Gibbs films of semi-fluorinated alkanes at the surface of alkane solutions

Adsorbed monolayer phases of semi-fluorinated alkanes F(CF2)(12)(CH2)(m)H with m = 12, 14, 16, and 18 (denoted F12Hm) at the free surface of their solutions in H-12 and H-16 solvents (dodecane, bicyclohexyl, and hexadecane) were studied by surface tension measurements and surface-sensitive X-ray techniques. Solutions in the two H-12 solvents exhibit sharp first-order phase transitions from a gaslike state at high temperature to a condensed state at lower temperatures. In the condensed film, the fluorocarbon blocks of the F12Hm, molecules are close-packed in two-dimensional hexagonal arrays (cross-sectional area, ca.28 circle/A(2)), but the in-plane order is rather short ranged, having a positional correlation length of only ca. 20 circle/A. The electron density profiles rho(e)(z) reveal that the mass centers of the fluorocarbon blocks are not aligned in a plane but distributed over a region of ca. 20-30 circle/A. The short-range order of these films is attributed to the packing frustration of the different diameters of the fluorocarbon and hydrocarbon blocks of the F12Hm molecules. Gibbs films Of F12H16 on hexadecane exhibit not a sharp transition but a gradual increase of the surface concentration below 25degreesC. This anomalous behavior is attributed tentatively to a solvation of the alkane block of the F12H16 molecules by the hydrocarbon solvent, which will prevent a close packing of the fluorocarbon blocks.