Ionic liquids - the quick guide

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There are about 600 common industrial solvents in use today – and most are bad news environmentally. The development of ionic liquids is a relatively new field that could make some of those solvents obsolete and change the way the chemical industry works. And that’s important. Almost a hidden science these days, chemistry is rarely the stuff of excitement and investment. Until, that is, you look at such big issues as the environment. And that’s why ionic liquids are hot: mostly because they are cool.

What is it? | The technology | Where's the buzz? | Any money in this?

What is it?

Ionic liquids are a new sort of chemistry. Though dating back to 1914, the field has been abuzz with excitement for the last five years or so. The annual output of new scientific papers, often a good guide to where the action is, runs at around 2,000.

Sigma-Aldrich – purveyors of kit, chemicals and other supplies to scientists around the planet – has come up with a neat description. “Ionic liquids are organic salts with melting points under 100 degrees, often even lower than room temperature.”

Come again? Go back to basic chemistry: table salt, or sodium chloride. In that familiar compound, a chlorine atom attracts an electron away from the sodium atom; the two atoms, now oppositely charged ions, cling together. If heated enough, the salt melts and flows. Ionic liquids have different ingredients – often organic – to start with, but they work much the same way. And, most importantly, they often flow at room temperature. That means they can be used in all kinds of practical ways – for instance, binding with heavy metals to purify polluted water. But their properties get stranger still.

A key report, Accelerating Ionic Liquid Commercialization: Research Needs to Advance New Technology, adds: “The thermodynamics and reaction kinetics of processes carried out in ionic liquids are different from those in conventional media.” There are “new opportunities for catalytic reactions, separations, electrochemistry, and combined reaction/separation processes”.

In other words, forget all that stuff you learned at school.

The technology

Ionic liquids can make some industrial solvents unnecessary. Volatile solvents, often organic chemicals, are in the regulators’ sights. The US and the EU are steadily tightening the screws, banning them from ever more products and processes. That is why gloss paint, for example, now comes dissolved in water.

Ionic liquids will dissolve a wide range of organic molecules. Chemists have devised all sorts of clever reactions that can happen at room temperatures in ionic liquids. Better yet, they let you do reactions that just do not work in ordinary solvents. They do not release noxious fumes into the environment. And once you have bought your ionic materials, you just keep on using them over and over again.

But for all the environmental potential, some chemists shy away from using the “green chemistry” tag for ionic liquids. Ionic solvents may not evaporate – as the scientists put it, they have no effective vapour pressure – but you still have to dispose of them at the end of their lives. You can’t just flush them down the drains. You can't even set light to ionic liquids – they don't burn.

Ionic liquids can also be interesting as materials in their own right, not just as something used in processing. The proponents of ionic liquids also reckon that they can create tailor-made solvents that match the needs of a particular reaction process.

Where’s the buzz?

Academics are piling into ionic liquids. One of the best-known groups, publishing more papers than most and top of the citation league, is at Queen’s University Belfast. The team is run by Professor Ken Seddon. He was enthusing about ionic liquids long before anyone else took them seriously. For several years now, the university has run a consortium of academics and businesses under the QUILL banner – it stands for Queen’s University Ionic Liquids Laboratory.

Seddon sees opportunities for ionic liquids in two key areas. “The first is when you have a lousy process that needs improving,” he says. More important, though, is where you have “chemistry that you would like to do, but conventional approaches simply won’t achieve it.”

Seddon’s conviction and enthusiasm, not to mention his science, has sucked in a growing band of chemists from different areas to try their brand of chemistry in ionic liquids. One group plays with enzymes, another with polymers, two of the fundamental areas of chemistry.

By coincidence, a team at another Queen’s University, this time in Canada, recently published a paper with a group from Georgia Institute of Technology in the journal Nature on “switchable solvents”. These new ionic liquids complicate the picture. They are different: Expose them to nitrogen gas, and they change back into regular (non-ionic) liquids. “It’s a potential tool for benign and economical processing in the manufacture of high-value-added specialty chemicals, such as pharmaceuticals,” says Charles Eckert, a professor in the School of Chemical and Biomolecular Engineering at Georgia Tech.

Any money in this?

BASF certainly thinks there is business in ionic liquids. It claims to be the first company to establish a commercial process built around ionic liquids. The company describes BASIL, as it calls it, as “‘smart’ process technology” that “economically solves several chemical processing problems associated with acid production during some chemical reactions”. BASF also has a new business unit, Biosionics, with a raft of products. It also has a partnership with Sigma-Aldrich.

There are start-ups in the sector, most notably in Germany, rare in Europe in still having a large chemistry industry. IoLiTec, based in Aachen, opened for business in November 2003.

Solvent Innovation, based on BioCampus Cologne, is another German start-up in ionic liquids. Degussa has a minor share.
The whole model for the business is that you need an ionic liquid tailored for the chemical process you want to do. Someone has to devise that liquid. In theory, says one of the players, about 10¹⁸ different ionic liquids are conceivable. So far you can buy a few hundred.