Volatile Times for the Very First Ionic Liquid: Understanding the Vapor Pressures and Enthalpies of Vaporization of Ethylammonium Nitrate

• Published in: Chemistry : a European journal 2014-09, Vol.20 (37), p.11640-11645
• Main Authors: Emel'yanenko, Vladimir N., Boeck, Gisela, Verevkin, Sergey P., Ludwig, Ralf
• Format: Article
• Language: English
• Subjects: Boiling points; Chemistry; DFT calculations; Electrolytes; Evaporation; Fuel cells; Ion pairs; Ionic liquids; Ions; protic ionic liquids; proton transfer; Solvents; Vapor pressure; Vaporization; Vapour pressure; volatility
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201403508

A hundred years ago, Paul Walden studied ethyl ammonium nitrate (EAN), which became the first widely known ionic liquid. Although EAN has been investigated extensively, some important issues still have not been addressed; they are now tackled in this communication. By combining experimental thermogravimetric analysis with time of flight mass spectrometry (TGA‐ToF‐MS) and transpiration method with theoretical methods, we clarify the volatilisation of EAN from ambient to elevated temperatures. It was observed that up to 419 K, EAN evaporates as contact‐ion pairs leading to very low vapour pressures of a few Pascal. Starting from 419 K, the decomposition to nitric acid and ethylamine becomes more thermodynamically favourable than proton transfer. This finding was supported by DFT calculations, which provide the free energies of all possible gas‐phase species, and show that neutral molecules dominate over ion pairs above 500 K, an observation that is in nearly prefect agreement with the experimental boiling point of 513 K. This result is crucial for the ongoing practical applications of protic ionic liquids such as electrolytes for batteries and fuel cells because, in contrast to high‐boiling conventional solvents, EAN exhibits no significant vapour pressure below 419 K and this property fulfils the requirements for the thermal behaviour of safe electrolytes. Overall, EAN shows the same barely measurable vapour pressures as typical aprotic ionic liquids at temperatures only 70 K lower. Up in the air! How the first reported ionic liquid evaporates into the gas phase has been studied by combined experimental and theoretical methods to provide evidence that ethylammonium nitrate (EAN) evaporates as ion pairs up to 419 K. Above this temperature, enhanced proton transfer from the cation to the anion leads to the evaporation of molecules into the gas phase. The high enthalpies of vaporization and the relatively low boiling point can thus be explained at the molecular level.