Interactions between Friction Modifiers and Dispersants in Lubricants: The Case of Glycerol Monooleate and Polyisobutylsuccinimide-Polyamine

Abstract

The structural and frictional properties of 10 wt% solutions of the amphiphilic molecules glycerol monooleate (GMO) and polyisobutylsuccinimide-polyamine (PIBSA-PAM) in squalane are studied using molecular dynamics simulations in bulk and under confinement between iron-oxide surfaces. GMO is a friction modifier, PIBSA-PAM is a dispersant, and squalane is a good model for typical base oils. A range of liquid compositions and applied pressures is explored and the formation and stability of reverse micelles is determined under quiescent and shear conditions. Micellization is observed mainly in systems with a high GMO content, but PIBSA-PAM may also form small aggregates on its own. In the confined systems under both static and shear conditions, some surfactant molecules adsorb onto the surfaces, with the rest of the molecules forming micelles or aggregates. Shearing the liquid layer under high pressure causes almost all of the micelles and aggregates to break, except in systems with around 7.5 wt% GMO and 2.5 wt% PIBSA-PAM. The presence of micelles and adsorbed surfactants is found to be correlated with a low kinetic friction coefficient, and hence there is an optimum composition range for friction reduction. This work highlights the importance of cooperative interactions between lubricant additives.

Molecular adsorption, self-assembly, and friction in lubricants

“…In recent work, the interaction between OFMs and dispersants (used to keep soot in solution) has been studied in MD simulations, and some significant effects on friction have been uncovered. The results of this work (by two of the authors, GT and PJC) are currently being prepared for publication [66].…”

Effectiveness of glycerol-monooleate in high-performance polymer tribo-systems

“…The difference can be structural and/or chemical. As GMO and OA can form micelles in oil, [5][6][7] micellar layers, rather than OFM monolayers, may form on PAI. If the hydrolysis of GMO is pressure dependent, these layers may consist of mainly GMO, rather than OA, in PAI-PAI contacts due to its lower pressure.…”

“…It has been suggested that GMO, its hydrolysis products (oleic acid and glycerol), and water formed micelles or aggregates in n-heptane and toluene under both static and shear conditions. [5][6][7] These micelles adsorbed on the surface. The nature of the GMO adsorbed layer is surface dependent.…”

Experimental and simulation study of the high-pressure behavior of squalane and poly-α-olefins

“…There have been many simulation studies of the rheological properties of squalane, and these have mainly focused on the diffusion and (Newtonian) shear viscosity under ambient conditions, 24,25 the non-Newtonian (shear-thinning) regime at high pressure, [26][27][28] and the rheological and tribological properties of the confined fl uid, ei ther on it s ow n, [29][30][31] or with additives. 32,33 In the MD work by Porras-Vazquez et al, 18 referred to above and discussed in Section III A, the liquid-solid phase boundary was sketched out over the temperature range 235-500 K, and at pressures of up to about 5 GPa.…”