IONIC INTERACTIONS IN ELECTROACTIVE SELF-ASSEMBLED MONOLAYERS OF FERROCENE SPECIES

The electrochemical and interfacial behavior of two types of electroactive self-assembled monolayer systems is investigated at gold electrodes. The first type is a ferrocene-based surfactant (a redox surfactant) derived from (dimethylamino)methylferrocene via quaternization of the amino group with various n-alkyl bromides. These have a long alkyl chain with 16 or 18 carbons in the chain pendent from the cationic ammonium group. We refer to these as C16 and C18. The second type is a ferrocene-based dimeric species with a disulfide functional group capable of providing a permanent anchor to the Au electrode, thus endowing monolayers of this species with exceptional stability toward desorption. The electrochemical quartz crystal microbalance (EQCM) is used to monitor the mass changes that occur at the electrode surface during the redox processes of these two species. For the redox surfactants, we have previously shown that C12 and C14 (two shorter chain derivatives) desorb as a consequence of oxidation. For C16 this same desorption occurs, but the rate is low enough to be monitored by changes in scan rate and depends on coverage. For C18, desorption following oxidation does not occur on the cyclic voltammetric timescale at room temperature but does proceed at a measurable rate at elevated temperatures. For cases in which the desorption of the redox surfactant following oxidation is slow compared to the cyclic voltammetric scan rate, the EQCM results suggest that anion incorporation into the monolayer occurs prior to desorption. For the disulfide species, for which desorption following oxidation is not possible due to the strong Au-S interaction, the EQCM results suggest that reversible anion association with the monolayer occurs following oxidation to the ferricenium state. The conditions under which ions become incorporated into or associated with charged monolayers are discussed, as are some possible consequences of this phenomenon.