Interactions of caffeine and theophylline with p-cresol: UV studies

UV absorption studies demonstrated the formation of weakly bonded charge transfer complexes between caffeine and theophylline with p-cresol in chloroform. The transitions involved were detected at wavelengths longer than those of the single pure substances. Equilibrium constants from the Benesi-Hildebrand equation could be measured together with other thermodynamic constants and molar extinction coefficients. In general, the equilibrium constants were very small while the entropies of formation were quite high. Even though the equilibrium constants of caffeine--p-cresol were independent of wavelength over a narrow range, the apparent enthalpies of formation of both complexes indicated wavelength dependence.