BEREKET DALGA, ABEBE BELAY^#
Applied Physics Program, School of Applied Natural Science, Adama Science and Technology University

The self-association of caffeine, chlorogenic acid and their hetero-association with methylene blue were studied in aqueous solution by UV/Visible spectrophotometry. The dimerization constants of caffeine and chlorogenic acid analyzed using dimer model at the wavelengths of 272.8 and 324 nm were found to be 565.33 and 1027 M^–1 respectively. The hetero-association constants of methylene blue with caffeine and chlorogenic acid analyzed using Benesi-Hilde brand approach at the wavelength of 664 nm were found to be 8418 and 14542 M^–1 respectively. Thermodynamic parameters such as Gibbs free energy, enthalpy and entropy of dimerization reactions of the compounds were also investigated using Vant’s Hoff equation at the temperature range of 295–301 K for self-association and 295–302 K for hetero-association. The change of enthalpy calculated for caffeine, chlorogenic acid and the complexes of MB-CAF and MB-CGA at the temperature of 301 K are –12.86 ± 0.073, –14.24 ± 0.839, 5.12 ± 0.09, and 3.61 ± 0.012 kJ·mol^–1, respectively. The obtained thermodynamic parameters indicated that self-and hetero-association of the compounds are mainly electrostatic and hydrophobic interactions played a major role in the binding reactions respectively.
Key words: Caffeine, chlorogenic acid, methylene.

Corresponding author’s e-mail: abebebelay96@gmail.com.

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