HCL VAPOR-INDUCED STRUCTURAL REARRANGEMENTS OF N-ALKANOATE SELF-ASSEMBLED MONOLAYERS ON AMBIENT SILVER, COPPER, AND ALUMINUM SURFACES

Exposure to HCl vapor of highly organized self-assembled monolayers (S AMs) of n-alkanoic acids on native oxide-covered Ag, Cu, and Al surfac es results in a variety of structural changes strongly dependent on th e specific SAM, Using infrared spectroscopy (IRS) and wetting measurem ents, it was observed that while SAMs on Al surfaces show no effects o f I-ICI exposure over periods of minutes, exposure of SAMs on Ag and C u over just periods of seconds induces protonation of the carboxylate head groups and severe reorganization of the film into different struc tural forms of the initial n-alkanoic acid. For the kg SAMs of n-tride canoic acid (C13) and shorter chains, disordered slates af the acid ar e produced which upon ambient storage revert back to a carboxylate sta te with much less order than the initial SAM. However, for C14 and lon ger chains, protonation occurs to give crystalline forms of the acid. For the C16 SAM, the original structure of all-trans chains, tilted at 19-20 degrees from the surface normal, converts to discreet monoclini c crystallites, estimated from the IRS data to be similar to 5 nm in t hickness and oriented with the {010} and/or {001} surface planes paral lel to the substrate surface. After 7 days ambient storage. reversion to the original SAM structure occurs, bur for longer chains (e.g., C24 ) reversal is incomplete. While generally similar results are observed for SAMs on copper, the specific deprotonation and reorganization rat es are faster than On Ag and only partial reversibility is observed. T hese different behaviors are interpreted in terms of a combination of differences in intermolecular interactions, headgroup-substrate intera ctions, and reactivities of the metal surfaces toward HCl and ambient O-2.