Surface Crystallization and Phase Transitions of the Adsorbed Film of F(CF2)12(CH2)16H at the Surface of Liquid Hexadecane

Abstract

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(CF 2 ) 12 (CH 2 ) 16 H (abbreviated as F 12 H 16 ), at the air/hexadecane solution interface was studied by measuring the surface tension of hexadecane solutions of F 12 H 16 as a function of temperature and bulk concentration under atmospheric pressure. Above the surface freezing temperature of H 16 , our system F 12 H 16 + H 16 exhibits gaseous, expanded, and condensed monolayer phases of F 12 H 16 . In the domain of the gaseous film the entropy of surface formation per unit area, Δs, 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 Δs decreases sharply with increasing bulk concentration. Thus, the expanded film of F 12 H 16 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 (F 12 H 16 ) is cut off by the surface freezing of H i6 . It appears that F 12 H 16 is effectively insoluble in the solid monolayer of H 16 and that the F 12 chains of the F 12 H 16 molecules form a close-packed film in the condensed state. The point at which the gaseous film coexists with the condensed phase of F 12 H 16 and the solid monolayer phase of H 16 represents a surface heteroazeotrope (surface eutectic) and is located at about 17.4 °C, i.e., ca. 0.2 K below the surface freezing of pure H 16 (17.64 °C).