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Supercritical gas is a state of matter with properties of both a liquid and a gas, only created above a material’s critical point. It has industrial applications, including extracting caffeine from coffee beans and oxidizing hazardous waste.
Supercritical gas, more correctly known as supercritical fluid, is a distinct state of matter that has properties of both a liquid and a gas. A supercritical gas can be considered an intermediate state between liquids and gases. Supercritical gases are created only above the thermodynamic critical point of a material. This usually means high pressures, often between about 40 and 80 atmospheres, although it varies greatly. Some critical points, especially water, reach 218 atmospheres and above. Supercritical fluids have low viscosity and high diffusivity.
Supercritical gases can diffuse into and through solids like a gas, but also dissolve some materials like a liquid. Supercritical fluids are found in at least two areas of nature. The first is around hydrothermal vents and submarine volcanoes on the ocean floor, where the pressure is so great that the creation of a supercritical gas is possible. The pressure underwater at 3 km (1.8 mi) is 300 atmospheres, more than enough. Another natural supercritical gas is the lower atmosphere of Venus, which is very dense and hot (93 atmospheres, 735 K) at the surface. Oddly, this means that only two states of matter can be found on the surface: solid and supercritical gas. There are no conventional liquids and gases.
Supercritical fluids have at least a dozen industrial applications. One is supercritical fluid mining, where supercritical fluids are used to extract material from a surrounding matrix. This is used to extract the caffeine from the coffee beans. Supercritical carbon dioxide sucks the caffeine right out of them. Similarly, supercritical fluids are used in supercritical drying, which extracts a liquid from a matrix. This is used in the preparation of biological samples for electron microscopy, as well as in the synthesis of airgel, a material which holds 12 Guinness World Records for properties such as lightness and insulation.
Supercritical water oxidation is used to oxidize hazardous waste, making it safe for disposal. The process is also used to synthesize micro- and nanoparticles for various pharmaceutical applications. The reverse of supercritical fluid extraction is called impregnation, whereby a material is introduced into a matrix rather than removed. The list goes on. As you can see, the applications of supercritical fluids are numerous.
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