Effect of molecular geometry on boundary layer lubrication

Simulation studies of the friction between layers of dialkyl surfactants have been performed using a non-equilibrium molecular dynamics method. The model layers at a temperature of 298 K and a normal pressure of 210 MPa are sheared at a relative velocity of 1 ms-1. The friction coefficient has been studied as a function of the molecular geometry by simulating bilayers of C10C18 (asymmetrical) and C18C18 and C10C10 (symmetrical) surfactants. In all cases the hydrocarbon chains are attached to a positively charged dimethylammonium head-group which interacts with a negatively charged surface. At a head-group area of 50 angstrom2 per molecule, the friction between the layers of asymmetrical surfactants is greater than that between layers of symmetrical surfactants at approximately the same normal pressure. At 77 angstrom2 the friction between the C18C10 layers remains higher than that of the C18C18 layers but is now lower than that of the C10C10, where the surface structure is highly disordered and the two layers are separated by only 15.8 angstrom. The friction between the layers correlates well with the amount of layer overlap as defined by the common area under the chain density profiles. These observation, which are in broad agreement with the experimental measurement on similar dichain surfactants are rationalized in terms of the translational, orientational and conformational structures of the layers.