Adsorption mechanism of neutral and protonated HQAP on Fe(110): a DFT, Monte Carlo and molecular dynamics study

The adsorption of 8-quinolinol-5-azoantipyrine (HQAP) on Fe(110) was investigated to clarify the interfacial behavior of representative neutral and carbonyl-protonated models. Although pKa-based speciation indicated that multiply protonated species dominate under strongly acidic conditions, neutral HQAP-AE and monoprotonated HQAP-OcarbonylH+ were retained for a restricted molecular comparison. Monte Carlo searches identified low-energy starting configurations, and molecular dynamics and radial distribution function analyses showed persistent Fe-N/O proximity over 2000 ps. The nearest-Fe contact of the azo nitrogen on the antipyrine side was shorter in the protonated model over both analysis windows. Other differences depended on the site and time window. The results support persistent multipoint interfacial contacts without demonstrating covalent bonding, chemisorption or a general ranking of adsorption strength.

Density functional theory (DFT) calculations were performed using the DMol³ module in Materials Studio 2023 with the GGA-PBE functional and DNP basis set; aqueous calculations used COSMO. Monte Carlo (MC) searches using Adsorption Locator supplied the initial adsorbed configurations for molecular dynamics (MD) using Forcite and COMPASS III. The molecular charge of HQAP-OcarbonylH+ was retained and balanced by an additional chloride counterion. Production comprised two consecutive 1000 ps NVT segments at 303 K using the Andersen thermostat and a 0.25 fs time step. Radial distribution functions (RDFs) and nearest-Fe distances were compared over 1000-2000 ps and the common final 1500-2000 ps window.