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A supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point. It can diffuse through solids like a gas, and dissolve materials like a liquid. Additionally, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties to be "tuned". Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. Carbon dioxide and water are the most commonly used supercritical fluids.

Figure 1. Carbon dioxide pressure-temperature phase diagram

Figure 2. Carbon dioxide density-pressure phase diagram

In 1822, Baron Charles Cagniard de la Tour discovered the critical point of a substance in his famous cannon barrel experiments. Listening to discontinuities in the sound of a rolling flint ball in a sealed cannon filled with fluids at various temperatures, he observed the critical temperature. Above this temperature, the densities of the liquid and gas phases become equal and the distinction between them disappears, resulting in a single supercritical fluid phase. In Table 1, the critical properties are shown for some components, which are commonly used as supercritical fluids.

The observations by de la Tour can be explained by looking at the phase diagram of a pure component, e.g. carbon dioxide. In Figures 1 and 2, two projections of the phase diagram of carbon dioxide are shown. In the pressure-temperature phase diagram (Fig. 1) the boiling line is observed, which separates the vapor and liquid region and ends in the critical point. At the critical point, the densities of the equilibrium liquid phase and the saturated vapor phases become equal, resulting in the formation of a single supercritical phase. This can be observed in the density-pressure phase diagram for carbon dioxide, as shown in Figure 2, where the critical point is located at 304.1 K and 7.38 MPa (73.8 bar). With increasing temperatures, the liquid-vapor density gap decreases, up to the critical temperature, at which the discontinuity disappears. Thus, above the critical temperature a gas cannot be liquefied by pressure. However, at extremely high pressures the fluid can solidify, as visible at the top of Figure 1. By definition, a supercritical fluid is a substance above both its critical temperature and pressure. In a practical sense, the area of interest in supercritical fluids for processing and separation purposes is limited to temperatures in the vicinity of the critical point, where large gradients in the physical properties are observed. The changes near the critical point are not limited to density. Many other physical properties also show large gradients with pressure near the critical point, e.g. viscosity, the relative permittivity and the solvent strength, which are all closely related to the density. At higher temperatures, the fluid starts to behave like a gas, as can be seen in Figure 2. For carbon dioxide at 400 K, the density increases almost linearly with pressure.

For engineering purposes, supercritical fluids can be regarded as “hybrid solvents” with properties between those of gases and liquids, i.e. a solvent with a low viscosity, high diffusion rates and no surface tension. In the case of supercritical carbon dioxide, the viscosity is in the range of 20–100 µPa·s (0.02-0.1 cP), where liquids have viscosities of approximately 500–1000 µPa·s (0.5-1.0 cP) and gases approximately 10 µPa·s (0.01 cP), respectively. Diffusivities of solutes in supercritical carbon dioxide are up to a factor 10 higher than in liquid solvents. Additionally, these properties are strongly pressure-dependent in the vicinity of the critical point, making supercritical fluids highly tunable solvents. Of the components shown in Table 1, carbon dioxide and water are the most frequently used in a wide range of applications, including extractions, dry cleaning and chemical waste disposal. In polymer systems, ethylene and propylene are also widely used, where they act both as a solvent and as the reacting monomer.

One of the most important properties of supercritical fluids is that their solvating properties are a complex function of their pressure and temperature, independent of their density. This means that (taking a very simplistic approach) raw materials containing CO₂ soluble products can be selectively extracted or selectively precipitated to obtain ultra-pure extracts. Although the details are much more complex than this, it remains the dominant chemical-free technology for the production of decaffeinated coffee, nicotine-free tobacco, and many of the world's best spice extracts.

Supercritical water/steam is used as the working fluid in many new steam-electric power plants, where they offer very high thermal efficiency. Supercritical water reactors (SCWRs) are promising advanced nuclear systems that offer similar thermal efficiency gains.

• Supercritical carbon dioxide

1. R.C. Reid, J.M. Prausnitz and B.E. Poling, The properties of gases and liquids, 4th ed., McGraw-Hill, New York, 1987.
2. W. Wagner and A. Kruse, Properties of Water and Steam, Springer-Verlag, Berlin, 1998.

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