Kinetics of complex formation of Fe(III) with caffeic acid: Experimental and theoretical study

Kinetic study on the complexation of caffeic acid with ferric chloride was performed in aqueous solution at pH 9.0. The complex was characterized by IR, UV-Vis spectroscopy and FE-SEM techniques. Kinetic data were obtained and used to model the reaction kinetics for the disappearance of the caffeic acid. The operational variables have been studied in the temperature range from 25 to 45 °C, initial iron and caffeic acid concentration from 0.6 mg/100 ml (ranging from 2.0 ml to 8 ml/100 ml). The complexation reaction was found to be a first-order with rate constants for k₁ (formation) 4.7 × 10^-3 s^-1. Additionally, the effect of concentration and temperature on the complexation reaction was investigated. Apparent kinetic parameters of the complex formation of Fe(III) with caffeic acid were found as; pre-exponential factor A (-1.17 min^-1), enthalpy of activation (ΔH°)# (-2188 J mol^-1), entropy of activation, (ΔS°)# (-7.34 J mol^-1 K^-1) and Gibbs free energy of activation, (ΔG°)#(-0.68 J). The apparent activation energy of the complexation reaction was evaluated to be 289.29 J mol^-1 which is consistent with the chemistry of Fe(III) with polyphenols which are supposed to mimic the interaction of Fe(III) with transferrin in biological media. Various theoretical parameters of the studied complex such as hardness, electronegativity, softness, total energy, dipole moment and point group symmetry were calculated employing Density functional theory (DFT) and found as 0.04465, 0.2130, 22.39, 0.5201 × 10^-8 eV, 15.13 Debye, and C1, respectively.