Structure and Friction of Stearic Acid and Oleic Acid Films Adsorbed on Iron Oxide Surfaces in Squalane

Abstract

The structure and friction of fatty acid surfactant films adsorbed on iron oxide surfaces lubricated by squalane are examined using large-scale molecular dynamics simulations. The structures of stearic acid and oleic acid films under static and shear conditions, and at various surface coverages, are described in detail, and the effects of unsaturation in the tail group are highlighted. At high surface coverage, the measured properties of stearic acid and oleic acid films are seen to be very similar. At low and intermediate surface coverages, the presence of a double bond, as in oleic acid, is seen to give rise to less penetration of lubricant in to the surfactant film and less layering of the lubricant near to the film. The kinetic friction coefficient is measured as a function of shear rate within the hydrodynamic (high shear rate) lubrication regime. Lubricant penetration and layering are observed to be correlated with friction coefficient. The friction coefficient with oleic acid depends only weakly on surface coverage, while stearic acid admits more lubricant penetration, and its friction coefficient increases significantly with decreasing surface coverage. Connections between film structure and friction are discussed.

Friction Modifier Additives

“…Relatively little attention was generally paid to the way the head group bonded to the substrate. More recently simulations of specific OFMs have been carried out both to study how their packing on surfaces influences friction [103] and also how monolayers behave when separated by a high pressure/high shear rate film of base oil [104]. A limitation of all work to date is that because of the very small time steps needed to model molecular motion accurately and the relatively slow rate of molecular diffusion [105], it is not yet possible to simulate the process of self-assembly to form an adsorbed film or the equilibrium between solution and surface film.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…Doig et al [241] used NEMD simulations to examine the structure and friction of stearic and oleic acid OFM films adsorbed on iron oxide surfaces and lubricated by squalane under hydrodynamic conditions (0.1 GPa). At high surface coverage (2.59 nm −2 ), the measured properties of stearic acid and oleic acid films were very similar.…”

Comparative Adsorption of Saturated and Unsaturated Fatty Acids at the Iron Oxide/Oil Interface

“…Interestingly, Doig et al, who have performed molecular dynamics simulations on oleic and stearic acid/hematite/squalene systems, predict that the main difference arising from the unsaturated chain is to make the oleic acid sit more upright on the surface, whereas the stearic acid will sit at an angle 28 . This is supported by the PNR results reported here, which indicate a layer thickness close to that of the extended oleic acid molecule, whereas PNR layer thicknesses for palmitic acid suggest it sits at an angle 17 .…”