A Molecular Dynamics Simulation of the Surface-Ordered Phase of n-Alkanes : the Effect of Chain Length

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Behavior of realistic n-pentadecane, n-C_<15>H_<32>; n-tridecane, n-C_<13>H_<28> and n-undecane, n-C_<11>H_<24> on the free surface of the melt is investigated searching for the effect of chain-length on the surface-ordered phase of n-alkanes, with larger simulation cells than that in our previous work. By very slow heating in each sample we find a temperature range (those are 325-340K, 305-320K and 288-293K, for n-C_<15>H_<32>, n-C_<13>H_<28> and n-C_<11>H_<24>, respectively) where the middle layers prefer to be in the melt state with both free surfaces remaining crystalline; these observations are considered to correspond to the surface freezing recently found in n-alkanes. It is observed that the orientation of molecular axes in the surface monolayer is nearly perpendicular to the surface, and the molecules form regular hexagonal side packing similar to that observed in the rotator phases in crystals. It is found that the temperature of surface freezing, the translational disorder in the surface normal direction and the transverse disorder within the surface depend clearly on the chain length: the longer the chain, the wider the surface freezing temperature range and the larger the translational disorder.
社団法人日本物理学会の論文
2002-04-15