Tunable surface phases in alcohol-diol melts

Surface crystallization is studied in mixed alcohol-diol melts by x-ray diffraction and surface tensiometry. A reversible transition, having no bulk counterpart, from a bilayer to a monolayer surface phase, tunable by either diol concentration φ or temperature T, is observed. The molecular tilt is found to vary with φ in the bilayer phase. The structure of both surface phases is determined in detail. A simple theory, assuming a linearized φ dependence of the free energies of the various interfaces in each phase, accounts well for the observed (φ, T) phase diagram.