Adsorption of Normal-Alkanes on Fe(110), FeO(110), and Fe₂O₃(0001): Influence of Iron Oxide Surfaces

Abstract

A comparative analysis of adsorption of six normal-alkanes (C N H 2N+2 , N = 4, 6, 8, 10, 12, 16) on Fe(110), FeO(110), and Fe 2 O 3 (0001) was carried out using classical molecular dynamics (MD) simulation. A realistic model system for adsorbed alkanes was employed using the COMPASS force field (FF), while the appropriate relaxed surfaces and an effective interfacial potential were obtained from ab initio calculations. The results show that butane molecules orient randomly on Fe(110) and Fe 2 O 3 (0001) surfaces, but they preferentially orient in the (010) direction on FeO (110) at low temperature. Additionally, alkanes adsorb physically on Fe(110), FeO(110), and Fe 2 O 3 (0001), in the following decreasing order Fe(110) > FeO(110) > Fe 2 O 3 (0001). The adsorption energies per saturated carbon site decrease with an increase of molecular chain length, and this propensity is similar for different surface potentials. In contrast, the saturated carbon density is insensitive to the surface potentials and shows an increasing trend for short alkane chains, but it remains steady for longer chains.

Thin film lubrication of hexadecane confined by iron and iron oxide surfaces: A crucial role of surface structure

“…27,28 The employed atomic charges for iron oxides, as presented in Table II, are a little higher than those obtained from our previous study using density functional theory (DFT) calculation. 12 However, this charge discrepancy does not affect significantly the current model systems due to the minor contribution of electrostatic energy in the interaction between hexadecane film and iron oxide surfaces. 12 Although Lennard-Jones (LJ) potential is applicable for metallic surfaces, it is not accurate at reproducing the surface properties compared to the embedded atom model (EAM) potential.…”

“…12 However, this charge discrepancy does not affect significantly the current model systems due to the minor contribution of electrostatic energy in the interaction between hexadecane film and iron oxide surfaces. 12 Although Lennard-Jones (LJ) potential is applicable for metallic surfaces, it is not accurate at reproducing the surface properties compared to the embedded atom model (EAM) potential. 29 Therefore, a many-body force-field using FinnisSinclair EAM (EAM/FS) potential 30 was applied for pure iron.…”

“…26. The 9-6 parameters derived from quantum methods, which had been developed from our previous study, 12 were employed to describe the interfacial interaction between the lubricant and the surfaces.…”

“…Our recent study has overcome these limitations by deriving an interfacial force-field from quantum mechanics/molecular mechanics methods (QM/MM potential) characterized for alkane on iron and iron oxide surfaces. 12 A MD simulation was therefore carried out to compare the tribological and structural properties of hexadecane between these tribo-pairs.…”

“…23 Our previous results revealed that n-alkanes prefer to adsorb on pure iron surface rather than iron oxides. 12 When considering the roles of both of surface corrugation and wall-fluid interaction strength, the question whether the lower wall-fluid interaction strength of iron oxide surfaces can yield a better tribological performance is still a controversial issue. Further, another question emerges: How do the iron, iron oxides, as well as their different surface orientations affect the behavior of the thin film?…”