DFT Study of Cl − Ingress into Organic Self-Assembled Monolayers on Aluminum
Résumé
We address the mechanism by which organic layers on aluminum substrate hinder the penetration of Cl − toward the metal substrate. Localized corrosion by chlorides on Al and its alloys is a major problem, and organic molecules that form self-assembled monolayers on metal substrates may provide efficient corrosion protection. In one of our previous works, we established experimentally that long-chain n -alkyl carboxylic acids form protective layers against Cl − corrosion on Al substrates. In a different work, we identified, using implicit models of the organic layer and metal substrate, two essential effects by which organic layers hinder the penetration of Cl − ions toward the metal substrate. The first effect is due to the inferior solvation of ions in the organic layer compared to that in an aqueous solvent. The second effect is due to the electric field at the electrochemical interface, and the extent to which it affects the penetration of Cl − depends on the electrode potential and the thickness of the organic layer. Both effects are related to a low dielectric constant of the self-assembled monolayer. In the present study, we continue our investigation and explicitly model the organic monolayer and Al substrate using density-functional-theory calculations. To this end, we consider organic monolayers consisting of either dodecanoic- or hexanoic-acid molecules. Current calculations confirm the findings of the simplified implicit models, i.e. the energy barrier for the Cl − penetration increases with the thickness of the organic monolayer and with Cl − concentration in the monolayer. Furthermore, we propose a new mechanism by which Cl − penetrates the organic monolayer. Due to the considerably inferior solvation of Cl − in the organic layer compared to that in water, calculations suggest that it is energetically easier to locally “open” the organic monolayer by creating a hole large enough to accommodate water molecules and Cl − . The presence of water molecules ensures a stronger Cl − solvation and a better electrostatic screening between anions. While the energy barrier for the Cl − penetration via the local “opening” mechanism is suggested to be smaller than for the penetration of Cl − into dense homogeneous organic monolayer, it is still significant enough to pose a considerable kinetic barrier for the penetration of Cl − from the aqueous solution into the organic monolayer at room temperature.