Lasting salty solution

March 31, 2000

Liquid salts used as solvents could revolutionise industrial processes. Geoff Watts looks at work on the reusable, green alternative.

Leave the top off a jar, barrel or tank of dry-cleaning fluid, nail varnish re-mover or any other solvent and your liquid asset will, likely as not, begin to evaporate. In the home, this is irritating; industrially, it represents economic waste and environmental pollution on a vast scale. No less than a third of all the solvents used by industry simply vanish into thin air.

A hitherto obscure group of chemicals has the potential to revolutionise industrial solvents and avoid this waste. "Ionic liquids" are salts with a very low melting point. At room temperature most of the salts we are familiar with, including the sodium chloride we sprinkle on our chips, are solid. They eventually liquefy on heating, but usually at very high temperature - something like 800x C in the case of common table salt.

At London's Imperial College, chemistry lecturer Tom Welton can show you bottles of salts that are liquid at ordinary, everyday temperatures. "In nature there aren't many ionic liquids to be found," says Dr Welton. "But we have managed to make hundreds of them. They used to be a curiosity. But when people realised they might be useful, they started looking for them."

Ionic liquids make splendid solvents. Domestically, we use solvents for cleaning, thinning or dissolving things. Industrially, they play an essential role in almost every chemical process. "If you want two chemicals to react, you can't just put a pile of one powder with a pile of another powder and hope they link together," says Dr Welton. "For a chemical reaction to occur, the molecules have to come into close physical contact. To get them to mix intimately on a molecular level, you dissolve them in a solvent."

Water, of course, is a useful solvent: cheap and easily available. But it will not dissolve everything, and it has other drawbacks. "It's very reactive," says Dr Welton, "especially with the kind of materials that the chemical industry is interested in. And because water is a natural solvent that's around us all the time, it's difficult to contain. If you have a spillage of a water solution, you have a real danger that it will mix with water in the local environment."

Water is at least non-toxic; many other common solvents are not. "On the whole," Dr Welton points out, "solvents are small molecules that are not bound to each other very strongly. So they're often volatile. Every time you smell a solvent it's because molecules of it have got inside your nose."

Regularly inhaling these small and reactive compounds may not, in the long term, do you a lot of good. Industry needs solvents with none of the drawbacks of water and the rest of the molecular solvents. The properties of ionic liquids make them well-suited to slaking at least part of that commercial thirst.

Like their solid counterparts, liquid salts are made up of positively and negatively charged sub-units, or ions. In salt, the sodium ions have the positive charge and the chloride ions the negative. "Because of the way that ions bind together, they can't escape from each other," says Dr Welton. "At room temperature they don't evaporate at all."

At Queen's University, Belfast, Ken Seddon has spent the past couple of decades working on ionic liquids. His aim is to take them from the laboratory to the industrial pilot plant. His group, the Queen's University Ionic Liquid Laboratories (Quill) has the backing of a score of chemical and drugs companies, each contributing Pounds 20,000 annually. In return for this, they get a say in how Quill targets its research.

There is general agreement among academics that industry is now alive to the potential of ionic liquids. The French Petroleum Institute has developed a pilot plant that uses them in a process for linking small molecules to form larger ones.

Professor Seddon says that many standard chemical processes operate as well or even better when the reactants are dissolved in ionic liquids. For example, chemical reactions that need to be catalysed will often occur at a lower temperature and with a better yield if the reactions takes place in an ionic solvent. At 80x C the Friedel-Crafts reaction - a key step in, among other things, the cracking of petroleum - takes eight hours and produces an 80 per cent yield. Using ionic liquids at 0x C, the same reaction will take place in 30 seconds with a 98 per cent yield of a more homogeneous product.

Charles Gordon, of the department of pure and applied chemistry at Strathclyde University, sees ionic liquids as having promise in many important chemical processes, especially catalytic reactions. But he does not envisage a complete switch. "I don't think they'll ever be cheaper than organic solvents," he says. "At the moment they're maybe ten to twenty times as expensive. On the other hand, they are easier to re-use."

Ionic liquids need not be lost or destroyed during use. "Therefore they can be considered as capital expenditure rather than as a consumable," says Professor Seddon.

Ionic liquids have become one of the players in what is referred to as "green chemistry" - less polluting and more sustainable. This has an obvious appeal to an industry all too aware of the public's suspicion of its activities.

Ionic liquids do have drawbacks. With most solvents, recovering whatever has been dissolved is straightforward: warm the mixture and the solvent will evaporate. Extracting something dissolved in an ionic solvent is not so simple. That said, Dr Welton has few doubts about the potential of ionic liquids - not least because of the extent to which their properties can be manipulated according to need. They are, he believes, the solvents in which many an industrial problem will eventually dissolve.

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