Effect of Hydrophobic Chain Structure on Phase Transition and Domain Formation of Hybrid Alcohol Films Adsorbed at the Hexane/Water Interface

The phase transition and domain formation of the adsorbed film of two kinds of hybrid alcohols (CF₃(CF₂)_m-1(CH₂)_nOH, FmHnOH), 2-perfluorooctylethanol (F₈H₂OH) and 2-perfluorohexylhexanol (F₆H₆OH), as a mixture at the hexane/water interface was investigated by interfacial tensiometry and X-ray reflection. The interfacial tension γ versus total molality m curve of pure F₈H₂OH has a break point at high concentration, and thus, the mean area per molecule A changes discontinuously at high interfacial pressure π, corresponding to the phase transition between expanded and condensed films. The Fresnel divided reflectivity R/R_F versus Q_z plots in the expanded state was well-fitted by the domain model for incoherent interference to determine the interfacial coverage, which is the fraction of the interface covered by the condensed phase. This indicates that the expanded film is heterogeneous and consists of a condensed F₈H₂OH domain, the size of which is larger than the X-ray coherence length (∼5 μm). In the mixed system, the discontinuous change in A at the phase transition point becomes small with increasing the bulk composition of F₆H₆OH X₂ in the mixture, and eventually the A value changes continuously; i.e, the phase transition becomes obscure in X₂ ≥0.6. This behavior was linked to an increase in interfacial coverage with X₂. Furthermore, the R/R_F versus Q_z plot was fitted by the domain model for coherent interference, suggesting that the size of the domain is smaller than 5 μm. These results are probably due to the reduction of domain line tension by preferential adsorption of F₆H₆OH at the F₈H₂OH domain boundary.