A supercritical fluid is a substance that has reached sufficiently high temperature and pressure to be beyond a “critical point” on the phase diagram (see “Macroscopic Properties”). This “critical” value of temperature and pressure will be different for each substance, and it corresponds to a set of conditions beyond which the distinction between the liquid and gas phases of matter is no longer clear. That is to say, beyond this value, the density and other properties of the substance can be changed continuously, and readily, with changes in the temperature and pressure of the system.

This makes for useful green solvents because having the ability to tune the density, solubility properties, and diffusivity of the supercritical fluid allows for reaction or extraction conditions to be sensitively manipulated.

Let’s consider one supercritical fluid that has drawn particular interest: carbon dioxide, or CO₂. Some of the advantages of CO₂ as a supercritical fluid are that it cannot be oxidized, it is aprotic, and it does not tend to participate in reactions involving free radicals. This makes carbon dioxide robust toward undergoing chemical reactions itself, and it also means that it is relatively benign as a contaminant (ignoring its role as a greenhouse gas, for the moment). However, CO₂ is a gas at ambient temperature and pressure, and thus, to serve as a good solvent, it must be used at elevated pressures and/or temperatures. At varied temperatures and pressures, supercritical CO₂ is also capable of dissolving a wide range of chemical compounds and is miscible with gases in almost any proportion. Supercritical CO₂ can also often be recycled as a solvent and thus does not tend to generate large amounts of waste. Indeed, carbon dioxide can also be used as a solvent in its liquid form, but it then loses several of the advantages with regard to the tunability of its properties that we mentioned above.