In 1822 Cagniard de la Tour and in 1869 Thomas Andrews showed that there is a critical temperature above which a single substance can exist neither as a liquid nor as a gas, but as a fluid in an intermediate state, called a supercritical fluid. At the critical point the densities of both liquid and gas become equal while the distinction between liquid and gas disappears and the substance is now described as a fluid. Gaseous compounds become supercritical when compressed to a pressure higher than the critical pressure (Pc) above the critical temperature (Tc). The term supercritical fluid corresponds to any type of fluid that exists in the region above the critical point which is known as the supercritical region. The most widely used supercritical fluid is carbon dioxide because its supercritical state is easily achieved at Tc of 31ο C and Pc of 73 bars.

​

Supercritical carbon dioxide (scCO2) is non-flammable, non-toxic, inexpensive and environmentally friendly. It has high solubility for non-polar organic molecules. The use of scCO2 allows facile separation of reactants, catalysis and products and can be used as a substitute for environmentally less acceptable solvents. Chemical inertness, its low critical point, excellent wetting characteristics, low viscosity and a highly tuneable solvent behavior are some of the most important physical and chemical properties of supercritical carbon dioxide.