Self-assembled monolayer mechanism for corrosion inhibition of iron by imidazolines

Some of the most effective corrosion inhibitors for oil field pipeline applications are the oleic imidazoline (OI) class of molecules. However, the mechanism by which OIs inhibit corrosion is not known. We report atomistic simulations (quantum mechanics and molecular dynamics) designed to elucidate this mechanism. These studies lead to the self-assembled monolayer (SAM) model for corrosion inhibition, which explains the differences in corrosion inhibition efficiency for various OI molecules. The SAM model of OI inhibitors involves the following critical elements: (i) The function of the OI is to form a self-assembled monolayer on the native oxide surface of iron; this serves a protective role by forming a hydrophobic barrier preventing migration of H₂O, O₂, and electrons to the Fe surface. (ii) The imidazoline head group serves as a sufficiently strong Lewis base to displace H₂O from the Lewis acid sites of the iron oxide surface. (iii) These head groups self-assemble on the surface to form an ordered monolayer on the iron oxide surface. [√3 x √3 for the (001) cleavage surface of α-Fe₂O₃.] (iv) The long hydrophobic tail (e.g., 2-oleic acid) tilts to form a tightly packed hydrophobic monolayer. [For α-Fe₂O₃(001) the tilt angle is about 72° with respect to the surface normal.] (v) This hydrocarbon tail must have a sufficient length to cover the surface. [For α-Fe₂O₃-(001) the chain length must be 12 or more carbon atoms.] (vi) The hydrophobic tail and the pendent group (e.g., -CH₂CH₂NH₂) must lead to an octanol/water partition coefficient (log P) below a critical value in order to rapidly form the monolayer. This SAM model should be useful in developing both alternative environmentally benign corrosion inhibitors and higher temperature corrosion inhibitors.