Towards optimizing extraction and purification technologies: Partial molar volumes and solubilities of caffeine and cannabidiol in high-pressure CO2

Both caffeine and cannabidiol (CBD) are target solutes in industrial supercritical CO 2 selective extraction processes. Although solubilities of caffeine in CO 2 have been studied for decades, no apparent molar volume data have been measured, which provides pressure dependence for solubility equilibria. For CBD, solubility data are scarce, and volumetric data unavailable. To this end, apparent molar volumes of caffeine and CBD in CO 2 were measured from T = (298 – 313) K and p = (8.0 – 12.5) MPa; and solubilities of caffeine in CO 2 were measured from T = (298 – 314) K and p = (9.0 – 21.0) MPa. Apparent molar volume minima were observed at the pressure corresponding to the second-order phase transition for each mixture, and molar volumes plateaued at higher pressures. Fluctuation solution theory (FST) was fit to both measured solubility (Caffeine AARD = 0.425, CBD AARD = 0.616) and apparent molar volumes (Caffeine RMSD = 947.59 cm 3 ⋅ mol −1 , CBD RMSD = 395.69 cm 3 ⋅ mol −1 ). A temperature-dependent coefficient was introduced to the FST equation to fit both solubilities and densities. Measured caffeine solubility data were also used to fit to six different empirical models, where the model proposed by Jouyban et al . (2002) showed the best agreement to measurements reported in this study (AARD = 0.13) and in the open literature (AARD = 0.47). A retrograde solubility region is described using FST calculations for both caffeine and CBD in CO 2 , which matches experimental data. This retrograde region could be exploited in future process optimization.