The Chemisorption of Organosulfur Compounds on Gold Surfaces: Construction of Well-Defined Organic Solids.

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([EQ]CH sub 2) sub n -X ) sub 2[EN] ([EQ]n=15-19[EN], X=CH3, CHCH2, CH2OH, CONH sub 2, and CO sub 2 H) on a Au(111) single crystal surface. Using this chemisorption system, monolayer densities approaching that of the bulk solid are obtained. The adsorption of polar molecules, such as water and methanol, as well and nonpolar ones (benzene and hexane being representative examples), on top of these model surfaces was studied using temperature programmed desorption, reflection infrared, and X-ray photoelectron spectroscopies. The subtle effects of both intra- and interlayer interactions are clearly seen in our results. The bonding of non-polar molecules is dominated by dispersive forces and is weak in all cases. Heats of adsorption, therefore, show very little dependence on coverage. In contrast, polar molecules interact weakly with non- polar surfaces, but bond much more strongly to polar materials. In this latter case, we find a very strong coverage dependence of the heat of adsorption which is dominated by interlayer hydrogen bonding. The relevance of these microscopic observations to important macroscopic properties such as wetting will be discussed.