Molecular Dynamics Simulations of Glycerol Monooleate Confined between Mica Surfaces

Abstract

The structure and frictional properties of glycerol monooleate (GMO) in organic solvents, with and without water impurity, confined and sheared between two mica surfaces are examined using molecular dynamics simulations. The structure of the fluid is characterized in various ways, and the differences between systems with nonaggregated GMO and with preformed GMO reverse micelles are examined. Preformed reverse micelles are metastable under static conditions in all systems. In n-heptane under shear conditions, with or without water, preformed GMO reverse micelles remain intact and adsorb onto one surface or another, becoming surface micelles. In dry toluene, preformed reverse micelles break apart under shear, while in the presence of water, the reverse micelles survive and become surface micelles. In all systems under static and shear conditions, nonaggregated GMO adsorbs onto both surfaces with roughly equal probability. Added water is strongly associated with the GMO, irrespective of shear or the form of the added GMO. In all cases, with increasing shear rate, the GMO molecules flatten on the surface, and the kinetic friction coefficient increases. Under low-shear conditions, the friction is insensitive to the form of the GMO added, whereas the presence of water is found to lead to a small reduction in friction. Under high-shear conditions, the presence of reverse micelles leads to a significant reduction in friction, whereas the presence of water increases the friction in n-heptane and decreases the friction in toluene.

Molecular adsorption, self-assembly, and friction in lubricants

“…The DREIDING force field [36] and OPLS force field (in both all-atom and united-atom formulations) [37,38] have been used in the work reviewed here. For model base oils, checks are made against experimental measurements of properties such as the density or viscosity, and deviations under ambient conditions are typically no more than 5% [31]. As will be shown in Sec.…”

“…Alternatively, the surfactant can remain in the liquid layer as a complete reverse micelle, with the polar head groups forming a core, and the non-polar tail groups immersed in the base oil. Any water in the system will prefer to be at the inorganic surfaces and/or in the cores of reverse micelles [30,31].…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…For some OFMs, such as glycerides, it remains unclear whether close-packed monolayers form on the surface, as experimentally observed for fatty acids [221][222][223], or whether they have a different friction reduction mechanism. Bradley-Shaw et al [234] used NEMD simulations to study the stability of reverse micelles, formed by glycerol mono-oleate (GMO) OFMs in organic solvents [235], confined and sheared between mica surfaces. The GMO reverse micelles were found to be metastable in the absence of shear for all of the systems studied.…”

Diffusion behaviour of water confined in deformed carbon nanotubes

“…Fluids under nanoscale confinement exhibit properties not observed in the bulk [1,2]. In the case of water this shows an even larger impact.…”