Miscible–Immiscible Transition of Deep Eutectic Solvents in Water Switched by CO2

It is very important to develop "smart" solvents that can integrate the advantages of homogeneous and heterogeneous reactions into one chemical process for enhanced reaction rate and minimal environmental impact. In designing these solvents, the applicability of the systems to real processes strongly depends on the phase behavior of the systems. In this work, a novel class of CO2-driven miscible–immiscible transition of deep eutectic solvents (DESs) has been explored. These DESs are composed of alkanolamine compounds as hydrogen-bond acceptors (HBAs) and phenolic compounds as hydrogen-bond donors (HBDs). They exhibit unique phase behavior with H2O: miscible with water in the absence of CO2 but immiscible with water in the presence of CO2 at room temperature and ambient pressure. Reversible liquid–liquid phase transition of such DES–H2O mixtures can be realized by alternative bubbling of CO2 and N2 under ambient conditions. Spectroscopic investigations show that the mechanism behind the miscible–immiscible transition of DESs involves breaking of the hydrogen bonds between HBDs and HBAs by H2CO3 generated by bubbling CO2 through an aqueous solution because the acidity of H2CO3 is stronger than that of HBAs. Further, the HBAs react with H2CO3 to form ammonium salts. The salting-out effect of ammonium salts leads to the formation of an ammonium salt-rich top phase and an HBD-rich bottom phase. When CO2 is removed by bubbling of N2, a reversible process is achieved.