Surface freezing and molecular miscibility of binary fluoroalkanol-alkanol liquid mixture

The surface freezing (SF) of liquid 6-perfluorohexyl hexanol (F₆H₆OH)–1-dodecanol (C₁₂OH) mixture was studied by surface tension, external reflection absorption FTIR (ERA-FTIR), and X-ray reflectivity (XR) measurements. The surface tension vs. temperature curve of pure C₁₂OH liquid shows a sharp break point at the surface liquid (SL)–SF phase transition temperature T_s. The surface tension of pure F₆H₆OH, on the other hand, decreases monotonically with increasing temperature. The entropy of surface formation in the SF state of C₁₂OH is large negative due to a well-ordered structure at the surface. The electron density profile determined by XR showed that the C₁₂OH molecules form bilayer stabilized by the hydrogen bonding between hydroxyl groups facing each other through water molecule intercalated in between them. In the F₆H₆OH–C₁₂OH system, the estimation of surface composition indicated that the SL state is enriched in F₆H₆OH compared to bulk liquid. The negative excess entropy in the SL layer suggests that F₆H₆OH molecules form domains at the surface. Furthermore, the surface composition and electron density profile in the SF state indicated that a small amount of F₆H₆OH mix with C₁₂OH in the SF bilayer in which F₆H₆OH molecules preferentially exist in the upper layer of the bilayer.