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Surface freezing is studied in molten binary mixtures of alkanes and alcohols of different lengths using X-ray surface scattering and surface tension measurements. A crystalline monolayer (for alkanes) or bilayer (for alcohols) is formed at the surface a few degrees above the bulk freezing temperatures. The behaviour is found to be dominated by the length difference of the two components, Δn. For small Δn the surface properties and structure vary continuously with concentration between those of the pure components. For large Δn, however, the variation is discontinuous, exhibiting surface segregation. Several new phenomena, not observed in the pure components, are also found: a new surface crystalline structure in alkanes, a suppression of surface freezing for some compositions in both materials, and the inducement of surface freezing in alcohols which do not show the effect when pure. A Flory-Huggins theory based on competition between entropic mixing and a repulsive interaction due to chain length mismatch accounts well for the observed phenomena in alkanes, but requires modifications when applied to alcohols, probably due to the more complex additional headgroup interactions. |
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