Molecular ordering and domain morphology of molecularly thin triacontane films at SiO2/air interfaces

The molecular ordering and the domain morphology of molecularly thin triacontane Rims at SiO2/air interfaces has been investigated at room temperature by scanning force microscopy, X-ray diffraction, and X-ray reflexion. Our results suggest that the room temperature structure is influenced by the surface freezing behaviour of triacontane, i.e. its solidification adjacent to the interface already some degrees (approximate to 69 degreesC) above the bulk melting temperature (approximate to 66 degreesC). Depending on the amount of alkane available either a closed monolayer topped by separated chunks of bulk (solidified droplets) or, in the case of only submonolayer alkane coverage fractal, seaweed-shaped domains are observed. In both cases, adjacent to the interface the molecules are oriented upright and packed in a rectangularly centered unit cell with a lateral correlation of much more than 200 molecules. A fractal dimension of 1.64 +/- 0.06 indicates that the domains are formed by diffusion-limited aggregation.