Kinetics of reactions in microheterogeneous aqueous systems

Abstract

Kinetics of reaction involving two low-spin iron(II) di-imine complexes, Fe(phen)²⁺₃ and Fe(hxsbH)²⁺, are reported where the solvent media include three microemulsions and a range of aqueous 2-butoxyethanol mixtures. In certain systems containing Fe(hxsbH)²⁺ the rate of reaction follows zero-order kinetics for a considerable part of the reaction. The kinetics are accounted for using a reaction scheme in which the complex metal ion is adsorbed at an interphase between organic-rich and water-rich domains. The same model is used to account for the marked acceleration in the rate of reaction between Fe(phen)²⁺₃ and hydroxide ions in these systems. The kinetics of a scheme in which reaction takes place in both domains are examined, the distribution of the substrate being at all times in equilibrium between the two microphases. In such cases a plot of In k(obs.) against 1/T can be S-shaped, where k(obs.) is the overall first-order rate constant. Effects of changes in acid, acid concentration and added surfactant on the rate of reaction in the microemulsions are reported.