Wetting behavior above the liquid-crystal-isotropic transition in a homologous series

An ellipsometric study of the wetting behavior at the free surface above th e isotropic to nematic or isotropic to smectic-A transition of nine homolog ous compounds with even alkyl chain lengths n in the range from four to twe nty carbon atoms is presented. All compounds show a pretransitional increas e of the nematic or smectic surface coverage as the bulk isotropic to liqui d-crystal transition is approached from above. The behavior of the nematic compounds (n=4 to 10) can be interpreted, within the framework of a Landau model, as complete wetting. In short nematic homologs the divergence of the nematic coverage is strongly reduced by a decrease of the nematic suscepti bility of the isotropic phase. The elastic coefficient L of the Landau mode l shows a pronounced increase with increasing n, resulting in the occurrenc e of a discontinuous prewetting transition in the shortest smectic homolog (n=12) that is still describable by the nematic Landau model. In the longer smectic homologs (n=14 to 20), layering steps appear in the pretransitiona l increase of the coverage. The results indicate probable partial wetting f or the longest homolog, whereas for the other smectic compounds the distinc tion between complete and partial wetting is difficult on the basis of elli psometry.