Loosely Packed Hydroxyl-Terminated SAMs on Gold
By Berron, Brad & Jennings, G. Kane
Published in Langmuir
2006
Abstract
We report the preparation of loosely packed hydroxyl-terminated self-assembled monolayers (SAMs) on gold by the adsorption of bis(11,11'-dithioundecyl)perfluoroheptanoate and base-mediated cleavage of the fluorocarbon terminal group. As shown through complementary characterization methods, the partially fluorinated SAM exhibits a structure in which the outer surface contains mostly -CF3 groups, the fluorocarbon groups are slightly canted on average, and the hydrocarbon chains underneath are in a fluidlike state. Upon cleavage of the fluorocarbon group, the hydroxyl-terminated alkyl chains relax into an increasingly canted, fluidlike state. The resulting monolayer packing exposes both methylene and hydroxyl functionalities, yielding an intermediate surface energy (θa(H2O) ≈ 68°). As compared to a densely packed hydroxyl-terminated SAM prepared from bis(11-hydroxyundecyl)disulfide, the cleaved films are thinner because of the greater average chain cant and exhibit a ∼50% higher capacitance and a factor of 5 lower charge-transfer resistance. The addition of THF to the electrolyte solution as a cosolvent intercalates into the loosely packed SAM to double the charge-transfer resistance and increase the capacitance by ∼20% but does not affect the capacitance of the densely packed SAM. The loosely packed SAM is also more easily exchanged upon exposure to a solution of n-docosanethiol.