DFT study of aqueous-phase adsorption of cysteine and penicillamine on Fe(110): Role of bond-breaking upon adsorption

Abstract

The bonding of the cysteine amino acid and the structurally similar penicillamine drug on Fe(110) was characterized by means of DFT calculations, because both molecules were identified to inhibit corrosion of mild steel in acidic HCl solution and it is commonly assumed that effective adsorption is important for achieving the inhibitory effect. Calculations indicate that intact adsorption of protonated (cationic) molecules from the aqueous-phase onto Fe(110) is not favorable, whereas the adsorption of other molecular forms is very strong. Fe(110) displays a strong tendency towards the abstraction of H atoms from the molecule and it is the SH bond that breaks first upon molecular adsorption, followed by OH bond-cleavage of the carboxylic group and finally by NH bond-cleavage of the NH3+ group; the extent of H abstractions depends on specifics. This H abstraction tendency is so strong that, even at a very low pH, protonated molecules will undergo at least the cleavage of the SH bond on the surface. Calculations suggest that adsorption and H abstraction affinities of cysteine and penicillamine are strong enough to displace O(ad)and OH(ad)from the surface by transforming them into water.