Carbon dioxide has had a bad press - as the climate-change negotiators meeting in The Hague this week can testify - but chemists may have found a new and important use for this waste gas, at least when it can be compressed enough to form an unusual liquid with intriguing properties. Carbon dioxide becomes really interesting under high pressure when it turns into a liquid - called supercritical carbon dioxide, or sc-CO2. In this state, CO2 has some remarkable properties as a solvent. And with carbon dioxide in plentiful supply - most of it comes from industrial fermentation processes - chemists are cashing in on sc-CO2's solvent properties to turn vital but energy- consuming and polluting industrial processes into environmentally friendly chemical reactions.
Apart from the fact that every breathing creature on earth exhales it, carbon dioxide is best known for two reasons: global warming and rock concerts. Though its growing concentration in the atmosphere from fossil- fuel burning is causing an increased greenhouse effect with rising average global temperatures, carbon dioxide's popularity with rock bands stems from one of its more bizarre properties. At around -80C, carbon dioxide freezes to a white solid, "dry-ice" or "cardice".
However, above this temperature dry-ice does not melt to a clear liquid, like ordinary ice. Instead, it turns directly into a cloudy gas (chemists say it sublimes), and, being heavier than air, it drops rapidly to the floor, spreading like some cold, eerie ectoplasm. Combined with a laser light show, the effect is, well, gothic.
Unlike water, carbon dioxide has no liquid state at ordinary atmospheric pressures, only solid and gas. But under high pressure (just over 70 atmospheres), at 31C carbon dioxide turns into something halfway between a liquid and a gas.
So, like a liquid sc-CO2 behaves as a solvent (gases can't do this), but like a gas, it easily spreads over a surface, getting into every nook and cranny much more easily than ordinary liquids.
Both these properties have already been exploited; sc-CO2 is being used to decaffeinate coffee and clean intricate machinery of oily grime. The big advantage here, other than its inertness and non- inflammability compared with the usual organic cleaning fluids, is that, when its work is done, easing off the pressure turns sc-CO2 back into a gas, leaving behind the dissolved substance (caffeine or grime) and no environmentally-difficult solvent waste to dispose of.
The gas can then be recompressed into a liquid, ready to use again, so it is not added to the CO2 already in the atmosphere. Chemists are now finding ways to exploit sc-CO2's solvent properties to make their reactions much more environmentally friendly. Chemistry, because of the industries based around it, is one of the major wealth producers of any industrialised nation. To give some idea how important chemistry is, prior to its break- up, the performance of ICI was a major indicator of the UK economy's health.
The crucial point is that most chemistry is about what happens when chemicals react together in solution. For this, solvents are necessary: they dissolve chemicals, ensuring that they thoroughly mix and react. The problem for chemists is what to do with waste solvents after a reaction. Because they are usually toxic, volatile, sometimes flammable liquids, not easily managed or contained and used in huge quantities, most of the solvents in general use can damage the …