Evidence for Density-Dependent Changes in Solute Molar Absorptivities in Supercritical CO₂: Impact on Solubility Determination Practices

Optimization of supercritical fluid extraction conditions requires accurate information on solute solubility under a given set of experimental conditions. In situ electronic absorbance spectroscopy is used commonly to determine solute solubilities, and one assumes that the solute molar absorptivity (ε) is constant over a broad density range when monitoring at a particular wavelength. Using UVvis spectroscopy, we have found that ε for anthracene and pyrene in supercritical CO₂is density dependent Systematic increases in e of 1.3-2.7-fold (30-170%) are observed as CO₂density increases from 0.3 to 0.9 g/cm³123 4567. We account for the observed density dependence in terms of solute-solvent dielectric interactions. These results illustrate the pitfalls associated with using in situ spectroscopic techniques for the determination of solute solubilities in supercritical fluids.