Abstract: Anti-rust oils (AROs) are essential for the long-term storage of metal equipment, especially in harsh environments. However, intrusive saline aerosols and moisture can cause severe ARO degradation and corrosion beneath the oil film. To address this issue, we synthesised a series of homologous N-butyl-C_n-ammonium dodecylbenzene sulfonate (C₄C_nNH₂⁺⋅^-SO₃PhC₁₂, C₄C_nNSPD, n = 4, 8, 12, 16, and 18) ionic liquids (ILs) to investigate how molecular structure influences steel corrosion under oil films. We found that the length of the cationic alkyl chain is critical for their inhibitory activity, with the long-chain IL (C₄C₁₆NSPD) creating a denser interfacial film (794.2 ng/cm²; 21.9 nm thick) and a lower critical micelle concentration (CMC) of 0.05 wt.%, only 1/20 that of short-chain ILs (C₄C₄NSPD). SAXS analysis revealed that C₄C₁₆NSPD IL forms spherical reverse micelles with a diameter of 32 nm in white oil. Meanwhile, C₄C₄NSPD disrupts micellar structures, suggesting an alkyl-chain-dependent mechanism of micelle assembly. C₄C₁₈NSPD can achieve 99.5% inhibition efficiency in oil films against NaCl droplet invasion. Theoretical calculations show that anions in ILs adsorb to steel surfaces via electrostatic coordination and π-π stacking, while long-chain cations strengthen bilayer stability via van der Waals interactions and steric shielding. Thus, the long-chain ILs form stable reverse micelles in white oil, effectively trapping invasive saline droplets. This synergistic mechanism of ordered interfacial adsorption and micellar water arrest enhances the anticorrosion performance of IL-based anti-rust oils.

Key words: Atmospheric corrosion, Ionic liquid, Antirust oil, Reverse micelle, Adsorption