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-assembled onto gold electrodes. The aggregates were characterized via their effect on the redox properties of several ferrocene derivatives immobilized 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 the redox potential for ferrocene oxidation in a positive direction relative to the potential observed in the absence of alcohol. The magnitude of the shift in ferrocene redox potential is a function of the alcohol concentration in solution, reaching a maximum for solutions saturated with alcohol, and of chain length for the n-alcohols, reaching a maximum for 1-decanol (the longest alcohol tested). Aggregates form only when the monolayer presents a hydrophobic surface to the contacting solution; use of an alkanethiol with a hydroxyl terminal substituent or of a ferrocene derivative containing a quaternary ammonium group completely quenches the effect. A structural model is proposed in which aggregate layers formed from saturated alcohol solutions are approximately 1.1-1.5 nm thick and relatively disordered, with properties that are not greatly different from those of the bulk alcohol liquids.