Surface crystallization and phase transitions of the adsorbed film of F(CF2)(12)(CH2)(16)H at the surface of liquid hexadecane

The interplay of the surface freezing phenomenon of hexadecane (abbreviated as H-16) with the monolayer phase behavior of a semifluorinated alkane, 1-(perfluorododecyl)hexadecane F(CF2)(12)(CH2)(16)H (abbreviated as F12H16), at the air/hexadecane solution interface was studied by measuring the surface tension of hexadecane solutions of F12H16 as a function of temperature and bulk concentration under atmospheric pressure. Above the surface freezing temperature of H-16, our system F12H16 + H-16 exhibits gaseous, expanded, and condensed monolayer phases of F12H16. In the domain of the gaseous film the entropy of surface formation per unit area, Delta s, is almost independent of temperature and bulk concentration, and its magnitude is similar to that of pure H-16. In contrast, in the domain of the expanded film hs decreases sharply with increasing bulk concentration. Thus, the expanded film of F12H16 in the present system manifests a unique behavior, indicating the formation of an ordered structure comparable to that of a condensed film. At lower temperature, this evolution of surface phases of the amphiphile (F12H16) is cut off by the surface freezing of H-16. It appears that F12H16 is effectively insoluble in the solid monolayer; of H-16 and that the F-12 chains of the F12H16 molecules form a close-packed film in the condensed state. The point at which the gaseous film coexists with the condensed phase of F12H16 and the solid monolayer phase of H-16 represents a surface heteroazeotrope (surface eutectic) and is located at about 17.4 degrees C, i.e., ca. 0.2 K below the surface freezing of pure H-16 (17.64 degrees C).