STRUCTURAL COMPARISON OF SELF-ASSEMBLED MONOLAYERS OF N-ALKANOIC ACIDS ON THE SURFACES OF SILVER, COPPER, AND ALUMINUM

Self-assembled monolayers of n-alkanoic acid, (CH3(CH2)(m)COOH, m = 2-18, 22), were formed on the native oxide surfaces of Ag, Cu, and Al. The structure of the monolayers was probed with optical ellipsometry, contact angle measurement, and reflection absorption IR spectroscopy. The results suggest that the structure of the adsorbed film strongly depends on the metal substrate, as well as the chain length of the acid involved. The metal substrate dominates the binding geometry of the head group and probably the packing density. The chain length also affects the packing density and the conformation of the molecular chain. The ultimate structure is a compromise of all interactions. Acids dissociate completely on all three of the metals and form carboxylate overlayers. On the surface of silver, the two oxygen atoms of the carboxylate bind to the surface nearly symmetrically, and the molecular chain extends trans zigzag for all the acids examined. The same tilt angle (estimated between 15 and 25-degrees from surface normal) was suggested for all the acids. While on the surface of aluminum and copper, the carboxylate binds to the surface asymmetrically, and for shorter chain acids, the molecular chains are disordered and liquid-like. As the chain length increases, the cohesive interaction increases and the packing becomes denser. Eventually, the molecular chains pack straight up for the long chain acid. In doing so, some strained or gauche conformations near the carboxylate head group develop as the binding geometry, which is preset by the metal surface, can not accommodate such an orientation. Such strain is more serious on an aluminum surface and less so on a copper surface.