The surface-ordered phase of n-nonadecane: A molecular dynamics simulation

Behavior of realistic n-nonadecane, ${\mathrm{C}}_{\mathrm{19}}{\mathrm{H}}_{40},$ on the free surface of the melt is investigated by the molecular dynamics simulation. A crystalline thin film made of three layers of lamellas is constructed and used as an initial state of the heating simulation, where we devise a special heating scheme in order to accelerate the rate of convergence to the thermal equilibrium. By very slow heating we find a temperature range (385–410 K) where the middle layer prefers to be in the melt state with both surface layers remaining crystalline; this observation is considered to correspond to the surface freezing recently found in n-alkanes. It is found that the molecules in the surface monolayer align their axes nearly perpendicular to the surface and form well-defined hexagonal packing. It is also found that the molecules in the surface monolayer show large center-of-mass fluctuations, translational and transverse, along the surface normal and parallel to the surface, respectively.