Structures of Self-Assembled Monolayers of n-Alkanoic Acids on Gold Surfaces Modified by Underpotential Deposition of Silver and Copper: Odd−Even Effect

Abstract

The structures of n-alkanoic acid monolayers (CH3(CH2)mCO2H, 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 underpotential 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, f...