Gibbs Films of Semi-Fluorinated Alkanes at the Surface of Alkane Solutions

Abstract

Adsorbed monolayer phases of semi-fluorinated alkanes F(CF 2 ) 1 2 (CH 2 ) m H with m = 12, 14, 16, and 18 (denoted F 1 2 H m ) at the free surface of their solutions in H 1 2 and H 1 6 solvents (dodecane, bicyclohexyl, and hexadecane) were studied by surface tension measurements and surface-sensitive X-ray techniques. Solutions in the two H 1 2 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 F 1 2 H m molecules are close-packed in two-dimensional hexagonal arrays (cross-sectional area, ca. 28A 2 ), but the in-plane order is rather short ranged, having a positional correlation length of only ca. 20 A. The electron density profiles ρ 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 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 F 1 2 H m molecules. Gibbs films of F 1 2 H 1 6 on hexadecane exhibit not a sharp transition but a gradual increase of the surface concentration below 25 °C. This anomalous behavior is attributed tentatively to a solvation of the alkane block of the F 1 2 H 1 6 molecules by the hydrocarbon solvent, which will prevent a close packing of the fluorocarbon blocks.