Progress in environmentally friendly ionic liquids aqueous two-phase systems

Abstract

Aqueous two-phase systems(ATPSs) were usually formed as a result of mutual incompatibility of two polymers or one polymer and one salt above a certain concentration. ATPSs have advantages over conventional extraction systems using organic solvents,since the bulk of both phases consists of water. Ionic liquids(ILs) are liquid salts at the temperatures below boiling point of water. The particular interest is their highly tunable properties by varying the chemical structures of the ions comprised. Thus,by the rational design of cation and anion,an IL with desired characteristics can be created for a particular application. In 2003,the excellent ability of ILs to induce ATPSs was demonstrated in the presence of inorganic salts. Due to their high water content and no use of volatile compounds,the ILs-based ATPSs have been used to extract/separate amino acids,drugs,low molecular mass compounds,radiological isotopes,proteins and enzymes. However,it is known that among a large number of IL-based ATPSs,toxic and poorly biodegradable ILs have been widely used. On the other hand,high concentrations of inorganic salts are always used in the ILs-based ATPSs. This will produce a great amount of highly salty water in the applications,thus resulting in environmental problems. In this paper,environmentally friendly IL-based ATPSs,such as anion functionalized IL-salt ATPSs,IL-sugar ATPSs,IL-polymer ATPSs and CO2 induced ATPSs were systematically analyzed and reviewed. Among these ATPSs,anion functionalized ILs were used to replace poorly biodegradable imidazolium or pyridinium halide ILs,and high concentrations of inorganic salts were replaced by some new phase-forming materials,such as sugars,polymers and CO2 induced organic amines. From the viewpoint of sustainable chemistry,the researches described above could not completely solve the environmentally friendly problems of IL-based ATPSs,however,these innovative achievements would play an important role in the development of environmentally friendly IL-based ATPSs. Finally,challenges in the research and development of environmentally friendly IL-based ATPSs were discussed and suggestions for further research were provided.