Surface phases of semi-fluorinated alkane melts

An ordered monolayer is found to form at the free surface of semi-fluorinated alkane melts, F(CF 2 ) 12 (CH 2 ) n H, a few degrees above the bulk freezing point. X-ray and surface tension measurements show the monolayer to assume one of two different structures, depending on n . For n ∼ 10 the structure is fully crystalline, while for n ∼ 18 it has only short-range in-plane order. This can be assigned to packing frustration of the different diameter F and H blocks, for chain lengths where their respective intermolecular interactions become comparable.