Loosely packed hydroxyl-terminated SAMs on gold

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 -CF3́ 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 hydroxylterminated 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(H ₂O) ≈ 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.