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 (VC₄) or undecyl (VC₁₁) linkages between the repeating viologen (4,4′-bipyridinium) subunits. Both polyelectrolytes were found to adsorb at nuetral pH on monolayers prepared by the self-assembly of either 3-mercaptopropionic (HSC₂COOH) or 8-mercaptooctanoic (HSC₇COOH) acids on golds. However, the adsorption was optimized by using the more hydrophobic polyelectrolyte (VC₁₁) and the longer thiol (HSC₇COOH). The gradual deprotonation of the HSC₇COOH monolayer in the pH range 6-11 enhances the adsorption of the VC₁₁ polyelectrolyte. From these data, an apparent pK_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 - polyelectrolytes interfacial structures did not block effectively the reduction or dissolved Ru(NH₃)₆³⁺ ions on the underlying gold electrode.