A rare surface freezing phenomenon is observed in molten normal alkanes, using x-ray and surface tension measurements. An ordered monolayer forms on the surface of the liquid alkane at temperatures up to 3°C above the bulk freezing temperature T f . The structure of the monolayer was studied in detail for a wide range of molecular lengths and temperatures. The single layer formed persists down to T f . The rare surface phase exists only for carbon numbers of 16рnр50. The molecules in the layer are hexagonally packed and show three distinct ordered phases: two rotator phases, with molecules oriented vertically (16рnр30) and tilted towards nearest neighbors (30ϽnϽ44) and one crystalline phase with molecules tilted towards next-nearest neighbors (nу44). The temperature dependence of the surface tension and the range of existence vs carbon number are satisfactorily accounted for within a simple theory based on surface energy considerations.

We present results of a detailed x-ray scattering study on the rotator phases of normal alkanes: CH3–(CH2)n−2–CH3 (20≤n≤33). We have characterized a new tilted rotator phase and determined the temperature and chain length dependence of the distortion, tilt, and azimuthal order parameters which characterize the time-space averaged structures of the five rotator phases. We have shown that there is no strong even–odd chain length effect on the phase diagram within the rotator phases and have shown the continuity of that phase diagram in the 26-27 carbon vicinity.

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