A multi-scale investigation of the anti-corrosion and diffusion properties of a natural dye extract on aluminum in NaCl solution using DFT, RDF, FFV, and MC/MD simulations

• Bio-derived inhibitors (Law, Gal, Dex) exhibit strong adsorption on Al(111) surfaces. • DFT and Fukui descriptors reveal enhanced local reactivity at active adsorption sites. • COSMO-RS analysis highlights dominant hydrophobic and hydrogen-bonding interactions. • Dextrose achieves the most compact film with minimal free volume and superior barrier effect. • Molecular orientation and dense film structure restrict chloride ion diffusion effectively. Aluminum is widely used in engineering applications but remains highly susceptible to chloride-induced degradation, highlighting the need for sustainable corrosion protection strategies. This study investigates three bio-derived molecules Lawsone (Law), Gallic acid (Gal), and Dextrose (Dex) extracted from Henna natural dye as environmentally friendly inhibitors for aluminum in NaCl medium. A multi-scale computational framework integrating Density Functional Theory (DFT), COSMO-RS, Fukui functions, Monte Carlo (MC), and Molecular Dynamics (MD) simulations was applied to elucidate their adsorption, diffusion, and film formation mechanisms. DFT results showed that Law possessed the smallest HOMO–LUMO gap (1.742 eV), indicating high reactivity, while COSMO-RS σ-profiles revealed predominant non-polar regions enhancing hydrophobic interactions. MC simulations confirmed strong adsorption of Law with interaction energies of –57.20 kcal mol⁻¹ (vacuum), –169.43 kcal mol⁻¹ (aqueous), and –333.73 kcal mol⁻¹ (co-adsorption). Gal exhibited the highest aqueous affinity (–183.73 kcal mol⁻¹), and Dex achieved the lowest fractional free volume (35.56%), forming the most compact and stable protective film. Radial distribution analysis confirmed chemisorption, while MD trajectories indicated parallel molecular alignment and reduced chloride diffusion (Dex > Gal > Law). Molecular electrostatic potential and Mulliken charge analyses localized reactive oxygen sites responsible for surface bonding. Overall, the synergy between strong adsorption, hydrogen bonding, and dense film architecture effectively restricts ion migration, providing atomic-scale insight into the inhibition mechanism of Henna-derived molecules and offering a framework for designing next-generation eco-friendly aluminum protection systems.