Two-dimensional distribution of liquid hydrocarbons facing alkanethiol monolayers visualized by frequency modulation atomic force microscopy

Self-assembled monolayers of 1-dodecanethiol were prepared on Au(1 1 1) films and immersed in liquid hydrocarbons: n-dodecane, n-hexadecane and 1-phenyloctane. Topographic protrusions of individual CH3 groups of the thiols were identified using a frequency-modulation atomic force microscope operated in the liquids. The pN-order, tip-surface force was determined as a function of the tip-surface distance and lateral coordinate to visualize structured liquids over the monolayers. Oscillatory modulations appeared on one-dimensional force–distance curves corresponding to liquid layers. On the other hand, the force distribution was homogenous along lateral coordinates, since any functional groups that produce specific interactions were absent at the interfaces. The layer-to-layer spacings were insensitive to the alkyl chain length, suggesting that the n-alkanes were oriented with their molecular axis parallel to the surface. A larger distance was observed with phenyloctane, which was ascribed to the large excluded volume of the phenyl group.