Fundamental Studies of the Interactions of Adsorbates on Organic Surfaces.

We have undertaken fundamental studies which characterize the important interactions of adsorbates on organic solids. Model substrates were prepared by the molecular self-assembly of organosulfur compounds of the general structure (S(CH sub 2 ) sub n - X) sub 2 ~(n=15-19,~X=CH sub 3 , ~ CONH sub 2 , ~CO sub 2 H, etc.) on a Au(111) single crystal surface. The adsorption of both polar and nonpolar molecules on top of these model surfaces was studied. The bonding of nonpolar molecules is dominated by dispersive forces and is weak in all cases. In contrast to this behavior we find that polar molecules, which interact weakly with the nonpolar surfaces, bond much more strongly to the polar materials. Our results also suggest an important role for structural transformations in the adsorbed overlayer in these latter cases. The potential relevance of these microscopic observations to important macroscopic properties such as wetting is discussed.