Bulk structure and surface activity of semifluorinated alkanes

Semifluorinated alkanes of different even numbered hydrocarbon chain lengths and their solutions in alkanes were studied by scattering methods. The pure substances pass through several solid phases over a broad temperature range. In this study we analyzed the underlying molecular structures by means of powder X-ray diffraction. The molecular orientation of SFA molecules in their solid phases is found to correspond to smectic liquid crystalline phases. The second part of the present paper reports on recent results on surface-induced ordering effects, obtained by surface sensitive methods applied to binary systems of SFAs and alkanes. A kink in the temperature dependence of the surface tension of such solutions suggests the forming of a Gibbs him, i.e. a crystalline monolayer of SFA molecules at the solutions surface. Specular X-ray reflectivity measurements and Brewster angle microscopy conducted on the system strongly support this suggestion. It is also found that the concentration and chain length of the solute strongly influence the temperature at which crystallization of the surface film occurs. (C) 2000 Elsevier Science B.V. All rights reserved.