Conformational dynamics of γ-APS on the iron oxide surface: an adsorption kinetic study using XPS and ToF-SIMS

The formation and structure of thin films of γ-aminopropyltriethoxysilane (γ-APS) on mechanically polished iron oxide surfaces have been investigated. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) have been used to study the time dependence of the surface interaction and the subsequent film structure, following adsorption from the liquid phase. It is shown, for the first time, that γ-APS exhibits the phenomenon of oscillatory adsorption, whereby the surface coverage of the silane species oscillates as a function of time. By analyzing the time dependence of the known bonding moieties of the γ-APS molecule, the dominant molecular orientation of the adsorbate is shown to change as a function of time. A model of possible adsorbate conformations is presented and used to describe the observed adsorption behaviour.