Clay modified electrodes: Part 9. Electrochemical studies of the electroactive fraction of adsorbed species in reduced-charge and preadsorbed clay films

Abstract

Clay modified electrodes (CME) were prepared with natural (untreated) and reduced-charge (prepared by treatment with Li+) montmorillonite. Both the values of ct, the total concentration, and c*, the electroactive concentration of adsorbed M(bpy)32+ (M = Ru, Os, or Fe), were found to be a factor of 10 smaller in the reduced-charge clay, yielding very similar c*/ct, ratios, even though the cations were restricted to the external surfaces in the reduced-charge clay. The electrochemical inactivity of the intercalated cations by itself can therefore not account for the low values of the c*/ct, ratio found in CME. The number of defects in the films appears to be more important than the specific adsorption sites in determining what proportion of the adsorbates will be electroactive. No evidence of mediated electron transport by adsorbed species was seen in films prepared with MV2+ pre-exchanged clay. However, at low scan rates, electron transfer from lattice Fe(II) resulted in initial ipa/ipc -values larger than unity for adsorbed Fe(bpy)32+. This effect was also observed for adsorbed Ru(bpy)32+ , but not for Os(bpy)32+, in both the natural and reduced-charge montmorillonite films.