Influence of the Iron Oxide Acid−Base Properties on the Chemisorption of Model Epoxy Compounds Studied by XPS

In this work, the influence of the iron oxide acid−base properties on the adsorption of model epoxy compounds was examined. To study this, iron oxide layers with a different surface hydroxyl fractions were prepared in controllable and reproducible conditions. The surfaces were characterized by X-ray photoelectron spectroscopy (XPS). An expression for the hydroxyl fraction of the oxide films was deduced, starting from the measured XPS intensities. Two model epoxy compounds characteristic of an epoxy/amide system were adsorbed on the oxides: N,N'-dimethylsuccinamide and N-methyldiethanolamine. Additionally, an amine molecule without alcohol groups, N,N'-diethylmethylamine, was adsorbed to investigate the role of alcohol functionalities on the amine adsorption mechanism. The interaction between the oxide layers and the nitrogenous model compounds were studied by examination of the O 1s and N 1s XPS photopeaks. The data showed that the amine and amide nitrogen adsorbed via two different bonding modes: via Lewis-like acid−base interactions and via Bronsted-like interactions or protonation. A direct correlation was found between the protonation level of the adsorbed nitrogenous molecules and the hydroxyl fraction in the outer oxide layer. Additionally, it was noted that the protonation level depended on the mobility and flexibility of the adsorbed molecules. It was also observed that the presence of alcohol groups in the amine molecular chain had a beneficial effect on the number of adsorbed amine molecules.