Interfacial Structure of Primary and Tertiary Liquid Alcohols over Hydrophilic Thiolate Monolayers

COOH- and OH-terminated thiolate monolayers were self-assembled on gold films, immersed in aliphatic alcohols, and probed using frequency-modulation atomic force microscopy. The tip-surface force was observed over the monolayers as a function of the vertical and lateral coordinates. The alcohols were structured over the monolayers to form liquid layers, as evidenced by the force strengths modulated along the vertical coordinate. The liquid layer of primary C-4 and C-8 alcohols was not homogeneous along the lateral coordinate over the COOH-terminated monolayer, perturbed by hydrogen bonding across the interface. On the other hand, tertiary C-5 and C-8 alcohols presented laterally homogeneous liquid layers. Over the OH-terminated monolayer, even primary octanol presented as laterally homogeneous layers. The laterally homogeneous or inhomogeneous distribution of the liquid alcohols was interpreted as the alcoholic OH groups being more or less able to engage the monolayers via hydrogen bonds, respectively.