Miscibility of alkanol and fluoroalkanol in Langmuir film at the air/water interface

The miscibility of 1,1,2,2-tetrahydrohenicosafluorododecanol (TFC₁₂OH) and 1-tetradecanol (C₁₄OH) in Langmuir film at the air/water surface is studied by surface pressure π versus area per molecule A isotherms, Brewster angle microscopy (BAM), and grazing incidence X-ray diffraction (GIXD) performed at 298.15 K under atmospheric pressure. Plotting the transition pressure, which is given by a kink point on the π versus A isotherm, against composition X₂, provides a two-dimensional phase diagram of this system. The diagram is of an eutectic type and therefore, the TFC₁₂OH and C₁₄OH molecules are practically immiscible in the condensed phase. The BAM images at mixed compositions are complicated but their composition dependency is understood in terms of the phase diagram. Furthermore, in the GIXD measurement at X₂=0.5, two sharp peaks were observed on the in-plane intensity plot above the eutectic pressure. By comparing their peak positions to those of the pure component systems, it is concluded that one of the peaks originates from the pure TFC₁₂OH condensed state and the other from the pure C₁₄OH condensed state. This microscopic information strongly supports our macroscopic results obtained from the phase diagram.