I dont expect it to extract an THCa isolate, CO2 isnt specific enough to only target one cannabinoid (it’s always a gradient), but am looking for super or subcritical parameters that will produce a THCa heavy extract from undecarbed plant material. This is where fractionation comes into play.
I’ve heard other people insinuate that subcritical is better for this, and doing it at a low enough solvent temp that it doesnt incidentally decarb the THCa during extraction, but nothing specific beyond that.
This is my best guess at it so far: do a terpene pull first at normal supercritical parameters (we use ~1150psi, 105˚F solvent temp for a solvent density of ~0.269 g/cm^-3). Then increase the solvent density greatly to around 0.9 g/cm^-3 (subcritical) to try to target the cannabinoids. This would be around 1200psi and 50˚F solvent temp. Our cannabis is THCa dominate, so while it will extract many different cannabinoids and cannabinoid acids, hopefully it will be THCa rich. I’m expecting the supercritical fluid to pull waxes/lipids first, so i will likely need to harvest after 1 hour fraction increments at these parameters to see how the extract changes over time. Just not sure if it will pull a lot of other desirables like chlorophyll along the way as it is undecarbed.
going back to my original question, the @Photon_noir post about Extraction & van der Waals Forces is excelllent and extremely thorough and the closest thing i’ve come across on this website that describes what is going on at a molecular level. I’m still trying to decode it 
. He say’s " Simply put, propane and CO2 are also, for these same reasons, harder and much harder , respectively, to induce into polarity". Nonetheless, supercritical CO2 will pull a significant water fraction during a terpene pull. I think this has to do with the “Debye force”, but not sure how to apply that in practice. Despite being a rigid non-polar molecule, it can definitely extract polar molecules, and therefore it is possible to tune the solvent to extact a relatively polar molecule like THCa…