Molecular dynamics simulation study of model friction modifier additives confined between two surfaces

We have used molecular dynamics simulations to investigate the molecular energetics and orientation of surfactant-like model "friction modifier" (FM) additives with and without hydrocarbon solvent and confined between idealized atomistic surfaces. The normal load, fluid layer thickness, and additive surface concentration dependencies agree favorably with those measured experimentally for model fluids using a surface forces apparatus. The simulations predicted either a single or multiple free energy wells with increasing surface separation, depending on the FM concentration. With no solvent added, pure FMs showed oscillations in normal pressure and free energy with increasing surface separation; stable states corresponded to successive layers of FM molecules being expelled from the region between the adsorbed films. In the case of FM/hydrocarbon solution, only a single stable position was found. The equilibrium structure was also found to depend on the head group structure of the FMs.