MICELLAR EFFECTS ON THE OXIDATIVE ELECTROCHEMISTRY OF LIPOPHILIC VITAMIN-C DERIVATIVES

The anodic oxidation of ascorbic acid (VC) and its lipophilic derivati ves ascorbyl-6-caprylate (VC-8), double dagger 6-laurate (VC-12) and 6 -palmitate (VC-16) have been studied by cyclic voltammetry at a glassy carbon electrode in the presence of cetyltrimethylammonium bromide (C TAB) and sodium dodecyl sulfate (SDS) micelles, The peak potentials (E -pa) and peak currents (i(pa)) have been found to be remarkably depend ent on the lipophilicity of the VCs and on the character and concentra tion of the surfactant. Making VC lipophilic remarkably shifts its pea k potential to negative values, the E-pa being 200, 70, -85 and -125 m V (vs, SCE) in aqueous solution at pH 6.8 for VC, VC-8, VC-12 and VC-1 6, respectively. In micellar solutions the E-pa and i(pa) change abrup tly around the critical micellar concentration (CMC) of the surfactant s and reach a plateau above the CMC, The E-pa at the plateau is shifte d to more positive values in SDS micelles, It is shifted to more negat ive values in CTAB micelles except in the case of VC-16, whose E-pa is shifted to the positive direction. The effectiveness of the micellar effect decreases in the order VC > VC-8 > VC-12 > VC-16. Sharp current maxima appeared in CTAB micellar solution below its CMC for VC-8, VC- 12 and VC-16, demonstrating adsorption of these lipophilic VCs at the electrode surface and formation of premicellar aggregates. The electro n-transfer rate constants and diffusion coefficients have been calcula ted from the cyclic voltammograms. From these data it is concluded tha t the hydrophobic! lipophilic interaction of the hydrocarbon tail and the electrostatic interaction of the ascorbate anion moiety of the VCs are the dominant factors controlling their electrochemical behaviour in micellar solutions.