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

Abstract

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.

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

“…In a previous publication, the adsorption of epoxy-amide model compounds revealed that a higher density of hydroxyls at the oxide surface results in a higher amount of Brönsted interactions (protonation) leading to a larger downward Volta potential shift, [5] see Fig. 3.…”

“…As observed after molecular adsorption, this results in a decrease of the substrate work function and decrease of the Volta potential. [5] At the buried oxide-coating interface, the formation of an interfacial dipole will have the same effect on φ ox Pol . φ ox Pol will decrease when the basic organic groups interact with the acid groups of the oxide surface.…”

“…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.…”

“…[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 obtained Volta potential variation induced by the application of coatings on differently treated steel substrates is analyzed.…”

In situ study of the deterioration of thiazole/gold and thiazole/silver interfaces during interfacial ion transport processes

“…After the sample is transferred from ambient air to the humid SKP chamber, the water activity at the MBT/Au interface increases [9]. This may induce a change of the interfacial dipole moment [38,39], if the monolayer adsorption geometry adjusts to the increased interfacial water amount. Fig.…”