Surface freezing in chain molecules: Normal alkanes

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 degrees 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 16 less than or equal to n less than or equal to 50. The molecules in the layer are hexagonally packed and show three distinct ordered phases: two rotator phases, with molecules oriented vertically (16 less than or equal to n less than or equal to 30) and tilted towards nearest neighbors (30<n<44) and one crystalline phase with molecules tilted towards next-nearest neighbors (n greater than or equal to 44). 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.