Interface Dipoles Observed after Adsorption of Model Compounds on Iron Oxide Films: Effect of Organic Functionality and Oxide Surface Chemistry

In this article, the interface dipole formed after adsorption of model compounds on thin iron oxide films is studied by scanning Kelvin probe (SKP). Amine and amide molecules with the same organic functionalities as an epoxy/amide coating system and a carboxylic acid molecule representing a maleic anhydride grafted or copolymerized polyolefin polymer were used as adsorbants. First, the effect of the functionality type on the Volta potential of a thermally formed iron oxide was investigated. It was shown that after carboxylic acid adsorption, the surface potential shifted in positive direction, whereas the amine and amide molecules induced a negative potential shift. Second, the Volta potential shift after adsorption of the amine and amide molecules on oxides with different amounts of surface hydroxyls was studied. A changing overall dipole moment as a function of the hydroxyl amounts was observed. Lewis acid−base interactions and protonation took place between the amine or amide functionality and the oxide surface. It was found that the Volta potential shift was mainly affected by the number of protonated amine or amide molecules. The individual interface dipole moments of the two binding types could be deduced from the Volta potential shift, assuming that the individual dipole moments of the two binding types contributed linearly to the overall dipole moment. For the protonation reaction, an interface dipole of −1.28 and −1.54 D was observed for the respective amine and amide molecule, whereas for the Lewis acid−base interaction, a dipole of +0.10 and +0.14 D was found.