The supercritical fluid commonly utilized is carbon dioxide (CO2). The state diagram of CO2 visualizes the various phases (solid, liquid, gas) depending on pressure and temperature. CO2, at 31,1°C and 73,8 bar, is in its supercritical state, in which there is no distinction between liquid and gaseous phases (as shown in the following picture).
Increasing the temperature and keeping the pressure constant (73,8 bar), CO2 remains in the supercritical state, and so happens when pressure is increased and temperature is constant (31,1°C): this individuates two rays – respectively parallel to the pressure and temperature axes – defining the zone in which CO2 is in the supercritical state; in particular, within this state, the possible combinations of pressure and temperature are shown to variate CO2 solubilizing properties. The reasons for the choice of this supercritical solvent are of economic (CO2 is cheap), environmental (CO2 is not toxic, it does not harm the ozone layer, it does not pollute and it does not contaminate the extracts) and technical (CO2 critical conditions can be reached easily) concern. SC-CO2 assumes the characteristics of a non polar solvent and it is comparable to n-Hexane; it has the characteristic to solubilize compounds which are scarcely soluble in water due to their nature.