Molecular Interactions between Organized, Surface-Confined Monolayers and Vapor-Phase Probe Molecules. 8. Reactions between Acid-Terminated Self-Assembled Monolayers and Vapor-Phase Bases

We present the results of a study of the interactions between three different acid-terminated self-assembled monolayer (SAM) surfaces and three basic vapor-phase probe molecules. The SAMs are composed of 4-mercaptobenzoic acid (MBA), 3-mercaptopropionic acid (MPA), and 11-mercaptoundecanoic acid (MUA), and the vapor-phase probes are, in order of increasing solution-phase acidity, decylamine, pyridine, and pyrazine. Our results are based on data from surface infrared spectroscopy and thickness-shear mode mass sensors. We find that all three SAMs irreversibly bind approximately one monolayer of decylamine, although there are slight differences that correlate with the structural nuances of the SAMs. The MPA and MBA SAMs bind decylamine through an electrostatic interaction brought about by transfer of a proton from the acid to the base. Because the MUA SAM is more impenetrable than the others, complete proton transfer is hindered, and binding of decylamine arises through a combination of proton transfer and strong hydrogen bonding. In the presence of its vapor, pyridine adsorbs to MBA surfaces at near-monolayer coverage, but upon N2 purging about two-thirds of it desorbs. Only one-half monolayer of pyrazine, which is less basic than pyridine, adsorbs to the MBA SAM, and upon N2 purging, about two-thirds of it desorbs. The aliphatic acid SAMs follow a similar trend. The results of this study indicate that the extent of base binding correlates most strongly with the structural nuances of the acidic SAMs and the relative basicity of the vapor-phase bases. These results are relevant to SAM-based chemical sensors.