Molecular design and performance of emissive amide-containing compounds as corrosion inhibitors: synthesis, electrochemical evaluation, DFT calculations and molecular dynamics simulations

Abstract

Corrosion presents a significant challenge across various industries, resulting in considerable economic losses and safety risks. Organic compounds that contain aryl moieties and hetero atoms like nitrogen and oxygen have potential applications as efficient inhibitors and coating layers for the surface of metals. Herein, we investigate the corrosion inhibition of mild steel in 1.0 M H₂SO₄ using newly synthesized amide-containing compounds with naphthalene (naphthamide 6C-9C) or benzene (benzamide 6C-9C) structures. Characterization of these inhibitors via IR and NMR spectroscopy confirmed their chemical structures. Electrochemical analyses, including open circuit potential and potentiodynamic polarization tests, showed that these compounds significantly reduce the corrosion rate of mild steel. They achieved inhibition efficiencies up to 80% at optimal concentrations. The enhanced performance of these inhibitors is linked to their greater molecular weight and longer alkyl chains, which improve adsorption and surface coverage. Photophysical investigations revealed notable solvatochromic effects and red shifts in polar solvents, indicating strong interactions with the environment. Density Functional Theory (DFT) calculations provided further insights into the molecular structure, electronic distributions, and adsorption behavior, confirming the higher efficiency of series naphthamide 6C-9C compared to benzamide 6C-9C. Moreover, molecular Dynamics (MD) simulations corroborated the formation of stable protective layers on the metal surface. From the DFT calculations it is evidently that naphthamide 9C exhibited a smaller HOMO-LUMO energy gap compared to compound benzamide 9C, indicating higher reactivity and greater inhibitory efficiency. The integration of experimental and theoretical findings confirms that amide-containing naphthalene and benzene derivatives are highly effective corrosion inhibitors, suitable for industrial applications.

Scheme 1. Pathway for synthesis of the target products.

Fig. 1. Potential–time curves of mild steel with the amid compounds: (a) naphthamide 6C–9C and b) benzamide 6C–9C.

Fig. 2. Tafel plots polarization curves of mild steel exposed to different inhibitors at 200 ppm for amide compounds: (a) naphthamide 6C–9C and (b) benzamide 6C–9C.

Fig. 3. (a–d) The relation between the wavelength at maximum absorption of compounds 6a–d at different organic solvents (dioxane, THF, DCM, EtOH, ACN, DMF, and DMSO) at 100 μM; (a) benzamide 6C, (b) benzamide 7C, (c) benzamide 8C, and (d) benzamide 9C.

Fig. 4. (a–d) Normalized emission spectra of compounds 6a–d at different organic solvents (DCM, EtOH, THF, dioxane, ACN, DMF, and DMSO) at 100 μM (λ_ex = 287 nm); (a) benzamide 6C, (b) benzamide 7C, (c) benzamide 8C, and (d) benzamide 9C.

Fig. 5. Optimized structures of compounds naphthamide 9C and benzamide 9C calculated at the B3LYP-D3(BJ)/6-31G(d) level. Key structural parameters are annotated in the figure. Intramolecular interactions, analyzed using the Multiwfn program, are based on geometries optimized at the same computational level. van der Waals interactions are represented by green surfaces.

Fig. 6. Distribution of the HOMO and LUMO for compounds naphthamide 9C and benzamide 9C, along with their corresponding energies, computed at the B3LYP-D3(BJ)/6-31G(d) level.

Fig. 7. Molecular electrostatic potential (MESP) surface map of compounds naphthamide 9C and benzamide 9C, displaying electrostatic potential maxima (positive values) and minima (negative values), calculated at the B3LYP-D3(BJ)/6-31G(d) level.

Fig. 8. The optimized geometry of the dimers of molecules naphthamide 9C and benzamide 9C optimized at the B3LYP-D3(BJ)/6-31G(d) level. Intermolecular interactions are depicted as follows: blue color indicates a hydrogen bond, green surface for van der Waals interactions, and red for rings.

Fig. 9. Side and top views of the adsorption of naphthamide 9C and benzamide 9C on the Fe (110) surface in aqueous solution at the end of MD simulations of 1 ns.

Scheme 2. Suggested mechanism for the adsorption of N-alkyl-2-naphthamides (naphthamide 6C–9C) and 4-(dimethylamino)-N-alkylbenzamides (benzamide 6C–9C) on the surface of iron metal.