REDOX REACTIONS OF LIPOPHILIC COBALT COMPLEXES IN AQUEOUS SURFACTANT MEDIA - CORRELATION OF RATE AND ELECTROCHEMICAL-BEHAVIOR

The effect of added surfactants, sodium dodecylsulfate (SDS), Triton X -100 (C8Hl7C6H4(OCH2CH2)xOH, x = 9, 10), and cetyltrimethylammonium br omide (CTAB), on measured rate constants for electron transfer between Co(terpy)2(2+) and a series of cobalt(III) complexes, [Co(phen)3]3+, [Co(5-Cl-phen)3]3+, [Co(5-Me-phen)3]3+, [Co(5,6-Me2-phen)3]3+, [CO(5-S O3-phen)3], and [Co(dipic)2]-, varying in ligand structure and ionic c harge, have been determined by stopped-flow spectrophotometry. The obs erved rate catalysis or inhibition, resulting from micellar interactio ns, has been rationalized using the Berezin model. Information on mice llar binding, afforded by the kinetic data, correlates with electroche mical changes, particularly changes in anodic and cathodic peak curren ts and half-wave potentials, determined by cyclic voltammetry, that re sult on addition of surfactant solutions to the cobalt complexes. Stro ng hydrophobic interactions dominate the micellar binding of divalent cationic terpyridine and phenanthroline complexes with SDS micelles, a nd the enhanced stabilization of the reduced state in the micellar env ironment is reflected in the magnitude of the observed potential shift s. The presence of the polar SO3- substituent in the neutral sulfonate d cobalt(III) phenanthroline complex inhibits its association with ani onic SDS. Potential shifts for the anionic Co(dipic)2 complex in Trito n X-100 and CTAB also appear to reflect changes in the relative stabil ity of oxidation states in the surfactant media.