Monolayers of mercaptoundecanol and mercaptoundecanoic acid were prepared on Au(111) films, immersed in aqueous solutions, and probed by frequency-modulation atomic force microscopy. The pN-order tip-surface force was observed over the monolayers as a function of vertical and lateral coordinates, together with the topography of the monolayers. The observed force distribution was modulated between adjacent OH endgroups in the mercaptoundecanol monolayer, as opposed to on top of COOH and COO(-) endgroups in the mercaptoundecanoic acid monolayer. Models of the interfacial hydrogen bond between water and the endgroups were proposed. The force distribution was insensitive to the electrolyte composition. There was no qualitative sign of tip-induced confinement of water.

An R-Al 2 O 3 (011 j 2) wafer was immersed in an aqueous KCl solution of 1 mol L -1 and observed with a frequency-modulation atomic force microscope. The tip-surface force was precisely determined as a function of the tip-surface distance. The force-distance relationship contained oscillations accompanied with an exponentially decayed, electric double layer force. The force oscillations were ascribed to liquid water layers confined over the Al 2 O 3 surface.