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 ^∘{}C above the bulk freezing temperature Tf. 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 Tf. The rare surface phase exists only for carbon numbers of 16{}n{}50. The molecules in the layer are hexagonally packed and show three distinct ordered phases: two rotator phases, with molecules oriented vertically (16{}n{}30) and tilted towards nearest neighbors (3044) and one crystalline phase with molecules tilted towards next-nearest neighbors (n{}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.