What are they?
Deep eutectic solvents (DESs) are composed of two or more components, at least one HBD and one HBA, interacting by each other by self-associating forming an eutectic mixture with a melting temperature far below that of its constituents. Generally, it is accepted that the self-association occurs via hydrogen bonding interactions, where it is assumed that Van der Waals forces can also play a role. The preparation of a DES is simply mixing the HBD(s) and HBA(s) and no purification steps are needed. DESs are also called Low Transition Temperature Mixtures (LTTMs). In recent research for biomass separation also other novel solvents are being explored: Aprotic Ionic Liquids (AILs or ILs) and Protic Ionic Liquids (PILs).
Aprotic ionic liquids (AILs) are salts composed of a cation and an anion with a melting temperature below 100°C. They are commonly synthesized via a quaternization reaction on an amine, phosphide or sulfide in conventional solvents, after which one or multiple purification steps are needed.
Protic ionic liquids (PILs) are composed of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) behaving as a Brønsted acid and a Brønsted base due to large differences in the pKa’s of the constituents. Thus, proton exchange occurs where the HBD is deprotonated and becomes the anion, while the HBA is protonated and becomes the cation. The preparation of a PIL is simply mixing the Brønsted acid and the Brøndsted base. Thus, no purification steps are needed.