Micellar catalysis of metal complex formation. Part 2.—Kinetics of the reaction between Ni 2+aq and various neutral bidentate ligands in the presence of sodium dodecylsulphate micelles in aqueous solution

Abstract

The kinetics of the reaction between Ni²⁺_aq and the bidentate ligands 2,2′-bipyridyl and 4,4′-dimethyl-2,2′-bipyridyl have been investigated in micellar solutions of sodium dodecylsulphate using the stopped-flow method. The results are compared with those obtained previously for the reaction between Ni²⁺_aq and the ligand pyridine-2-azo-p-dimethylaniline (PADA). The kinetic analysis suggests that complex formation occurs in the region of the micelle surface by the same mechanism as in aqueous solution but the observed rate of reaction is considerably enhanced when surfactant is present due to the concentrative effect of the micelle surface on the reactants. This increase in the rate of reaction is quantitatively related to the partitioning of the ligand between the micelle surface and the aqueous solution regions. The kinetic analysis indicates that no significant ligand partitioning between the micelle surface and the interior of the micelle takes place. Since the rate constant for reaction at the micelle surface is independent of the ligand used, it is concluded that the rate-limiting step is release of a solvating water molecule from the metal ion. The rate constant for this process is similar to that measured in aqueous solution in the absence of micelles.