ALCOHOL AGGREGATION AT HYDROPHOBIC MONOLAYER SURFACES AND ITS EFFECT ON INTERFACIAL REDOX CHEMISTRY

The aliphatic alcohols 1-butanol, 1-hexanol, 1-octanol, 1-decanol, and 2,2,4-trimethylpentanol were found to aggregate from aqueous solution onto the surface of hydrophobic monolayers of alkanethiols self-assem bled onto gold electrodes. The aggregates were characterized via their effect on the redox properties of several ferrocene derivatives immob ilized as minority components in the monolayers and on the interfacial capacitance of the electrodes. Alcohol aggregation causes a decrease in the overall capacitance at the electrode and dramatically shifts th e redox potential for ferrocene oxidation in a positive direction rela tive to the potential observed in the absence of alcohol. The magnitud e of the shift in ferrocene redox potential is a function of the alcoh ol concentration in solution, reaching a maximum for solutions saturat ed with alcohol, and of chain length for the n-alcohols, reaching a ma ximum for 1-decanol (the longest alcohol tested). Aggregates form only when the monolayer presents a hydrophobic surface to the contacting s olution; use of an alkanethiol with a hydroxyl terminal substituent or of a ferrocene derivative containing a quaternary ammonium group comp letely quenches the effect. A structural model is proposed in which ag gregate layers formed from saturated alcohol solutions are approximate ly 1.1-1.5 nm thick and relatively disordered, with properties that ar e not greatly different from those of the bulk alcohol liquids.