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.
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