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

Abstract

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.

SKP as a tool to study the physicochemical interaction at buried metal–coating interfaces

“…2. [4] Although the variation of − coat across the different oxide films is relatively small compared with the potential shifts observed after adsorption of epoxy-amide model molecules (e.g. due to lower density of groups interacting with oxide surface), − coat is the highest for the hydroxyl-rich surfaces and the lowest for the hydroxyl-poor surfaces.…”

“…Recently, Wielant et al [4,5] made a detailed simulation of the adhesion of an epoxy-amide coating by adsorbing two different model compounds (N,N -dimethylsuccinamide and Nmethyldiethylamine) on differently treated steel surfaces. It was observed that the hydroxyl density at the oxide surface could be assumed as the most crucial surface property toward adsorption of these molecules.…”

“…The higher the amount of surface hydroxyls, the higher the number of protonated amide and amine functionalities. [4] By SKP, it was shown that the interface dipole which was formed as a result of the physicochemical bonds at the interface was determined by the protonation reaction. [5] In this work, the SKP is applied to characterize, in a nondestructive way, the physicochemical condition of different iron oxide-polymer interfaces.…”

The role of acid‐base properties in the interactions across the oxide‐primer interface in aerospace applications

“…Up to now, most research efforts focused on controlling the film morphology, such as the pore diameter, oxide layer and barrier layer thickness. However, numerous studies [3][4][5][6][7] provide evidence for the significance of the oxide chemistry in the interfacial bonding with an organic coating.…”

“…[9] Since most analysis techniques are not able to probe interactions at the buried interface, adhesion is often modelled by the adsorption of functional molecules representing the resin on the metal surfaces. [4,10,11] This type of measurements offers valuable information about bonding mechanisms on a molecular level. Additionally, the adsorption of various molecules on different surface oxides provides an indication of the nature of the oxide, such as its activity and its acid-base properties.…”