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 $\phi$ or temperature $T$, is observed. The molecular tilt is found to vary with $\phi$ in the bilayer phase. The structure of both surface phases is determined in detail. A simple theory, assuming a linearized $\phi$ dependence of the free energies of the various interfaces in each phase, accounts well for the observed ( $\phi ,T$) phase diagram.