Kinetic and thermodynamic compensation study of the hydration of faba beans (Vicia faba L.)

Abstract

This work applies kinetic and thermodynamic compensation to evaluate the kinetics of hydration of faba beans. A mechanism was proposed, consisting of a zero-order adsorption step followed by a first-order desorption step, with both reactions going through a transition state with a previous equilibrium stage.The kinetic constants were obtained from a previous study for pHs 3, 6, 9 and 12 and at temperatures of 20, 35, 50 and 65 °C. From these kinetic constants, the equilibrium constants were calculated using the Eyring equation for each pH value and temperature.The kinetic constants were fitted to the Arrhenius equation and the set of pair estimates for lnk0 and the activation energy followed the straight lines that cause the kinetic compensation, with isokinetic temperatures of −10.3 °C found for the zero-order adsorption step and 8.4 °C for the first-order desorption step.The equilibrium constants were fitted to the Van't Hoff equation and the set of pair estimates for the activation enthalpy and the activation entropy followed the straight lines that cause thermodynamic compensation. The isoequilibrium temperatures were obtained as −11.4 °C for the zero-order adsorption step and 7.5 °C for the first-order desorption step. As expected, the isokinetic and the isoequilibrium temperatures were very close for each step.The mechanism was concluded to be the same for the ranges of pH and temperature studied. Since all the isokinetic and isoequilibrium temperatures were lower than the working temperature values, the control was concluded to be entropic for all cases.Statistical compensation could be discarded for the zero-order adsorption step but for the first-order desorption step, the compensation was concluded to be as significant as the experimental propagation of errors.