Dear Bethany,
Generally, the specificity of an extraction is enhanced by a co-solvent. There are online calculators to make the data more accessible or tunable than from a Mollier diagram. Here are a couple:
CO2 Calculator—— A web computational tool
Carbon dioxide - Density and Specific Weight
This reference might be what you are looking for:
(PDF) Enzyme-assisted supercritical fluid extraction: an alternative and green technology for non-ex...
Chemically, solvation is an energy-minimization process, as is most of chemistry. Rules-of-thumb like polarity and more rigorous calculation of interaction energies are just trying to describe it. Even a plausible theory will still need the actual lab experiment to see if the predictions match the outcomes.
Generally, any time different chemical species are soluble in the same solvent it is difficult to separate them in solution. Usually a precipitation is required.
Best regards,
Steven
