Supercritical CO₂

Carbon dioxide has attracted a negative image as a “Greenhouse gas” over the last few years but it can be successfully used as a highly tunable solvent to replace traditional petrochemical solvents. Carbon dioxide is generated from many everyday activities but the growth in the production of biofuels such as bioethanol provides an economic and sustainable supply of high purity CO₂. The use of liquid or supercritical CO₂ provides an alternative to conventional organic solvents in a wide range of applications including extraction and fractionation of botanical materials, reactions with conventional or bio-catalysts, product cleaning and the production of micro particles. As a solvent, CO₂ is universally considered as a clean, green solvent and can be used for the preparation of “organic” extracts in many countries.

Carbon dioxide can be used in the liquid (sub-critical) or supercritical state. A supercritical fluid is a substance above its critical temperature (Tc) and pressure (Pc) which for CO₂ is 304 K (31.1 °C) and 7.4 MPa (73.8 bar), making the supercritical phase easily accessible compared to other solvents. Supercritical CO2 has properties that are intermediate between gaseous and liquid CO₂ exhibiting both high diffusivity and low viscosity with variable density and polarity achieved through changing pressure and temperature.