Supercritical fluids
Thomas Andrews, born 1813, demonstrated the continuity of the gaseous and liquid states showing that during changes between the two states, physical properties display no abrupt changes.
Credit: DOE
Almost 130 years ago, Irish chemist Thomas Andrews gave a lecture to the Royal Society, reporting for the first time that supercritical fluids dissolve some non-volatile solvents. Andrews described the ‘critical point’ as the end of the vapour pressure curve in a phase diagram. At temperatures and pressures above this point, many gases ‘dissolve’ certain solids and liquids.
By 1879, Hannay and Hogarth had characterised the behaviour of supercritical fluids in more detail. During another presentation to the Royal Society, they reported results from a series of experiments during which they dissolved solids with a fusing (melting) point much above the liquid solvent’s critical point. The solid remained diffused in the vapour even when they raised the temperature to some 55°C above the critical point and ‘considerably’ expanded the volume.
Chemists continued to experiment with supercritical fluids for almost a century, although they remained something of a curiosity until the 1970s, when rising energy costs led chemists to consider supercritical fluids as a cheaper alternative to liquid extraction and distillation.
Today, supercritical fluids are employed commercially to decaffeinate coffee and tea as well as to purify essential oils and flavours from herbs and other natural products.
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Excerpted with permission, www.rsc.org/chemistryworld.