Formation of Monolayer Films by the Spontaneous Assembly of Organic Thiols from Solution onto Gold

Long-chain alkanethiols, HS(CH₂)_nX, adsorb from solution onto gold surfaces and form ordered, oriented monolayer films. The properties of the interfaces between the films and liquids are largely independent of chain length when n > 10; in particular, wetting is not directly influenced by the proximity of the underlying gold substrate. The specific interaction of gold with sulfur and other "soft" nucleophiles and its low reactivity toward most "hard" acids and bases make it possible to vary the structure of the terminal group, X, widely and thus permit the introduction of a great range of functional groups into a surface. Studies of the wettability of these monolayers, and of their composition using X-ray photoelectron spectroscopy (XPS), indicate that the monolayers are oriented with the tail group, X, exposed at the monolayer-air or monolayer-liquid interface. The adsorption of simple n-alkanethiols generates hydrophobic surfaces whose free energy (19 mJ/m²) is the lowest of any hydrocarbon surface studied to date. In contrast, alcohol and carboxylic acid terminated thiols generate hydrophilic surfaces that are wet by water. Measurement of contact angles is a useful tool for studying the structure and chemistry of the outermost few angstroms of a surface. This work used contact angles and optical ellipsometry to study the kinetics of adsorption of monolayer films and to examine the experimental conditions necessary for the formation of high-quality films. Monolayers of thiols on gold appear to be stable indefinitely at room temperature but their constituents desorb when heated to 80°C in hexadecane. Long-chain thiols form films that are thermally more stable than films formed from short-chain thiols.