Adsorption behaviour and corrosion inhibition mechanism of the major active molecules in Xanthii extract as green corrosion inhibitors: a quantum chemical and molecular dynamics study

To establish a comparative molecular-level understanding of how structurally distinct active components in Xanthii extract contribute to corrosion inhibition on iron, seven major active molecules were investigated using quantum chemical calculations and molecular dynamics simulations. Caffeic acid and ferulic acid exhibited the highest electron-donating ability, with EHOMO values of −5.389 and −5.383 eV, whereas aloe emodin showed the lowest ELUMO (−4.028 eV) and the smallest energy gap (2.003 eV). Xanthiazone combined a relatively low ELUMO (−3.143 eV), a small energy gap (2.371 eV), and pronounced multipolarity. All molecules spontaneously adsorbed on the Fe surface in water but showed distinct configurations, ranging from flat-lying coverage to localised anchoring. RDF results showed first O-Fe peak distances of 2.11-3.13 Å, all shorter than that of water O atoms (3.29 Å), while the corresponding peak intensities ranged from 9.2–23.7, all exceeding that of water O atoms (6.50). MSD analysis showed that caffeic acid had the lowest diffusion coefficient (5.17 × 10−9 cm2 s−1), whereas tomentosin and xanthatin had the highest values (1.52 × 10−7 and 1.46 × 10−7 cm2 s−1). These results clarify structure-dependent adsorption mechanisms and help identify key corrosion-inhibiting components in Xanthii extract.