Lubricants are complex fluids consisting of a base oil and many different additives, and are used to control friction and wear between solid inorganic surfaces in relative motion. A review of recent work on molecular simulations of lubricants is given. It is shown that simulations can be used to uncover a lot of interesting behaviour, including additive adsorption, additive self-assembly, and a competition between the two. The specific examples to be discussed are: the adsorption of stearic acid and oleic acid in squalane on iron-oxide surfaces; the self-assembly of glycerol monooleate in bulk n-heptane; the adsorption and friction of glycerol monooleate in squalane on iron-oxide surfaces; and the conformations of functionalised copolymers in bulk n-heptane. The structures adopted by the additives can be correlated with the observed frictional properties, opening up the possibility of molecular-level design of new lubricants.

Molecular-dynamics simulations are used to explore bilayers formed by simple ionic surfactants at the mica–water interface, and to shed light on experimental observations.

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.

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