The Science Behind CO2
As cannabis connoisseurs seek higher purity and greater quality, previously obscure extraction methods have come into vogue for making different, and better, cannabis concentrates. One such method is supercritical fluid extraction, used for making so-called “CO2 oil.” As dabbers become more terpene-conscious, extract artists are using methods that allow for higher terpenes contents, with CO2 supercritical fluid extraction at the forefront of cannabis extraction.
Supercritical fluid extraction (SFE) uses the strange properties of gases that have been compressed beyond their “critical point.” Carbon dioxide (CO2) is the most frequently used gas because its critical point can be reached at around 90 °F, cool enough that delicate plant terpenes and cannabinoids don’t get deactivated. Furthermore, the shape of the CO2 molecule allows it to act as a nonpolar solvent, in the same way butane or hexane are nonpolar.
A supercritical fluid behaves both like a liquid and a gas; it diffuses through solids like a gas but dissolves compounds like a liquid. The fluid has almost no surface tension, so it easily penetrates the cuticle of cannabis’ trichomes dissolving all the oils in its path. If you want to see what supercritical CO2 (scCO2) looks like, go to this video. As the temperature rises the carbon dioxide becomes supercritical at 0:35 as the solution becomes turbid and opaque.
The dissolving powers of scCO2 change with different pressures; this allows the extractor to use different vessels at different pressures to separate extractions that have different components. Playing with the pressure allows complete separation of terpenes and cannabinoids, and everything in between.
Supercritical carbon dioxide runs through an extraction vessel at a certain temperature and pressure, after gathering the available terpenes, waxes and cannabinoids the solution passes to a separator vessel (under different condition of temperature and pressure) that can be used to change the composition of the extract. After the extraction is complete, a drop in pressure allows the CO2 to easily evaporate and be recovered.
As a general rule SFE using scCO2 gives extracts with a higher terpene content than typical BHO dabs. Whereas BHO might have anywhere between 0.5 % to 3.5 % terpenes depending on how it’s made, CO2 oil for consumption generally has around 8 -10 % terpenes by mass. One study from the University of Udine in Italy experimented with SFE to extract aroma compounds from industrial hemp for their use in cosmetics. With the focus on terpenes, not cannabinoids, they found that heavier sesquiterpenes like caryophyllene and humulene were less soluble at lower pressures. SFE allows an extractor to vary the terpene content depending on the setup.
Interestingly enough, researchers in the University of Leiden in Holland discovered that THC is the least soluble cannabinoid in scCO2, followed by CBG and CBD with CBN having the highest solubility. The highest concentration of THC they obtained in an extract was 50 %, but they started using a Bedrocan variety of cannabis that only had 13 % THC.
Researchers in the University of the Basque Country in Spain used real high-THC-producing strains of cannabis (Amnesia, AK-47, Somango and Critical) and experimented with different solvent systems. By using a supercritical fluid mixture of CO2 and ethanol they were able to extract around 95 % of the available THC in the first run.
Unpublished methods in the medical marijuana industry have been optimized for making cannabis extracts. Since SFE extracts more terpenes, the extracts are generally less viscous and more fluid then BHO, making them perfect for vaporizer pens.
Besides the advantages that SFE offers as far as selectivity (the ability to chose extracts with different components from the same starting material), CO2 is a “green solvent” unlike butane or hexane, which are petroleum derivatives.