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

• Published in: Langmuir Vol. 13; no. 18; pp. 4865 - 4869
• Main Authors: Hayami, Y, Findenegg, G. H
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 03.09.1997
• Subjects: Chemistry; Exact sciences and technology; Gas-liquid interface and liquid-liquid interface; General and physical chemistry; Surface physical chemistry; Organic adsorbate; Crystallization; Temperature effect; Fluorocarbon; Experimental study; Organic adsorbent; Hexadecane; Phase transformation; Concentration effect; Liquid liquid interface; Liquid liquid adsorption; Surface properties; Surface tension; Thermodynamic properties; Phase equilibrium
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la9702446

The interplay of the surface freezing phenomenon of hexadecane (abbreviated as H16) with the monolayer phase behavior of a semifluorinated alkane, 1-(perfluorododecyl)hexadecane F(CF2)12(CH2)16H (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 H16, our system F12H16 + H16 exhibits gaseous, expanded, and condensed monolayer phases of F12H16. 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 H16. In contrast, in the domain of the expanded film Δs 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 H16. It appears that F12H16 is effectively insoluble in the solid monolayer of H16 and that the F12 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 H16 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 H16 (17.64 °C).