Mechanism of surface freezing of alkanes

Using molecular dynamics simulation of octane (C₈) and nonadecane (C₁₉), we probe the mechanism of n-alkane surface freezing, the appearance of a crystalline monolayer above the liquid at a temperature T_sf above the bulk freezing point T_f. Formation of a crystalline monolayer occurs robustly in these systems. When T_f > T_sf, the surface frozen phase is metastable with respect to the solid but persists for long periods for study in simulations. Surface freezing of both C₈ and C₁₉ is driven by significant energy-lowering when alkane chains become ordered along the surface normal, and we elucidate the origins of this phenomenon. The degree of configurational disorder in the surface frozen layer relative to the solid is much larger for C₈ compared to C₁₉. From the Gibbsian viewpoint, we extract the excess energy and entropy of the liquid and surface frozen phases. We also consider the surface frozen layer as an intervening third phase, the viewpoint taken in previous theoretical analyses. Here, we find significantly increased entropy of the surface frozen phase of C₈ associated with configurational disorder, while the energy and entropy of the surface frozen phase of C₁₉ are marginally different from the bulk solid. Finally, by combining our previously determined solid-vapor surface free energies of C₈ and C₁₉ with liquid-vapor surface tensions from this work, we eliminate wetting as a possible mechanism for C₈ surface freezing, but it remains a possibility for C₁₉. We analyze the molecular structure of the liquid, surface frozen, and solid surfaces and discuss its relevance to thermodynamic properties.