Molecular simulation studies of nanoscale friction between phosphorylcholine self-assembled monolayer surfaces: correlation between surface hydration and friction.

We performed all-atom molecular dynamics simulations to study the friction between surfaces covered with two phosphorylcholine self-assembled monolayers (PC-SAM) under shear. PC-SAM surfaces with a sqrt7 X sqrt7R19 degrees lattice structure and a parallel arrangement of the head groups were used as model zwitterionic surfaces. They provide a full representation of the zwitterionic nature of phospholipid surfaces, which are believed to play an important role in the lubrication of biological joints such as knees and hips. The surfaces were immersed in aqueous solutions and kept in contact with two regions of bulk water. Sodium chloride and potassium chloride solutions at various concentrations were employed to study the effects of the presence of ions on friction. The results show a strong relationship between surface hydration and friction. Higher ionic concentrations or ions with shorter Debye lengths cause a larger disruption to the hydration around the zwitterionic surfaces, leading to larger friction forces. In addition, the results show that under nanoscale confinement, the friction coefficients of PC-SAM surfaces in pure water are directly proportional to both shear velocity and surface separation distance. These results are comparable to previously published experimental studies.