Surface freezing in chain molecules. II. Neat and hydrated alcohols

Abstract

Surface freezing is studied in dry and hydrated normal-alcohol melts by x-ray scattering and surface tensiometry. A single crystalline bilayer forms at the surface, for even carbon numbers only, at temperatures up to 1 °C (dry) or 2 °C (wet) above the bulk freezing, and persists without change down to bulk freezing. The packing is hexagonal, with untilted molecules for short chains and tilted molecules for long chains. The lattices of the upper and lower monolayers are shifted along the next-nearest-neighbor direction. Hydration is found to swell the bilayer by $\sim 2.5$ Å due to water intercalation into the bilayer, at a molecular water:alcohol ratio of $\sim 1:2.$ It also increases the transition temperatures, and the temperature and chain-length ranges for which surface crystallization is observed. These effects are accounted for quantitatively by considering the surprising increase in hydration upon freezing, and taking into account the Gibbs-rule-predicted water depletion at the surface in the liquid surface phase.