Supercritical fluids

A supercritical fluid is obtained when a fluid is handled above a certain pressure (critical pressure) and above a certain temperature (critical temperature). These two parameters define the so-called critical point of a fluid which is a substance-specific property. The critical point is where the boiling point curve and condensing point curve of a two-phase system meet. Consequently below this critical point a component can be transferred from the solid phase to the liquid phase and subsequently to the gaseous phase by increasing the temperature at constant pressure. The phase changes in this case are all connected to a certain energy which is needed to change the state of the component. Above the critical point there are no different phases and thus no distinction between liquid or gas. In this supercritical area some of the properties of the fluid are comparable to the gaseous phase, others to the liquid phase and can be adjusted with changing pressure and temperature.

Supercritical fluids can replace conventional solvents such as methanol, ethanol, methylene chloride etc. Since some of these solvents are toxic, efforts are now being made to avoid their use. An alternative to these solvents is provided especially by supercritical CO₂. Substances dissolve at a much higher rate in supercritical fluids than would normally be expected at a given vapour pressure. CO₂ has proven to be particularly well suited for the extraction of natural substances.

The low critical temperature of 31 °C allows gentle treatment of natural substances. Moreover, CO₂ is not only readily available, but may also be obtained at relatively low prices.