SELECTIVE BINDING OF DIVALENT-CATIONS AT THE SURFACE OF SELF-ASSEMBLED MONOLAYERS OF AN AROMATIC BIFUNCTIONAL MOLECULE STUDIED ON A QUARTZ-CRYSTAL MICROBALANCE

Self-assembled monolayers (SAMs) of a small aromatic bifunctional mole cule, 4-carboxythiophenol, formed on gold films have been used to sele ctively bind Cd2+ and Pb2+ ions at the surface of the SAMs. SAMs forme d with this molecule on Au have a surface rich in carboxylic acid func tionality which can be used to bind the above-mentioned cations throug h an ion-exchange process. The kinetics of ion exchange, as well as th e pH dependence of the binding of Cd2+ and Pb2+ ions on the SAM surfac e, have been followed using quartz crystal microbalance measurements. It is found that, at low pH where the carboxylic acid groups are expec ted to be unionized, significant mass increase is observed. At high pH , the percentage of ion exchange is found to be twice that expected fr om charge neutrality constraints (0.5 Cd2+/Pb2+ ion per carboxylic aci d group). This is tentatively explained as being due to incorporation of Cd(OH)(+) and Pb(OH)(+) ions. The titration curve obtained yields a n equilibrium binding constant of similar to 5.3 for both ions, which is close to that obtained for these ions in Langmuir monolayers and La ngmuir-Blodgett multilayer films.