Adsorption of Viologen-Based Polyelectrolytes on Carboxylate-Terminated Self-Assembled Monolayers

The adsorption of viologen-containing polyelectrolytes on the surface of carboxylate-terminated, self-assembled monolayers was investigated by voltammetric methods. The two polyelectrolytes used had either butyl (VC 4 ) or undecyl (VC 11 ) linkages between the repeating viologen (4,4‘-bipyridinium) subunits. Both polyelectrolytes were found to adsorb at neutral pH on monolayers prepared by the self-assembly of either 3-mercaptopropionic (HSC 2 COOH) or 8-mercaptooctanoic (HSC 7 COOH) acids on gold. However, the adsorption was optimized by using the more hydrophobic polyelectrolyte (VC 11 ) and the longer thiol (HSC 7 COOH). The gradual deprotonation of the HSC 7 COOH monolayer in the pH range 6−11 enhances the adsorption of the VC 11 polyelectrolyte. From these data, an apparent p K a of ∼8 for the monolayer −COOH groups was obtained. The overall degree of packing of the viologen subunits at the monolayer−solution interface was found to be rather poor. The combined monolayer−polyelectrolyte interfacial structures did not block effectively the reduction of dissolved Ru(NH 3 ) 6 3+ ions on the underlying gold electrode.