Molecular Simulations of n-Alkylamines as Boundary-Lubrication Additives at a Steel Surface: Adsorption and Structure

Boundary-lubrication additives protect steel surfaces through the formation of adsorbed molecular films that reduce direct surface contact. Among such compounds, n-alkylamines provide a simple model system for investigating relationships between molecular structure, adsorption thermodynamics, and film organization at solid interfaces. In this study, classical molecular dynamics simulations were employed to systematically examine the adsorption of n-alkylamines with different alkyl-chain lengths on an iron-terminated hematite (0001) surface, used as a representative model of steel. Adsorption was analyzed over the full range of surface coverages, from isolated molecules to dense layers. Energies of adsorption, surface coverages, intermolecular interaction energies, and structural descriptors of the adsorbed films were evaluated. Longer alkyl chains strongly enhance adsorption stability. Analysis of interaction energies in the first adsorbed layer revealed that lateral additive-additive interactions, dominated by van der Waals forces between alkyl chains, become comparable to or exceed additive-substrate interactions for long-chain molecules. Increasing surface coverage induces a transition from predominantly flat-lying configurations to densely packed tilted first adsorbed layers, accompanied by plateau regimes in the energy of adsorption and surface coverage. Predicted surface coverages for n-octadecylamine are in good agreement with available experimental measurements. These results provide molecular-level insight into how chain length and intermolecular interactions govern adsorption thermodynamics and film organization at solid interfaces, with relevance to both boundary lubrication and surface science.