2012
Volume 12, Number 1, pp. 23–28
Self-association, sodium ion complexation and optical transition probabilities of caffeic acid determined spectrophotometrically
Abebe Belay
Adama Science and Technology University, School of Natural Sciences, Department of Physics, P.O.Box, 1888, Adama, Ethiopia
The concentration-dependent self-association of caffeic acid and its complexation with sodium ions have been studied in water solutions at room temperature (298 K). The self-association and hetero-association were determined using a dimer model and the Benesi–Hildebrand approach. The equilibrium constant for the complexation of caffeic acid with sodium ions was 1.42 ´ 103 M–1. From the temperature dependence of the dimerization in the range up to 348 K the changes of Gibbs’ free energy, enthalpy and entropy of dimerization were calculated as –34.06, –63.20 k J mol–1 and –88.84 J K mol–1, respectively. The optical transition probabilities of caffeic acid were determined in different polar solvents by integrating the absorption coefficients over appropriate wavenumber regions. The calculated values of transition dipole moment and oscillator strength in water are 19 × 10–30cm and 0.49, respectively.
Keywords: Benesi–Hildebrand approach, caffeic acid, dimer model, nonlinear curve fitting, optical transition probability, thermodynamic properties