That there is a critical temperature above which a single substance can only exist as a fluid and not as either a liquid or gas, was shown experimentally 180 years ago by Baron Charles Cagniard de la Tour. He heated substances, present as both liquid and vapour, in a sealed cannon, which he rocked back and forth and discovered that, at a certain temperature, the splashing ceased. Later he constructed a glass apparatus in which the phenomenon could be more directly observed.


The phase diagram of a single substance

These phenomena can be put into context by the figure above, which is a phase diagram of a single substance, such as pure carbon dioxide. The diagram is schematic, the pressure axis non-linear and the solid phase at high temperatures occurs at very high pressures. The areas where the substance exists as a single solid, liquid or gas phase are labelled; as is the triple point where the three phases coexist. The curves represent co-existence between two of the phases.

If we move upwards along the gas-liquid co-existence curve, which is a plot of vapour pressure versus temperature, as shown in red in the figure above, both temperature and pressure increase. The liquid becomes less dense because of thermal expansion and the gas becomes denser as the pressure rises. Eventually, the densities of the two phases become identical, the distinction between the gas and the liquid disappears and the curve comes to an end at the critical point. The substance is now described as a fluid. The critical point has pressure and temperature co-ordinates on the phase diagram, which are referred to as the critical temperature, Tc, and the critical pressure, pc.

The disappearance of the distinction between the liquid and gas phases can be graphically illustrated by conducting a modern version of the Cagniard de la Tour experiment in which the meniscus between a liquid and a gas in a view cell disappears at the critical temperature. The figure below shows three schematic representations of a view cell in which this experiment is conducted at appropriate points on the liquid-gas co-existence curve.


Disappearance of the meniscus at the critical point

Cell (a) is at the lowest temperature and shows the liquid and gas phases with a meniscus between them. As the temperature and pressure rise and the density difference between the two phases becomes less, the meniscus becomes less distinct, as shown in cell (b). In practice the meniscus is no longer flat, because of temperature fluctuations and the small density difference. When the critical point is passed the meniscus disappears altogether, as shown in cell (c). Photographs of a real experiment of this type are shown below, in which the effects described can be seen in spite of the thermal turbulence present.

In recent years, fluids have been exploited above their critical temperatures and pressures and the term supercritical fluids has been coined to describe these media. The greatest advantages of supercritical fluids occur typically not too far above their critical temperatures. One compound, carbon dioxide (critical temperature 31°C and pressure 74 bar), has so far been the most widely used, because of its convenient critical temperature, cheapness, chemical stability, non-flammability, stability in radioactive applications and non-toxicity. Large amounts of CO2 released accidentally could constitute a working hazard, given its tendency to blanket the ground, but hazard detectors are available. It is an environmentally friendly substitute for other organic solvents. The CO2 that is used is obtained in large quantities as a by-product of fermentation, combustion, and ammonia synthesis and would be released into the atmosphere sooner rather than later, if it were not used as a supercritical fluid. The polar character of carbon dioxide as a solvent is intermediate between a truly non-polar solvent such as hexane and weakly polar solvents.

Although often pursued in practice for environmental reasons, the more fundamental interest in supercritical fluids arises because they can have properties intermediate between those of typical gases and liquids. Compared with liquids, densities and viscosities are less and diffusivities greater. The conditions may be optimum for a particular process or experiment. Furthermore, properties are controllable by both pressure and temperature and the extra degree of freedom, compared with a liquid, can mean that more than one property can be optimized. Any advantage has to be weighed against the cost and inconvenience of the higher pressures needed. Consequently, applications of supercritical fluids take place in particular niche areas.

Thermodynamic properties of supercritical fluids are discussed and a facility for calculating thermodynamic properties of carbon dioxide given on other pages. Carbon dioxide under certain conditions runs a small risk of causing catastrophic explosions, known as BLEVEs, and these conditions should be prudently avoided. Calculation of BLEVE conditions for carbon dioxide is discussed on another page.