19.3 Supercritical properties and terminology?
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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
19.3 Supercritical properties and terminology?
Supercritical fluids have some very unusual properties. When a compound is
subjected to conditions around the critical point ( which is defined as
the temperature at which the gas will not revert to a liquid regardless how
much pressure is applied ), the properties of the supercritical fluid become
very different to the liquid or the gas phases. In particular, the solubility
behaviour changes. The behaviour is neither that of the liquid or that of the
gas. The transition between liquid and gas can be completely smooth.
The pressure-dependant densities and corresponding Hildebrand solubility
parameters show no break on continuity as the supercritical boundary is
crossed. Physical properties fall between those of a liquid and a gas.
Diffusivities are approximately an order of magnitude higher than the
corresponding liquid, while viscosities are an order of magnitude lower.
These properties ( along with low surface tension ) allow SCFs to have
liquid-like solvating power with the mass transport characteristics of
a gas.
Potential Supercritical Fluids
Compound Critical Critical Density
Temperature Pressure
( C ) ( bar ) (g cm^-3)
Ammonia 132.4 112.8 0.235
Carbon dioxide 30.99 73.75 0.468
CFC-12 111.8 41.25 0.558
Dimethyl ether 126.9 52.7 0.271
Ethane 32.4 49.1 0.212
HCFC-22 96.15 49.90 0.524
HCFC-123 183.68 36.62 0.550
HFC-116 19.7 29.8 0.608
HFC-134a 101.03 40.57 0.508
Methanol 240.1 83.1
Nitrous oxide 36.4 72.54 0.453
Propane 96.8 42.66 0.225
Water 374.4 227.1
Xenon 16.6 58.38 1.105
Nitrous oxide is seldom used because early researchers reported explosions.
Note that using liquid CO2 at pressure ( as for the commercial extraction
of hops ) is still just liquid CO2 extraction, not supercritical CO2
extraction. There are several good general introductions to supercritical
fluids [8,9,10]
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