Monolayers of sulfur‐containing molecules at metal surfaces as studied using SERS: 3, 3′‐thiodipropionic acid and 3‐mercaptopropionic acid adsorbed on silver and copper

Abstract The modification of metal surfaces with self‐assembled thiol monolayers is the subject of intensive studies owing to both its fundamental interest in surface chemistry and its potential technological significance. In this work, the applicability of surface‐enhanced Raman scattering (SERS) to determine the conformation and p K a values of ω‐terminated thiol molecules was investigated. 3, 3′‐Thiodipropionic acid (TDPA) and 3‐mercaptopropionic acid (MPA) monolayers were spontaneously formed on silver and copper surfaces by adsorption from TDPA and MPA solutions, respectively. The structure of the monolayers formed was determined from SERS measurements. The SERS investigations showed that molecules forming TDPA monolayers prefer to adopt a gauche conformation of the—S—C—C chains, whereas in MPA monolayers formed from 1 m M MPA aqueous solution a significant part of the adsorbed molecules adopts a trans conformation. Formation on the metal surface of an MPA monolayer with high surface coverage of MPA makes dissociation of the carboxylic groups of MPA significantly more difficult. Analogous changes in the strength of similar adsorbed compounds have been reported by other groups. In contrast to those results, we found that the formation of monolayers from TDPA increases its acidic strength even at high surface coverage when monolayers are formed from relatively concentrated TDPA solutions. Probably the interaction of carboxylic groups with the metal surface is easier for TDPA than for MPA for geometric reasons. We consider that ionization of carboxylic groups increases the strength of this interaction. Copyright © 2005 John Wiley & Sons, Ltd.