The solubility of theobromine, theophylline, and caffeine in water and five organic solvents including methanol, ethanol, 1-propanol, ethyl acetate, and acetone was determined by a high performance liquid chromatography method at T = (288.15 to 328.15) K and atmospheric pressure. It was found that the solubility of theobromine, theophylline, and caffeine in these solvents increased with increasing temperature. The empirical Apelblat equation and universal quasichemical model were used to correlate the experimental solubility. The results showed that both models can satisfactorily correlate the solubility data. The crystal forms of the solutes in equilibrium with the saturated solution were analyzed using scanning electron microscopy and powder X-ray diffraction.

... It was dried in an air-circulating oven (Jinghong DNG-9070A, Shanghai, China) at T = 368.2 K
for 4 h. The purity (≥98%) was determined by HPLC. The reference substance of caffeine
(≥98%) was purchased from Chengdu Biopurify Phytochemicals Ltd. (China). ...