Surface micelles of semifluorinated alkanes in Langmuir-Blodgett monolayers

We have studied the molecular patterns formed in Langmuir-Blodgett (LB) films of semifluorinated alkanes CnF2n+1CmH2m+1 (FnHm diblocks, n = 6, 8, 10, m = 14, 16, 18, 20) by atomic force microscopy. All the compounds investigated formed surface micelles whose shape and dimensions varied with the molecular structure of the FnHm diblocks. Except for F6H16 and F8H14, which produced exclusively circular micelles, all of the other diblocks also formed elongated micelles that coexisted with circular micelles. The mean diameter of the circular micelles increased with the length of the hydrogenated segment of the diblocks. By contrast, increasing the length of the fluorinated segments did not have any detectable effect on the diameter. The elongated micelles became more numerous and longer for longer FnHm (both the length of the hydrogenated and of the fluorinated segments had a strong effect). The surface pressure of transfer had an influence on the morphology of the elongated micelles, the latter becoming fewer and shorter for higher surface pressures. A detailed X-ray reflectivity study conducted on F8H16 LB films showed that the hydrogenated segments are directed towards the silicon wafer, while the fluorinated segments point outwards toward the air. A disc-like model is proposed for the surface micelles.