Spectrophotometric Method for the Determination of Caffeic Acid Complexation and Thermodynamic Properties

The concentration dependent self-association of caffeic acid and its complexation with sodium ions have been studied in aqueous solutions at room temperature or 298 K. The self-association and hetero-association have determined by dimer model and Benesi-Hildebrand approach respectively. The dimerzation constants of caffeic acid in water and acetonitrile solutions are 2947 and 758 M-1 respectively. The equilibrium constant for the complexation of caffeic acid with sodium ions at the wavelength of 343 nm was 6860 M-1. The thermodynamic properties of self-association of caffeic acid were estimated from Van’t Hoff’s equation. The values of change in Gibb’s free energy, enthalpy and entropy are calculated at temperature of 328 K. The dimerization of caffeic acid are due to intermolecular hydrogen bonding by carboxylic acid groups of the compound and exothermic which characterize by relatively large negative of change in enthalpy. The intermolecular hydrogen bonding of such kind of compound is very important force in the formation of supramolecular liquid crystals.