Transfer rate of several tris( β-diketonato)iron(III) complexes at the bulk aqueous–Triton X-100 micellar interface: effects of substituent groups in the extractants

Abstract

The transfer rate of tris( β-diketonato)iron(III) complexes with acetylacetone, benzoylacetone, trifluoroacetylacetone, benzoyltrifluoroacetone, and 2-thenoyltrifluoroacetone at the water–Triton X-100 micellar interface was measured by stopped-flow spectrophotometry. The forward transfer rate was first-order with respect to the micellar concentration; the rate-determining step was concluded to be the capturing process of the collided complex at the micellar surface. The value of the rate constant varied with the substituent groups in the extractants; several effects of the substituent groups on the transfer rate were observed. The “mantlephobicity” of the trifluoromethyl group because of its low wettability, the hydrophobicity of the phenyl and thienyl groups in light of their bulkiness, the dilution of the negative efficiency of the trifluoromethyl group at the capturing by enlarging the complex surface with these bulky groups, and the “mantlephilicity” of the thienyl group from its interaction with the hydrogen-bonded network of the micellar “mantle” were estimated. In contrast, the backward rate was independent of the micellar concentration, and the order could be explained by the distribution of the tris-complexes between the “core” and “mantle” of the micelle. The “core–mantle” double-layer structure of the micelle would be an important key for the behavior at the interface.