Understanding the Conformational Dynamics of Organosilanes:  γ-APS on Zinc Oxide Surfaces

The oscillatory adsorption and orientation of thin films of γ-(aminopropyl)triethoxysilane (γ-APS) on zinc oxide surfaces have been investigated using X-ray photoelectron spectroscopy. Two different values of solution pH were chosen, one above and one below the isoelectric point (IEP) of the substrate. At both values of pH, films of organosilane molecules aligned perpendicular to the surface are produced. Moreover, varying the electrokinetics of the adsorption process radically alters the molecular orientation during oscillatory adsorption. When γ-APS is adsorbed at pH 6.4 (i.e., lower than the IEP of the metal substrate) the molecules are oriented with the silanol groups at the interface, whereas at pH 10.4 the molecules are oriented with the amine groups at the interface. The oscillation in the adsorbed coverage of γ-APS is correlated with either a disordering of the film or a realignment of the γ-APS molecules parallel to the surface.