Electrochemical and surface analytical studies of the self-assembled monolayer of 5-methoxy-2-(octadecylthio)benzimidazole in corrosion protection of copper

5-Methoxy-2-(octadecylthio)benzimidazole (MOTBI) monolayer was self-assembled on fresh copper surface obtained after etching with nitric acid at ambient temperature. The optimum conditions for formation of self-assembled monolayer (SAM) were established using impedance studies. The optimum conditions are methanol as solvent, 10 mM concentration of the organic molecule and an immersion period of 24 h. The MOTBI SAM on copper surface was characterized by contact angle measurements, X-ray photoelectron spectroscopy and reflection absorption FTIR spectroscopy and it is inferred that chemisorption of MOTBI on copper surface is through nitrogen. Corrosion protection ability of MOTBI SAM was evaluated in aqueous NaCl solution using impedance, electrochemical quartz crystal nanobalance, potentiodynamic polarization and weight-loss studies. While bare copper showed a charge-transfer resistance (R_ct) value of 1.89 kΩ cm² in 0.20 M NaCl aqueous environment, the R_ct value for SAM covered copper surface is 123.4 kΩ cm². The MOTBI SAM on copper afforded corrosion inhibition efficiency of 98-99% in NaCl solution in the concentration range and in the temperature range studied. The SAM functions as a cathodic inhibitor. Quantum chemical calculations showed that MOTBI has relatively small ΔE between HOMO and LUMO and large negative charge in its benzimidazole ring, which facilitate formation of a polymeric Cu⁺-MOTBI] complex on copper surface.