moderate temperatures, without requiring a reaction step. Furthermore, the addition of an agent able to disrupt hydrogen bonding interactions can lead to the recovery of at least one of the initial compounds. 3,4,11,12 The starting materials are usually cheaper, toxicologically well characterized, and often derived from renewable resources, which results in low cost and more environmentally friendly products. 3,4,11,12 As in ILs, innumer ous combinations of the starting materials can be attempted, and therefore, DES may also be classified as "designer solvents". Since DES have fewer restrictions ...