Phase transition diagrams of ionic liquids with liquid/gas phase boundaries.

The article focuses on the experimental determination and analysis of liquid-to-gas phase transition diagrams of ionic liquids (ILs) by constructing saturation vapor pressure curves under reduced pressure. Using a vacuum-level tunable thermogravimetry (TG) setup and a method to evaluate evaporation coefficients based on the Hertz–Knudsen–Langmuir equation, the study overcomes challenges related to the low volatility of ILs and the difficulty in measuring their evaporation dynamics. The evaporation coefficient was found to depend strongly on pressure but showed minimal dependency on molecular structure or temperature, enabling accurate extraction of saturation vapor pressures and boiling points of various ILs. The resulting phase diagrams provide valuable thermodynamic data for ILs, confirming their extremely low vapor pressures at ambient conditions and supporting their potential applications in vacuum and space environments. This work establishes a methodological baseline for further thermodynamic studies and computational predictions of IL evaporation behavior.