INTERMOLECULAR INTERACTIONS BETWEEN SURFACTANTS COADSORBED ON ELECTRODES

Reactions of electroactive species adsorbed on electrodes from surfact ant solutions are influenced by coadsorbed nonelectroactive surfactant . Theory and methods are described to provide Frumkin interaction para meters between all amphiphilic species adsorbed onto the electrode in such situations, using cyclic and square wave voltammetry (SWV) and as suming reversible electron transfer. A simple equation to estimate sur face concentrations from reversible SWV curves is presented. The metho d was applied to the electroactive amphiphile dimethyl(ferrocenylmethy l)hexadecylammonium ion (Fc-C16) adsorbed onto glassy carbon electrode s from aqueous solutions containing hexadecyltrimethylammonium bromide (CTAB). Results were consistent with head-down orientations of these surfactants on a glassy carbon electrode. While mixed adsorbate layers were formed, a small preference for adsorption of Fc-C16 was found in the presence of CTAB. Interaction parameters between Fc-C16/Fc-C16, F c-C16/CTAB, reduced and oxidized forms of Fc-C16, and CTAB/CTAB were a ttractive, suggesting favorable hydrophobic interactions. Interactions of the +2 charged, oxidized Fc-C16 with itself and with CTAB were rep ulsive, suggesting that Coulombic head group repulsion overcomes hydro phobic interactions between these species.