Molecular interactions of caffeine with benzoic acid, o- and p- methoxybenzoic acids and o- and p- nitrobenzoic acids

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Abstract
The solubility of the complexes formed by caffeine with benzoic acid and substituted benzoic acids has been studied. An insoluble complex is formed by o-methoxybenzoic acid, whereas p-methoxy-, o- and p-nitrobenzoic acids and benzoic acid itself form soluble complexes. The phase diagrams have been analysed to evaluate the approximate stability constants for 1:1 interaction. The stabilities increase as the electron-withdrawing power of the benzoic acid substituent decreases, and there is a correlation between log K values and pKa values (or σ constants) of the acids. This indicates that the binding strength is related either to the polarity of the carboxyl group or to the electron density of the aromatic ring. The implications of these results are discussed in terms of the mechanism of complex-formation.