Experimental Apparent Vapor Pressures and Thermal Behavior of Carboxylate Protic Ionic Liquids

Protic ionic liquids (PILs), which are based on amines and carboxylic acids, are known to have lower thermal stability than other classes of ionic liquids (ILs). Moreover, unlike conventional ILs, some PILs possess non-negligible vapor pressures, which extend their applicability to processes where distillability is required. However, despite their distillability, vapor pressure data for PILs remains scarce in the literature. In this work, the apparent vapor pressure and thermal behavior of four PILs were investigated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Both techniques provide a practical alternative to complex vapor pressure measurement techniques, enabling rapid screening with minimal sample requirements. The apparent vapor pressure was more affected by the cation; diethanolammonium PILs had lower vapor pressure than monoethanolammonium PILs. Additionally, observations of mass change rates in TGA provided estimates of decomposition temperatures and supported the formation of amides as the most probable decomposition products.