Molecular dynamics simulation on pressure and thickness dependent density of squalane film

Molecular dynamics (MD) simulations using the polymer consistent force field (PCFF) were adopted to investigate the pressure and thickness dependent density of squalane film in a nanogap at 373 K, with three different initial film thicknesses, and for a wide range of pressures. The equivalent densities predicted by MD simulations were compared with the empirical data. Results show that the squalane atoms tend to form layers parallel to the confining substrates but the orientations of squalane molecules are irregular throughout the film. In addition, distinct excluded volumes are not found at the interfaces of the film and substrates. Furthermore, with the same initial film thickness h0, the film thickness h and compressibility decrease with increasing pressure, but the compressibility is similar for films with different initial film thicknesses. The equivalent densities predicted by MD simulations with the maximum initial film thickness (9.44 nm) are accurate to the values of Tait equation. The MD simulation with adequate initial film thickness can accurately and conveniently predict the bulk densities of lubricants.