Surface freezing in binary mixtures of chain molecules. II. Dry and hydrated alcohol mixtures

Surface freezing is studied in dry and hydrated alcohol mixtures by surface x-ray scattering and surface tension measurements. A crystalline bilayer is formed at the surface a few degrees above the bulk freezing temperature. The packing is hexagonal, with molecules aligned along the surface normal in all cases. The in-plane lattice constant reveals a qualitatively different behavior with composition for hydrated and dry mixtures. The simple theoretical approach used successfully for alkane and deuterated alkane mixtures accounts well also for the alcohol mixtures. The repulsive length-mismatch term opposing the mixing entropy term in the free energy of the mixtures is shown to have a universal behavior for all mixtures studied: protonated alkanes, deuterated alkanes, and dry and wet alcohols. This universality is somewhat counterintuitive in view of the different interactions (e.g., hydrogen bonding in alcohols) in the different mixtures.