Ionic liquids in the gas phase - Raman spectroscopy reveals molecular secret

Green solvents

At first glance ionic liquids would not seem unusual, heat sodium chloride to a high enough temperature and it will melt. But, ionic materials that are liquid at room temperature are much stranger. Room temperature ionic liquids (RTILs) have high lattice energy because they are composed of bulky ions that find it difficult to crystallise. Their ionic properties have, however, led chemists to choose them as non-volatile, non-combustible and generally non-toxic solvents to replace volatile organic compounds (VOCs). Their ionic character means that they can dissolve a wide range of materials as well as many that are insoluble in any conventional solvent organic or otherwise.

A wide range of RTILs have been studied over the last few years. They can be formed by coupling together two bulky ions. With so many possibilities it is almost possible to create a RTIL by design for a given application. However, there are still a few gaps in our understanding of these intriguing liquids.

Gas phase ionic liquids

Now, a gas-phase Raman study has been used to offer a clearer understanding of the nature of ionic liquids. The study reveals that in the gas phase each ion of the pair exists as a distinct molecule.

Ionic liquid pioneer Kenneth Seddon of The QUILL Centre, at Queen's University Belfast and colleagues there and in the Department of Chemistry, at the Technical University of Denmark, in Lyngby, Denmark, the Instituto de Tecnologia Qumica e Biologica, Universidade Nova de Lisboa, in Oeiras, Portugal, and the Department of Materials Science & Engineering, at Massachusetts Institute of Technology, Cambridge, Massachusetts, USA, have investigated the compound 1-methylimidazolium ethanoate, [Hmim][O₂CCH₃]. Gas phase Raman spectra identified several bands which were assigned and then interpreted using ab initio quantum mechanical calculations to predict the vibrational spectra. "The obtained data reinforce a previous interpretation, based on FT-ICR (Fourier transform ion cyclotron resonance) mass spectrometric data, that the vapour phase over [Hmim][O₂CCH₃] consists predominantly of two neutral molecules, monomeric ethanoic acid and 1-methylimidazole," the team explains. The researchers add that Raman has a major advantage over other spectroscopic techniques in that it can identify discrete species in situ as well as having the ability to function with sealed glass ampoules.

A class apart

The team explains that there are two classes of ionic liquids: aprotic ionic liquids which are composed solely of ions and protic ionic liquids in which the cation ([BH]⁺) and the anion (A^-) can exchange a hydrogen nucleus, a proton. For protic ionic liquids there exists an equilibrium between the species in which two neutral species (the conjugate base, B, and acid, AH) are thought to exist. Both classes are distinct from ionic solutions which contain a solvent that might evaporate independently of the ions, for ionic liquids there is no additional "solvent". It is the class of ionic liquid that the team hoped to investigate with Raman spectroscopy because the distinction between protic and aprotic is not entirely clear.

"Some protic ionic liquids present such a low equilibrium constant that, in practice, they are effectively fully ionized," the team explains, "others have such a high equilibrium constant that they are effectively a mixture of two molecular species."

The issue is not simply one of semantics. One of the main benefits of using RTILs as "green" solvents is due to their low vapour pressure, which means they do not have a tendency to evaporate. However, although most ionic liquids have extremely low vapour pressures even when warmed, some ionic liquids, the protic type in particular, do have a measurable vapour pressure. "Moreover," the team says, "their vapour pressures, and consequently their boiling points, strongly depend on the overall composition of the liquid, as determined by the equilibrium constant."

"Our results constitute the first spectroscopic study on the composition of the vapour of [Hmim][O₂CCH₃] and the work is a representative example of a protic ionic liquid in equilibrium with its vapour," the team says.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.