ABSTRACT
Adsorption mechanism of corrosion inhibitor molecules is not well understood. This chapter discusses the use of advanced molecular dynamics simulations to understand the adsorption of inhibitor molecules on metal–water and air–water interfaces. Free energy of inhibitor molecules near the interfaces is determined. It is found that in infinite dilution, the inhibitor molecules adsorb strongly to the metal–water interface without encountering any free energy barrier. The micelles of the inhibitors, on the other hand, experience a long-range free energy barrier to adsorption on the metal surface. At equilibrium, the adsorption morphology of inhibitors on metal–water interfaces is found to be a function of their hydrophobic tail length and the charge of their polar head groups. Cationic benzyl dimethylammonium inhibitors with long alkyl tail lengths (bda-12) show better surface coverage than with short alkyl tail lengths (bda-4). The bda-12 molecules adsorb in such a way that some molecules lie flat on the metal surface and others aggregate to form a hemi-micelle on top of the lying-down molecules. The bda-4 molecules adsorb by lying flat on the surface. Charge-neutral decanethiol inhibitor molecules adsorb in a much denser morphology by aligning in a bilayer morphology on the metal surface. An equimolar mixture of cationic and anionic inhibitor molecules shows a synergistic effect in adsorption. Besides a good adsorption tendency, other desirable characteristics of inhibitor molecules are their good aqueous solubility and a reduced tendency to form micelles. It is found that addition of a hydroxyl group to the terminal position of alkyl tail significantly enhances the solvation tendency of an inhibitor molecule and also reduces its micellization tendency.
