Simulating Surfactant–Iron Oxide Interfaces: From Density Functional Theory to Molecular Dynamics

Abstract

Understanding the behaviour of surfactant molecules on iron oxide surfaces is important for many industrial applications. Molecular dynamics (MD) simulations of such systems have been limited by the absence of a force-field (FF) which accurately describes the molecule-surface interactions. In this study, interaction energies from density functional theory (DFT) + U calculations with a van der Waals functional are used to parameterize a classical FF for MD simulations of amide surfactants on iron oxide surfaces. The Original FF, which was derived using mixing rules and surface Lennard-Jones (LJ) parameters developed for nonpolar molecules, were shown to significantly underestimate the adsorption energy and overestimate the equilibrium adsorption distance compared to DFT. Conversely, the Optimized FF showed excellent agreement with the interaction energies obtained from DFT calculations for a wide range of surface coverages and molecular conformations near to and adsorbed on α-Fe 2 O 3 (0001).

Scale-Dependent Friction-Coverage Relations and Non-Local Dissipation in Surfactant Monolayers

“…5a with the top surface containing nanoscale curvature are larger (particularly for r tip = 4 nm) than those from previous simulations with flat surfaces. 52 The same observation was made by Knippenberg et al 84 from NEMD simulations comparing friction from alkyl chains grafted on a diamond surface compressed by either a spherical fullerene tip or a flat amorphous carbon slab. Moreover, de Beer and Müser showed using NEMD that, for polymer brush-bearing surfaces, systems with nanoscale curvature provided additional dissipation mechanisms compared to those with flat surfaces.…”

“…to the formation of more solid-like monolayers, with reduced molecular interdigitation. 37,52 A reduction in µ with increasing Γ has recently been observed in macroscale tribology experiments by Fry et al 17 They used OFM molecules with different headgroups and tailgroups to form monolayers with different surface coverages on silica surfaces. 17 The surface coverage, obtained using ellipsometry and QCM, was found to correlate with the initial friction coefficient, with minimum friction occurring at maximum coverage.…”

“…49 The Lennard-Jones and Morse interactions were cut off at a distance of 1.2 nm. 52 A slab implementation of the particle-particle, particle-mesh algorithm 66 with a relative force accuracy of 10 −5 was used for the Coulombic interactions.…”

“…All of the amide headgroups remain attached to the surface under even the most extreme conditions studied. This is due to the strong chemisorption interactions between the amide headgroup and surface atoms, 52 which were parameterised to reproduce interaction energies from accurate DFT calculations. 51 X-ray photoelectron spectroscopy experiments have shown that, like the fatty amides investigated in this study, fatty amines also chemisorb on steel surfaces, 18 and thus the monolayers are likely to show similar durability.…”

A Combined Experimental and Atomistic Investigation of PTFE Double Transfer Film Formation and Lubrication in Rolling Point Contacts

“…So far, most atomistic simulations considered pure PTFE systems [16][17][18][19], with only few exceptions investigating PTFE in contact with other materials [20,21]. Despite the technological relevance of PTFE-lubricated steel contacts, to the best of our knowledge, there is no classical MD study in the literature that addresses the tribology and transport of PTFE in contact with iron oxide, a model frequently used for steel surfaces in atomistic simulations [22]. One possible reason for this could be the lack of a suitable force field to describe the interaction between iron oxide and PTFE.…”