The behavior of the ellipticity coefficent ρ and the Brewster angle θB (not shown in the Figures) show that formation of the dense H18F12 film causes an anchoring transition of the LC phase from homeotropic at the dilute H18F12 film to planar at the dense H18F12 film.
The change of the anchoring condition from homeotropic to planar has a striking effect on the nematic or smectic order which is present at the surface of the isotropic LC bulk phase. For 8CB, which shows nematic surface order, the ellipsometric data suggest that a thin planar nematic film remains when the dense H18F12 film forms. The planar nematic film grows in thickness as the bulk transition to the nematic phase is approached from above (cf. inset in Figure 7).
In the case of 12CB, which shows smectic surface order, our data suggest that the few (essentially one or two) smectic layers, which exist on the surface of the isotropic bulk phase, vanish when the dense H18F12 film forms, i.e., the smectic surface order is destroyed when the LC molecules change their alignment from homeotropic to planar.
In addition to the ellipsometry measurements, we have conducted AFM studies of the free surface of 8CB and 12CB droplets doped with H18F12 in order to clarify the structure of the dense film of the semifluorinated alkane. The AFM images show clearly that the dense film consists of a crystalline-like hexagonal packing of surface micelles, similar to those observed in transferred Langmuir films and spin-coated films of semifluorinated alkanes (see, e.g., M. P. Krafft, Acc. Chem. Res. 45, 514 (2012)). In addition to the hexagonal pattern of the surface micelles, we observe in some cases linear steps in the surface (arrows in Figure 10b) the height of which is clearly smaller than the smectic layer thickness. So far, we do not have an explanation for these steps. The formation of the surface micelles might be the reason for the observed anchoring transition from homeotropic to planar (see Figure 12).