Using atomic-resolution x-ray reflectivity we have investigated monolayers formed at the bulk alkanol-sapphire interface. Measurements were performed using high energy x-rays at 71 and 32 keV, sufficiently high that there is little absorption through the liquid. This system, with its relatively balanced inter-molecular and molecule-substrate interactions bridges the gap between self-assembled and surface-frozen monolayers, each dominated by only one interaction. For octadecanol the monolayer is comprised of densely packed surface-normal molecules, hydrogen-bound to the sapphire. Mediated by a partial desorption of the alkanols, the monolayers melt reversibly at a temperature about 30�C above the bulk, which we rationalized within a thermodynamic model comprising interfacial interactions, molecular elasticity, and entropic effects. Separating the substrate from the alkanol melt leaves the monolayer structurally intact.  Additional studies show that the monolayer is epitaxial with the sapphire substrate. The interfacial surface freezing behavior found is expected to be generic to weakly bound, self-assembled monolayers.

Missing link: Surface freezing meets self-assembly

243rd National Meeting (2012)

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