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, $\mathrm{F}({\mathrm{CF}}_2{)}_8-({\mathrm{CH}}_2{)}_{18}\mathrm{H},$ form a stable smectic bilayer phase, noted $M_1,$ with a total thickness of 3.3 nm, at an apparent molecular area about 0.3 ${\mathrm{nm}}^2,$ 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.