Water: A Very Short Introduction by John Finney

Author:John Finney
Language: eng
Format: epub
ISBN: 9780198708728
Publisher: OUP Oxford
Published: 2015-06-24T04:00:00+00:00

Going supercritical

We have seen that increasing temperature and pressure separately puts increasing strain on the directionality of the hydrogen bonding that dominates the structure of water. So what happens when we increase temperature and pressure together? It is well known that supercritical water (above 374°C and about 220 atmospheres pressure, where the density is about one-third of that of water under ambient conditions) behaves very differently chemically from ‘normal’ water. For example, molecules that are relatively insoluble in water at ambient conditions can become highly soluble, making supercritical water an effective medium for a number of processes such as treatment of hazardous waste and solvent extraction. So it is particularly interesting to try to find out how the structure differs under these more extreme conditions.

Questions we might ask include: how much is the first-neighbour four-fold motif distorted? What are the structural consequences of this? And does the hydrogen-bonding interaction that dominates water structure under normal conditions survive?

With respect to the network structure, the experimental evidence is pretty clear: as temperature is increased above 100°C (but keeping the liquid under increased pressure to prevent evaporation), the water network becomes increasingly distorted. It is largely gone by about 300°C, even though the density may be as high as 80 per cent of that of the ambient liquid. The basic tetrahedrally coordinated motif appears to have pretty well vanished.

With respect to how much hydrogen bonding has survived, the position is less clear, as extracting a clear fingerprint for a hydrogen bond from structural data is not easy. However, using what is thought to be a reasonable geometrical criterion for a hydrogen bond suggests that indeed most (though perhaps not quite all) of the hydrogen bonding has been lost by the time the density has fallen to about two-thirds of its ambient value, a conclusion which is in substantial agreement with that from spectroscopic techniques that pick up the presence of hydrogen bonds in a different way.

So it is no surprise that supercritical water behaves very differently from the water we normally experience.

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