Carbon dioxide in ionic liquid microemulsions.

A microemulsion is a thermodynamically stable dispersion of two immiscible fluids stabilized by surfactants. The hydrophilic head groups of the surfactants point to the polar phase, while the hydrophobic tails extend into an apolar phase. Owing to the capacity to host a variety of polar and nonpolar species simultaneously, microemulsions have been widely applied in protein delivery, drug release, catalysis, and nanomaterials synthesis. In general, the organic solvent (oil) and water are used as the two immiscible fluids in the formation of microemulsions. In recent years, especially with the development of green chemistry, supercritical CO2 (SC CO2) and ionic liquids (ILs), which are usually regarded as green solvents, have attracted much attention. In comparison with the conventional solvents (usually water and organic solvents), these green solvents have some unique properties. For example, SC CO2 is readily available, inexpensive, nontoxic, nonflammable, and has moderate critical temperature and pressure. Most importantly, the physical properties of SC CO2 can be adjusted by the pressure and temperature continuously. Furthermore, CO2 can be easily recaptured and recycled after utilization. ILs are an interesting class of tunable and designable solvents with essentially zero volatility, wide electrochemical window, nonflammability, high thermal stability, and wide liquid range. Such unique properties confer SC CO2 and ILs great potential of applications in chemistry and chemical engineering. The formation of microemulsions with SC CO2 or IL is very attractive owing to their unusual solvent properties. Up to now, various kinds of microemulsions containing SC CO2 or IL have been prepared, including water-in-SC CO2 [7] and CO2-in-water microemulsions, [8] IL-in-oil and oil-in-IL microemulsions, IL-in-water and water-in-IL microemulsions, IL-in-IL microemulsion, and IL-in-SC CO2 microemulsions. The creation of microemulsions with IL as the continuous phase and CO2 as the dispersed phase is very interesting and has not yet been reported. Herein, we demonstrate the first work for the formation of a CO2-in-IL microemulsion. This novel microemulsion has many advantages. For example, the size of the dispersed CO2 droplet can be tuned by the pressure of CO2; the properties of the continuous phase can also be tuned by the kind of ILs because of the tunable and designable features of ILs. These special properties give CO2-in-IL microemulsion various applications such as in material synthesis, chemical reactions, and extraction. It is well known that the amphiphilic surfactant can selfassemble in ILs to form different aggregates such as micelles, vesicles, or liquid crystals. In this work, the aggregation behavior of surfactant N-ethyl perfluorooctylsulfonamide (C2H5NHSO2C8F17, N-EtFOSA) in 1,1,3,3-tetramethylguanidinium acetate (TMGA) ([N-EtFOSA]= 3.0 wt%) was characterized by freeze fracture electron microscopy (FFEM). As shown in Figure 1, N-EtFOSA molecules aggregated into spherical micelles in TMGAwith an average diameter of 10– 20 nm.