Solubility of Caffeine in Supercritical CO2: A Molecular Dynamics Simulation Study.

The extraction of caffeine from green tea leaves and cocoa beans is a common industrial process for the production of decaffeinated beverages and pharmaceuticals. The choice of the solvent critically determines the yield of this extraction process. Being an environmentally benign and recyclable solvent, supercritical carbon dioxide (scCO2) has emerged as the most desirable green solvent for caffeine extraction. The present study investigates the solvation properties of caffeine in scCO2 at two different temperatures (318 and 350 K) using molecular dynamics simulations. Unlike in water, the caffeine molecules in scCO2 do not aggregate to form clusters due to relatively stronger caffeine-CO2 interactions. A well-structured scCO2 solvent shell envelops each caffeine molecule as a result of strong electron-donor-acceptor (EDA) and hydrogen-bonding interactions between these two species. Upon heating, although marginal site-specific changes in the distribution of nearest CO2 around caffeine are observed, the overall distribution is retained. At a higher temperature, the caffeine-CO2 hydrogen-bonding interactions are weakened, while their EDA interactions become relatively stronger. The results underscore the importance of the interplay of these interactions in determining stable solvent structures and solubility of caffeine in scCO2.