Sub- and Super-Critical Fluids
There are compressible gases that at a certain temperature and under a certain pressure, collectively their critical point, become fluids that are neither gas nor liquid but rather a “super-critical fluid”. While these fluids have been known for centuries it was only in recent decades that new discoveries showed some of them could be excellent solvents. Carbon dioxide is the most highly studied of these and has been demonstrated at large scale to be a useful solvent for everything from synthesis to extraction, cleaning, and analysis.
The green advantages of these supercritical fluids are numerous including lack of toxicity for water and carbon dioxide, lack of flammability, tunability, and the possibility of ‘infinite recyclability’ by cycling pressure. (Note: The use of carbon dioxide as a solvent does not require the generation of new CO2 and therefore does not contribute to greenhouse gas emissions.)