Caffeine solubility in supercritical carbon dioxide/co-solvent mixtures

In order to assess the effect of co-solvents on the solubility of caffeine in supercritical carbon dioxide, experimental solubility of caffeine in supercritical ethanol–carbon dioxide and isopropanol–carbon dioxide mixed solvents was obtained using a high-pressure semi-continuous flow apparatus. Caffeine solubilities in 5% ethanol/95% CO2, 10% ethanol/90% CO2 and 5% isopropanol/95% CO2 mixed solvents were determined at 323.2 and 343.2 K and pressures ranging from 15.2 to 30.0 MPa. The results indicate much higher solubilities of caffeine in these mixed solvents than those obtained in pure carbon dioxide. The large solubilities are attributed to the resultant chemical interaction forces (hydrogen bonding) and increase in dispersion forces in these systems upon the addition of co-solvents. The solubility data also show that the effect of co-solvents is more pronounced as the critical point of the mixed solvent is approached due to the higher compressibility of the fluid at this condition. The higher solubility enhancement with the addition of ethanol as compared to that with the addition of an equal amount (by weight) of isopropanol to carbon dioxide is related to the higher number of hydroxyl groups available for hydrogen bonding and possible steric effects in systems containing isopropanol.