Metal–ligand complex formation in the presence of ionic micelles. Equilibrium and kinetics of the reaction between copper(II) and benzoylacetone in the presence of ionic micelles of SDS or DTACl

Abstract

The equilibrium and kinetics of the metal–ligand complexation reaction between Cu^II and the chelating agent, 1-phenylbutane-1,3-dione (benzoylacetone), have been studied in micellar solutions of sodium dodecylsulphate (SDS) and dodecyltrimethylammonium chloride (DTACl). The apparent stability constant of the 1:1 complex passed through a maximum with the increase in the micellar concentrations for SDS solutions, while it decreased monotonically for DTACl solutions. The micellar effect on the apparent stability constant is related to the partitioning of reactants between the micellar phase and the aqueous solution. The observed rate of 1:1 complex formation was considerably enhanced by the presence of SDS micelles. The effect can be quantitatively interpreted by considering the reaction in the electric double-layer around the micelles, where Cu^II ions are concentrated by the electrostatic interactions. The rate constant for the reaction in this layer was found to be greater than that in aqueous solutions by a factor of ca. 18. This effect was deduced to be caused by the instability of the keto form of benzoylacetone.