INFRARED STUDIES OF WATER-ADSORPTION ON MODEL ORGANIC-SURFACES

The adsorption of water on methyl and carboxylic acid terminated monolayers of n-alkanethiols adsorbed on gold substrates was studied using reflection-absorption infrared spectroscopy. Despite the differences in substrate polarity, the adsorption of multilayer quantities of water at temperatures < 120 K yields films of amorphous solid water on both materials while adsorption at higher temperatures (> 130 K) leads to the formation of polycrystalline ice. The transformation of the adsorbed overlayers from the amorphous to the polycrystalline state was followed using temperature-programmed infrared spectroscopy. Our results suggest that there is a critical layer thickness (critical mass coverage) necessary for this transition to occur. We also show that "free" OH (OH not hydrogen bonded to neighboring water molecules) is present on both substrates and that it is both more plentiful and stable on the acid terminated surface.