Surface freezing in chain molecules: Normal alkanes

Abstract

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 \ifmmode^\circ\else\textdegree\fi{}C above the bulk freezing temperature ${\mathrm{T}}_{\mathrm{f}}$. 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 ${\mathrm{T}}_{\mathrm{f}}$. The rare surface phase exists only for carbon numbers of 16\ensuremath{\leqslant}n\ensuremath{\leqslant}50. The molecules in the layer are hexagonally packed and show three distinct ordered phases: two rotator phases, with molecules oriented vertically (16\ensuremath{\leqslant}n\ensuremath{\leqslant}30) and tilted towards nearest neighbors (3044) and one crystalline phase with molecules tilted towards next-nearest neighbors (n\ensuremath{\geqslant}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.