Theoretical Study of the Aliphatic-Chain Length's Electronic Effect on the Corrosion Inhibition Activity of Methylimidazole-Based Ionic Liquids

Density Functional Theory based studies about the inhibition of corrosion affecting the cathode hematite surface by coverage with an methylimidazole-based ionic liquid, IL, were performed. Inhibition performance is tuned through the length of aliphatic chains C-n attached to the imidazolium aromatic ring, where n = 1,..., 20. Frontier molecular orbitals and energy gaps for single ILs and a pristine (112) hematite surface, the energy difference between ILs and hematite-surface, Fukui indices for the ILs, and adsorption energies and charge transfers for ILs/hematite surface systems were calculated. Results show that adsorption activity is due to the donation/back-donation bonding among the imidazolium aromatic ring of the ILs and the exposed iron atoms of the hematite surface. Dispersive long-range interactions from the ILs alkyl tails allow a closer bonding of the corrosion inhibitor molecules to the hematite surface; the charge transfer between the IL and the hematite is in C-18. The optimal alkyl chain size is 18 and this is in full agreement with the reported experimental alkyl chain size that produces high inhibitory properties of related vinylimidazolium ILs.