A small amount of a co-solvent increases the ability of supercritical carbon dioxide to dissolve polar compounds. Neat supercritical CO2 has dissolving properties similar to hexane. This means that, by itself, carbon dioxide is very good for dissolving relatively non-polar materials. The addition of just a small quantity of co-solvent enhances the solubilizing power of the supercritical carbon dioxide, making it possible to extract much more polar molecules. Typical co-solvents include: methanol, ethanol, and water.
Co-solvent addition is typically done using an HPLC type pump. There are two traditional methods a co-solvent pump can be used. Firstly, as a co-solvent doping module where co-solvent is added to the sample to a desired % of the vessel’s overall volume and then the CO2 pump is actuated to bring the vessel up to the desired set pressure for extraction. Secondly, the CO2 and co-solvent pump are both actuated at the same time with the restrictor valve open to maintain a set ratio of co-solvent to CO2 in the sample vessel.
Is there a correlation to use with the flow rate of CO2 to maintain a fixed percentage of ethanol in the vessel? Base the amount of co-solvent in the system as a function of the vessel volume. If your vessel is a 100 ml and you want 5% ethanol in your extraction, you would want to add 5 ml of co-solvent to the vessel before you begin the extraction. Once the dynamic flow/extraction has begun, you want to replace the amount of co-solvent that is flushed out of the vessel with the carbon dioxide, maintaining a 5% level throughout the vessel. A Mass Flow Meter (a simple flow meter will suffice) can be used to gauge the amount of carbon dioxide flowing out of the vessel under your dynamic extraction flow step. Take 5% of this volume of carbon dioxide flow adding back that amount of ethanol to the vessel with the co-solvent pump. This is the least complex method of keeping a constant amount of co-solvent in the vessel throughout your extraction.