Self-assembly and friction of glycerol monooleate and its hydrolysis products in bulk and confined non-aqueous solvents

Abstract

Atomistic molecular dynamics simulations are used to study the self-assembly and friction of glycerol monooleate mixed with oleic acid, glycerol, calcium oleate, or water in n-heptane and toluene solvents. The aim is to determine how chemical degradation products of glycerol monooleate could lead to changes in structural and frictional properties. In bulk solution, almost all mixtures studied contain self-assembled reverse micelles. Under confinement between sheared mica surfaces, the reverse micelles disintegrate, but the distribution of molecules between the surfaces and the centre of the fluid layer depends sensitively on the chemical composition, with more polar mixtures showing stronger adsorption. The measured kinetic friction coefficient is correlated with the extent of surface adsorption: while degradation products lead to increases in the friction coefficient in most cases, all changes are more pronounced when there is less surface adsorption.

Molecular adsorption, self-assembly, and friction in lubricants

“…These types of structures can persist under confinement between moving surfaces (such as in engines) and compete with monolayer adsorption. The balance of self-assembly and adsorption has been shown to have an impact on the friction coefficient; this has been illustrated in the case of glycerol monooleate in various solvents, confined between inorganic surfaces [31,61]. Therefore, to establish structure-property relationships in lubricants, it is essential to understand first how molecular structure dictates the fundamental processes of adsorption and self-assembly.…”

“…3(a). Remarkably, GMO and related molecules form reverse micelles in bulk solution [57,58,59,60,30,31,61].…”

“…First, under engine conditions, the additive molecules can undergo a mixture of thermal and chemical degradation, e.g., hydrolysis. To assess the impact of such effects, the structural and tribological properties of GMO and its hydrolysis products were surveyed extensively [61]. The hydrolysis products included oleic acid and glycerol, and the solution of GMO and/or its hydrolysis products was confined between mica surfaces at P = 10 5 Pa and T = 298 K. Keeping the additive content fixed at 10 wt% in n-heptane, it was observed that hydrolysis leads to an increase in µ, and that after complete hydrolysis of GMO, the friction coefficient had increased by 50%.…”

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

“…40 The effects of GMO hydrolysis on the self-assembly, adsorption, and friction have also been explored. 41 As an ashless dispersant, PIBSA-PAM is not expected to form compact micelles, but aggregates with a small aggregation number (n < 10) might be observed; 4 in some PIBSA-based dispersants, this self-assembly competes with surface adsorption at high concentrations. 42 A single model compound, squalane (2,6,10,15,19,23-hexamethyltetracosane, RMM = 422.83 g mol −1 ) is used to simulate the base oil of the lubricant; the molecular structure is shown in Figure 1(c).…”

“…-and so the local shear rates can be much higher than one would estimate from engineering parameters. Figure 6(a) shows that at high shear rates, the friction coefficient increases logarithmically with shear rate, 1,3,20,40,41,[58][59][60][61][62][63][64][65] and this can be described by a very simple theory, which treats the response of the liquid to a shear force in the framework of an Eyring-like, thermally activated hopping theory. The resulting equation for the friction coefficient is 20…”