Azoles and their derivatives are among the often used organic corrosion inhibitors for copper. For this reason, the adsorption of four azole molecules—imidazole, triazole, tetrazole, and pentazole—on Cu(111) and Al(111) surfaces has been studied and characterized using density functional theory calculations. We find that the molecules weakly adsorb in an upright geometry through nitrogen atom(s). Molecular electronic structure is only weakly perturbed upon adsorption and the molecule–surface interaction involves the hybridization between molecular sigma-orbitals and metal states, yet the main contribution to bonding comes from the electrostatic dipole interactions due to a large dipole moment of azole molecules. With increasing the number of nitrogen atoms in azole ring the molecular electronegativity and chemical hardness linearly increase. The harder the molecule the more difficult the hybridization with metal states, which can explain why with the increasing number of nitrogen atoms in azole ring the molecule–surface bond strength decreases thus following the imidazole ) triazole ) tetrazole ) pentazole trend.
The effect of methyl, phenyl, and mercapto substituents on electronic structure of imidazole type inhibitors was characterized by density-functional-theory calculations. The most coherent trend is observed for chemical hardness. It is demonstrated that, in general, larger molecules are chemically softer provided that they belong to the same chemical type. The electronegativity is reduced by methyl and mercapto substituents and increased by phenyl substituent. It is further shown that phenyl substituent reduces the solvation free energy thus increasing the relative tendency of the molecule to get adsorbed on the surface, which may contribute to the increased inhibition effectiveness against corrosion of copper observed experimentally.
The gas-phase adsorption of triazole, benzotriazole, and naphthotriazole—considered as corrosion inhibitors—on copper surfaces was studied and characterized using density functional theory (DFT) calculations. We find that the molecule–surface bond strength increases with increasing molecular size, thus following the sequence: triazole ( benzotriazole ( naphthotriazole. This trend is explained in terms of molecular electronegativity and chemical hardness, which decrease monotonously as the molecular size increases. While the electronegativity of triazole is almost degenerate with the work function of Cu(111) surface, the electronegativity of larger acenotriazoles is smaller. The difference in electronegativity between the Cu(111) and the acenotriazoles thus increases with increasing the molecular size, which, together with decreasing the molecular hardness, results in larger molecule-to-metal electron charge transfer and stronger molecule–surface bonds.