The principle of the extraction of solids using supercritical CO2 relies on the strong variation of the solvation power of CO2 which occurs by simple variation the operating conditions (temperature and pressure). This allows to selectively extract molecules according to their chemical nature. Very weakly polar, CO2 turns out to be an excellent solvent of nonpolar or small polar molecules in supercritical conditions.
Once the desired compound is dissolved in supercritical CO2, the pure extract can easily be obtained by simple depressurization. This results in the separation of CO2, which becomes gas again,
from the extract, which is recovered in solid or liquid form. In theory, any porous solid material (plants, plastics, wood, can be treated by supercritical fluid extraction to recover valuable
compounds (oils, fragrances, pigments ...) or undesired substances (pollutants, residual solvents ...).
Subcritical water extraction can also be carried out to extract hydrophobic compounds (polyphenols, terpenes, tannins ...) from various plant materials (fruits, wood, algae ...).