Surface structure tuning in alcohol melts by bulk additives

Surface freezing, i.e. the formation of a solid bilayer at the surface of a pure alcohol melt above the freezing temperature, was recently discovered. We demonstrate here that the structure of the bilayer can be influenced by specific bulk additives. Two different additives, able to form hydrogen bo nds (water) and both hydrogen bonds and van der Waals interactions (alpha,o mega-diols), are used. The surface layer's structure is studied by surface- specific X-ray methods. We find that water swells the bilayer by intercalat ion into its center at a molecular water:alcohol ratio of similar to 1:2. F or the diolated alcohols a reversible monolayer-bilayer surface phase trans ition is observed, the first of its kind for surface-frozen layers. A conti nuous decrease of the molecular tilt with increasing diol concentration is also found for the tilted bilayer phases. (C) 2000 Elsevier Science B.V. Al l rights reserved.