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.

Supercritical CO2 1

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.

Liquid and supercritical CO₂ are both weak, aprotic solvents and a comparison of physical properties with other commonly used extraction solvents shows that for most parameters liquid and supercritical CO₂ compare most closely to n-hexane. The range of molecules that can be extracted is more limited than with traditional organic solvents because of its low polarity, but this can be mitigated by the use of more polar co-solvents. Ethanol and water are most commonly used as co-solvents at Suprex. The low polarity of the liquid and supercritical CO₂ however allows selective extraction of less polar molecules. Sequential extraction of fractions at increasing pressure and temperature followed by the use of a co-solvent can enable relatively pure extracts containing a narrow range of compounds to be produced. For reactions, the stability and unreactivity of CO₂ provides a versatile and tuneable solvent.

Supercritical CO2 2

From a legislative perspective carbon dioxide is universally accepted as a process solvent including for organic processing as illustrated by the EU regulations (Directive 88/344/EEC- as amended) summarised above and the Soil Association (clause 40.8.8 v16.4, 2011) regulations. Many of the current and new applications for supercritical CO₂ are driven by the replacement of current VOC usage and in many cases this results in a higher quality product without solvent residues. As the demand for natural products increases and the regulations on the use and disposal of solvents becomes more onerous, supercritical CO₂ presents a sustainable , economic and completely scalable alternative solvent.