Stability of co-crystals of caffeine with gallic acid in presence of coformers

摘要

Caffeine forms interactive complexes with several compounds. The formation and stability of caffeine's cocrystals with competing coformers has been investigated in terms of Gibbs free energy of coformer replacement from the solubility values and solubility product of co-crystals at a given temperature. The feasibility of replacement was checked experimentally, and analysis was done by XRD, FTIR, and DSC technique. The Gibbs free energy of coformer replacement at other temperatures was predicted by using solubility values with Gibbs-Helmholtz equation and correlating experimental solubility of coformer via nonrandom two-liquid (NRTL) model of liquid mixtures in terms of activity coefficient and thereby predicting the coformer solubility at other temperatures. The NRTL parameters for catechin, citric acid, oxalic acid, and gallic acid have been estimated. The thermodynamic approach to determine the coformer replacement is found to be in line with the experimental results. Practical applications Cooling of tea infusion leads to solid sediment formation called as tea cream, because of formation of many stable complexes of tea components. Several parameters such as temperature, concentration, and chemical composition, affect the formation of tea cream. Tea infusion contains caffeine along with different acids such as gallic acid, oxalic acid, and tannic acid, which can form stable co-crystals. This study attempts to estimate the stability of the cocrystals of caffeine in the presence of different acidic coformers as a function of temperature and thus prediction and minimization of tea cream formation.