Micellar effects on the oxidative electrochemistry of lipophilic vitamin C derivatives

Abstract

The anodic oxidation of ascorbic acid (VC) and its lipophilic derivatives ascorbyl-6-caprylate (VC-8),‡ 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 (CTAB) and sodium dodecyl sulfate (SDS) micelles. The peak potentials (E_pa) and peak currents (i_pa) have been found to be remarkably dependent on the lipophilicity of the VCs and on the character and concentration of the surfactant. Making VC lipophilic remarkably shifts its peak potential to negative values, the E_pa being 200, 70, –85 and –125 mV (vs. SCE) in aqueous solution at pH 6.8 for VC, VC-8, VC-12 and VC-16, respectively. In micellar solutions the E_pa and i_pa change abruptly around the critical micellar concentration (CMC) of the surfactants and reach a plateau above the CMC. The E_pa at the plateau is shifted to more positive values in SDS micelles. It is shifted to more negative 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 electron-transfer rate constants and diffusion coefficients have been calculated from the cyclic voltammograms. From these data it is concluded that 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.