Low-melting mixture solvents: extension of deep eutectic solvents and ionic liquids for broadening green solvents and green chemistry

Green solvents such as water and ionic liquids (ILs) are pillars of the great mansion of green chemistry and green processing. Initially proposed as a new family of ILs, deep eutectic solvents (DESs) have received fast development in the past two decades. In this contribution, DESs are reviewed critically and the concept is extended to low-melting mixture solvents (LoMMSs), which cover all kinds of materials including ionic compounds, molecular compounds, and metals. Six classes of LoMMSs are proposed as the new classification system and examples are given. Finally, several thermodynamic issues concerning LoMMSs are discussed. Two new concepts, robustness of LoMMSs and high-entropy LoMMSs, are proposed.