Clarification of the mechanisms of surface melting and surface freezing in n-alkane thin films by large-scale molecular simulation

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It is well known that n-alkanes exhibit surface freezing, where a solid monolayer forms at the surface of an isotropic liquid above its bulk melting temperature. This is a distinct contrast to a surface-melting phenomenon in the low-molecular substance. In this study, we investigated the melting processes of n-alkane thin films by molecular dynamics simulations. In the case of n-nonane, it is found that melting occurs from the surface, which indicates that n-nonane exhibits surface melting. In the case of n-nonadecane, on the other hand, we find a temperature range where the middle layer is in the molten states and both surface layers remain in the crystalline state (surface frozen state).