SOLUBILITIES OF THE XANTHINES CAFFEINE, THEOPHYLLINE AND THEOBROMINE IN SUPERCRITICAL CARBON-DIOXIDE

An apparatus for the experimental determination of the equilibrium solubilities of solids in dense gases is described. Determination of solubilities has been performed by using a static analytical method with direct coupling of an equilibrium cell to a supercritical fluid chromatographic (SFC) system. The solubilities of caffeine, theophylline and theobromine in supercritical carbon dioxide were measured at different temperatures (313, 333 and 353 K) and over a pressure range of 20-35 MPa. Although the chemical structures of the xanthines are very similar, their solubilities in supercritical carbon dioxide vary substantially. The solubilities of caffeine in CO2 are one order of magnitude higher than those of theophylline and two orders of magnitude higher than those of theobromine. Solubility values obtained in this work are compared with published data. Reasons for observed differences are discussed.