MOLECULAR-ORIENTATION MECHANISM OF N-PARAFFINS AND FATTY-ACID THIN-FILMS BY EVAPORATION METHOD

The structure of the evaporated thin films of n-paraffins and long chain fatty acids was studied by X-ray symmetrical reflection technique. The activation energy of evaporation of n-paraffins linearly increased with an increase in the carbon number. Sublimation of paraffins was observed at carbon numbers below 28. Paraffin and stearic acid molecules tended to orient perpendicular to a glass substrate (normal orientation). The normal orientation could be controlled by supercooling defined by the difference between the melting temperature of the sample and the substrate temperature. Mechanism of the orientation is closely related to molecular motion on the substrate.