SURFACE FREEZING IN CHAIN MOLECULES - NORMAL-ALKANES

A rare surface freezing phenomenon is observed in molten normal alkane s, using x-ray and surface tension measurements. An ordered monolayer forms on the surface of the liquid alkane at temperatures up to 3 degr ees C above the bulk freezing temperature T-f. The structure of the mo nolayer was studied in detail for a wide range of molecular lengths an d 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 hexagon ally 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) an d one crystalline phase with molecules tilted towards next-nearest nei ghbors (n greater than or equal to 44). The temperature dependence of the surface tension and the range of existence vs carbon number are sa tisfactorily accounted for within a simple theory based on-surface ene rgy considerations.