Aqueous-phase green synthesis of formate-based ionic liquids and their thermophysical properties

Abstract

In this study, the anion exchange reaction of an organic cation chloride with excess KHCO2 to synthesize formate-based ionic liquids (ILs) was performed in water, and the water-miscible IL products were easily obtained via salting-out phase separation caused by the excess KHCO2. According to the synthesis results, a set of heuristic rules could be generated as follows. First of all, the more “hydrophobic” ILs required less amount of excess KHCO2 for salting-out phase separation. Secondly, the amphiphilic ILs with a long cationic alkyl chain (e.g., octyl) are likely to form reverse micelles to encapsulate inorganic salt (i.e., KCl), resulting in relatively high residual contents of K+ and Cl−. Lastly, cooling would exert a positive effect on the salting-out phase separation and thus expand the application range of this method. For example, [C2-MIm]HCO2 was prepared at −18 °C, because it has to be phase-separated out by cooling. The basic properties of these ILs were also thoroughly characterized and analyzed, such as melting point, thermal decomposition temperature, density, refractive index, surface tension, viscosity and conductivity etc.