Self-assembled binary monolayers of n-alkanes on reconstructed Au(111) and HOPG surfaces

Self-assembly of n-heptadecane (n-C17H36) and n-hexatricontane (n-C36H74) was studied by means of scanning tunneling microscopy. A droplet of a solution Of n-C17H36 containing 0.6 wt % of n-C36H74 formed an ordered binary alkane monolayer on the herringbone Au(111) surface, which was composed of 30% n-C36H74 and 70% n-C17H36. The alkane molecules were arrayed in parallel to the [01(1) over bar] azimuth, and their intermolecular distance was 0.48 nm, which is equal to the distance of solid alkanes. In contrast, the alkane monolayer formed on a highly oriented pyrolytic graphite (HOPG) surface was predominantly composed of an n-C36H74 layer including only ca. 3% of n-C17H36 as an impurity. The n-C36H74 molecules on the HOPG surface were compressed along the molecular axis and in the perpendicular direction to the molecular axis. It is deduced that the intermolecular interaction is optimized when the surface structure is profitable for self-assembly of alkanes where no strong alkane-surface interaction is required.