Experimental evidence for an original two-dimensional phase structure: an antiparallel semifluorinated monolayer at the air-water interface.

We show the spontaneous formation of an antiparallel monolayer of diblock semifluorinated n-alkane molecules spread at the air-water interface. We used simultaneous measurements of surface pressure and surface potential versus molecular area and performed grazing x-ray reflectivity experiments to characterize the studied monolayer, which is obtained at almost zero surface pressure and precedes the formation of a bilayer at higher surface pressure. Its thickness, equal to 2.7 nm, was found to be independent of the molecular area. This behavior may be explained by van der Waals and electrostatic interactions.

Air-water interface-induced smectic bilayer

We show, using surface pressure versus molecular area isotherms measurements and x-ray reflectivity, that the long diblock semifluorinated n-hexaeicosane molecules, F(CF2)(8)-(CH2)18H, form a stable smectic bilayer phase, noted M1, with a total thickness of 3. 3 nm, at an apparent molecular area about 0.3 nm(2), though in the bulk the used molecules do not form smectic phases at any temperature. We discuss different molecular packing models according to our experimental data and deduce that molecules are antiparallel with fluorinated chains outwards and interleaved hydrocarbon chains inwards.