Soybean oil fatty acid aromatic amides as dual-functional inhibitors and antimicrobial agents: synthesis, characterization, thermal stability, and performance evaluation

In the present study, novel amide derivatives of soybean oil fatty acids were synthesized via amidation of hydrolyzed fatty acids with aniline and benzylamine, yielding aromatic amides with structurally distinct amine moieties. These compounds were designed as potentially effective and biodegradable corrosion inhibitors. Comprehensive physicochemical characterization of the amides was performed using FT-IR and ¹H/¹³C NMR spectroscopy, while their thermal stability was assessed through thermogravimetric (TG) derivative thermogravimetric (DTG) and differential thermal analysis (DTA). The corrosion inhibition performance of the synthesized compounds was investigated using electrochemical techniques in CO₂-saturated 1 wt% NaCl solution. Linear polarization resistance (LPR) measurements were employed to study the time dependence of the corrosion rate, whereas potentiodynamic polarization (Tafel) curves were recorded to determine the electrochemical parameters of the corrosion process. The LPR results indicated that the inhibition efficiency of carbon steel increased with increasing inhibitor concentration, reaching 93-96% at 100 ppm. The inhibition mechanism was further elucidated by examining the surface morphology of mild steel using scanning electron microscopy (SEM). SEM images revealed that inhibitor-treated samples exhibited a smoother and less damaged surface compared to the uninhibited system, indicating the formation of a protective film. Adsorption behavior followed the Langmuir adsorption isotherm, and the calculated Gibbs free energy of adsorption (- 39 to - 42 kJ mol⁻¹) suggested strong and spontaneous adsorption with a dominant chemisorption contribution. The antimicrobial activity of the compounds was evaluated against sulfate-reducing bacteria (Desulfovibrio desulfuricans) as well as Gram-negative bacteria (Escherichia coli and coliforms). The benzylamine-derived fatty acid amide demonstrated the highest activity, achieving complete inhibition of D. desulfuricans growth at 50 mg L⁻¹. The results indicate that soybean oil fatty acid amides exhibit dual functionality, combining anticorrosive and antimicrobial properties, and may serve as promising environmentally friendly inhibitors for corrosion protection in oil-field environments affected by microbial activity.