[BMIM]PF6, or 1-butyl-3-methylimidazolium hexafluorophosphate, is commonly used as an ionic liquid. As has been shown by the accepted answer to this question, sodium chloride that has been made to boil actually exists either as a monomer (Molecule of NaCl), or a dimer, rather than ions, in the gas phase. What happens to 1-butyl-3-methylimidazolium hexafluorophosphate, or any other ionic liquid, that has been boiled? Would it, like NaCl, also exist as a monomer? What would happen to an ionic liquid that has been placed into a vacuum chamber from which air had been removed?


2 Answers 2


Ionic liquids have very special properties, the most important of those of interest is that they are liquids at low temperature with extremely low vapour pressure. This is in fact the main reason they have caused excitement among chemists. Quoting (1) from a report comparing the volatility of various ILs:

An extremely low vapor pressure (e.g., ca. 100 pPa at 298 K for [C4mim][PF6] 1 compared with 3 kPa at 298 K for H2O 2) is one of the extraordinary properties of room temperature ionic liquids (RTILs), i.e., molten salts with melting points below 100◦C. As a consequence, RTILs such as [C4mim][PF6] at 298 K are liquids which do not evaporate significantly even under ultrahigh vacuum (UHV) conditions (i.e., for a pressure range 100 nPa...100 pPa [3]), which offers the possibility to use RTILs, e.g., as substitutes for volatile organic solvents [4, 5].

When an IL vaporizes it does so primarily as a neutral species, just like in the case of sodium chloride in the linked post, because charge separation (separation of oppositely charged ions) consumes a lot of energy.

One challenge in studying the vapor of ILs is that they have very high boiling points (typically >500K, see Ref 2) and degrade at higher T. By measuring the vapor pressure over broad T ranges it has been possible however to estimate the normal boiling point of some ILs by extrapolation, as explained in Ref. (3):

Moreover, due to the extremely broad temperature range, the estimation of the boiling temperature of [EMIm][NTf2] from thefit of the Clarke–Glew-equation can be considered as the first reliable boiling point of the archetypical ionic liquid obtained from the experimental vapor pressures measured in the most possible close proximity to the normal boiling temperature.


  1. http://arxiv.org/abs/1006.2090v1
  2. F. Mozaffari, Journal of Molecular Liquids 209 (2015) 657–661
  3. Ahrenberg et al., Phys. Chem. Chem. Phys., 2016,18, 21381-21390
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    $\begingroup$ Very interesting, I hadn't seen a measured vapor pressure of an ionic liquid before. Now if only I can find the vapor pressure of a polymer melt... $\endgroup$ Commented Jul 9, 2020 at 22:59
  • $\begingroup$ @nicolau it does not have to be ionic. Check out what happens when we try to render the triple-point pressure of gallium. $\endgroup$ Commented Jan 6, 2022 at 18:46

Ionic liquids are not magical liquids. They are just typical organic compounds with certain properties. These compounds generated a storm in the tea cup in the late 1990s and 2000s, but they are as toxic as other solvents. Certainly they have niche applications especially as a gas chromatography stationary phase, where they can be used at high temperature (up to 350 centigrade!).

Now you can guess based on the GC application, that ionic liquids cannot be distilled or vaporized. They start to decompose by complex path ways above these temperatures. We can easily see that in a gas chromatograph. Surprisingly, not much has been studied as what happens to ILs at high temperatures.

Only one recent study exists Dicationic ionic liquid thermal decomposition pathways. There are complex decomposition products in the absence of oxygen. Like any organic compound, they will burn in the presence of air. NaCl is another story. It is not an organic compound. In short do not compare NaCl with ionic liquids.


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