Multifunctional behaviour of surfactin at the biointerfaces: Corrosion inhibition, antioxidant activity, and colloidal properties.

In this study, surfactin was studied for the first time as a corrosion inhibitor for carbon steel in neutral media. Its physicochemical and colloid-chemical properties were thoroughly investigated, including surface tension, electrical conductivity, and critical micelle concentration (CMC). The antiradical activity of surfactin was evaluated via cyclic voltammetry using the DPPH radical, confirming its antioxidant potential. Electrochemical studies showed that surfactin reduces DPPH• radicals prior to electrode interaction, leading to decreased peak currents (from Ipc1 0.207-0.198 mA/cm², Ipa2 from 0.071 to 0.015 mA/cm²) and confirming its antiradical activity and corrosion inhibition potential.Quantum chemical analysis revealed that surfactin exhibits a HOMO energy of -10.49 eV, LUMO energy of 0.72 eV, a wide HOMO-LUMO gap (ΔE = 11.21 eV), high global hardness (η = 5.61 eV), and a positive electron transfer fraction to iron (ΔN = 0.19), indicating predominant electron-donating behavior and supporting its corrosion inhibition potential. Corrosion inhibition performance was assessed using both gravimetric and electrochemical methods. Polarization curves showed a substantial decrease in corrosion current density (icorr), down to 0.45 μA·cm⁻² after 72 h of immersion in a 3 % NaCl solution containing 1 g/L surfactin. The surfactin acted as a mixed-type inhibitor, effectively suppressing both anodic and cathodic reactions of corrosion process. Surface characterization using FTIR confirmed the formation of a protective film on the steel surface, attributed to the adsorption of surfactin molecules. SEM/EDS analysis further revealed morphological changes and confirmed the presence of a protective layer. These results highlight the promise of surfactin as a green and effective corrosion inhibitor for mild steel in neutral environments.