Thermodynamics of ionic liquid evaporation under vacuum

Area of Science:

• Physical Chemistry
• Materials Science

Background:

• The low volatility of ionic liquids (ILs) is a key property, but their evaporation dynamics under vacuum are not well understood.
• Understanding IL evaporation is crucial for applications requiring high thermal stability.

Purpose of the Study:

• To investigate the thermodynamics of ionic liquid evaporation under vacuum.
• To quantify key thermodynamic parameters and model the vaporized IL structure.
• To establish relationships between IL structure and evaporation behavior.

Main Methods:

• Thermogravimetry (TG) measurements under vacuum were used to study IL evaporation.
• Analysis of TG data to determine evaporation onset temperatures, enthalpies, entropies, and saturation vapor pressures.
• Thermodynamic modeling using referential data and IL evaporation entropies to determine vaporized IL structure.

Main Results:

• Evaporation enthalpies of ILs (110-140 kJ mol-1) were higher than typical molecular liquids.
• Entropies (>88 J mol-1 K-1) indicated ILs are exceptions to Trouton's rule.
• ILs were found to evaporate as intact ion pairs.
• Evaporation dynamics are systematically affected by IL chemical structure, molecular weight, and charge balance.

Conclusions:

• Ionic liquids evaporate as ion pairs under vacuum.
• A method and empirical equation were developed to estimate IL liquid phase upper limit temperature.
• The findings provide insights into IL thermal stability and aid in designing more stable ILs.