Micellar effects upon the reactions of complex ions in solution. Retardation of the base hydrolysis of cis-(azido)(imidazole)bis(ethylenediamine)cobalt(III) by sodium dodecyl sulphate

Abstract

The kinetics of base hydroloysis of cis-(azido)(imidazole)bis(ethylenediamine)cobalt(III) has been studied in the presence of sodium dodecyl sulphate at 40°C. Unlike the case for univalent anions, the surfactant strongly retards the base hydrolysis of the complex. The rate data have been satisfactorily analysed by the pseudo-phase ion-exchange model: [graphic ommitted] where Cb⁺ denotes the imidazole (1-) conjugate base, cis-[(en)₂Co(im)N₃]⁺, and W and M stand for the aqueous and micellar pseudo-phases, respectively. The pseudo-phase ion-exchange equilibium has also been verified by measuring the equilibrium constant using a spectrophotometric method. The second-order base hydrolysis path of Cb⁺ is, however, absent for the fully micelle-incorporated species, Cb⁺_M. Micellar binding of Cb⁺ also leads to a slowing down of its spontaneous aquation rate to a small extent (by a factor of 2), which may be ascribed to the difference in the polarity of the microenvironment of the reactant in the micellar pseudo-phase and the aqueous phase.