Abstract
X-ray surface scattering and surface tension measurements reveal surface freezing in molten mixtures of alkanes of two different lengths. A crystalline monolayer is formed at the surface a few degrees above the bulk freezing temperature. The structure of the monolayer has been determined on an angstrom scale. Two widely different patterns of behavior emerge, which depend on the length difference of the two components, Δ n. For small Δ n the surface properties and structure vary continuously with concentration. For large Δ n, however, the variation is discontinuous, exhibiting surface segregation. Furthermore, a new surface crystalline structure appears for a well-defined range of compositions and temperatures, and surface freezing is completely suppressed for another range. 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.
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