Structures of self-assembled monolayers of n-alkanoic acids on gold surfaces modified by underpotential deposition of silver and copper:: Odd-even effect

The structures of n-alkanoic acid monolayers (CH3(CH2)(m)CO2H, m = 5, 6, 11, 13-22) self-assembled on gold premodified by electrodeposition of a silver or copper adlayer are studied by contact angle measurements, IRAS (infrared reflectance-absorption spectroscopy), and XPS (X-ray photoelectron spectroscopy). Prior to immersion into the fatty acid-containing solutions, the substrates are prepared by under-potential deposition (upd), resulting in a sub-monolayer or full monolayer of silver or copper on gold. The adlayer promotes anchoring of carboxylate headgroup and assembly of n-alkanoic acids, which would otherwise exhibit no chemisorption on bare gold. The results show that the monolayers exhibit low wettability, all-trans methylene conformation, and a bidentate binding scheme of the carboxylate headgroups onto the upd-modified substrate. The odd-even effect of alternating peak intensity of the methyl-stretching modes is significant for monolayers on silver upd surface and, to a less extent, yet notably, for those on copper upd surface. These features are distinctly different from structures of n-alkanoic acid SAMs on bulk copper containing native oxides. We attribute the difference to the degree of surface oxide formation indicated by XPS which reveals that in ambient conditions the upd-modified surface is less oxidized by dioxygen than the corresponding bulk substrate.