Hydrophobic deep eutectic solvents: the new generation of green solvents for diversified and colorful applications in green chemistry

To achieve sustainable development goals, the exploitation of green solvents has been a research hotspot. Owing to the easier preparation process with 100% atom economy, low cost, potential biodegradability and negligible toxicity, deep eutectic solvents (DESs) are recognized as attractive substitutes of organic solvents and ionic liquids. However, the most synthesized DESs are miscible with aqueous media as well as hydrophilic organic solvents, which strictly restrict their applications in water-containing or lipophilic samples. Very recently, hydrophobic DESs (HDESs) have been discovered with hydrophobicity and good solvation abilities for organic and inorganic compounds. The emergence of HDESs overcomes the limitations of traditional hydrophilic DESs and extends the application scopes. In this review, recent progress on the properties and applications of HDESs in detection technologies for food safety and environment protection, separation and extraction processes, biocatalysis, energy utilization, enantioseparation, material modification and pharmaceutical fields since 2015 was reported. The contributions of HDESs lie in broadening the application scopes of DESs and alleviating some of the current industrial and social issues. A brief outlook on the future perspectives and unsolved challenges is also proposed. The aim of this review is to inspire the research enthusiasm to HDESs and promote the industrial development of HDESs.