Abstract
Interactions between caffeine and 21 un-and mono-substituted benzoic acids were investigated at pH 7.0 and 30°by means of an indirect method utilizing the spectral change due to the competition between congo red and benzoates in complexation with caffeine. The main molar composition of the complexes appeared to be 1 : 1, and apparent equilibrium constant and free energy change for the complex formation were evaluated. The relation between the free energy change and pKa for benzoates suggested that direct electrostatic forces between the carboxyl group in the benzoate molecules and the nitrogen of the 7-position in the caffeine molecule played a dominant role in the complexations.
