The interfacial tension γ of the hexane solution of a mixture of 1,1,2,2-tetrahydroheptadecafluorodecanol (FC10OH) and 1,1,2,2-tetrahydrohenicosafluorododecanol (FC12OH) against water was measured as a function of the total molality m and composition of FC12OH X 2 at 298.15 K under atmospheric pressure. The γ vs m curve has one or two distinct break points depending on X 2. By plotting the γ and m values at the break points and drawing the interfacial pressure π vs mean area per adsorbed molecule A curves, it was suggested that the triple point of adsorbed film, at which the three kinds of states (gaseous, expanded, and condensed states) coexist simultaneously, exists at a middle composition. The phase diagrams of adsorption (PDA) were constructed and the excess Gibbs energy of adsorption g H,E was calculated to examine the miscibility of FC10OH and FC12OH molecules in the adsorbed film. It was found that these alcohols mix almost ideally both in the gaseous and expanded states and nonideally in the condensed state. Furthermore, the mixing of FC10OH and FC12OH in the condensed film was accompanied by a positive g H,E value. This suggests that the difference in the magnitude of mutual interaction between the same species affects appreciably the miscibility of molecules in the condensed state where the adsorbed molecules are assumed to be closely packed and arranged regularly.

The mixed adsorbed film of 1H,1H-perfluorooctanol (DFC8OH) and 1H,1H,2H,2H-perfluorodecanol (TFC10OH) at the hexane/water interface was studied on the basis of interfacial tension measurement and its thermodynamic data analysis. An adsorbed film at any composition of the mixed system as well as those of pure DFC8OH and TFC10OH systems exhibits three states:  the gaseous, expanded, and condensed states. Construction of the phase diagram of adsorption clarified that DFC8OH and TFC10OH mix almost ideally in the gaseous and expanded states. On the contrary, the excess Gibbs energy of adsorption g H,E value evaluated in the condensed state was positive. These results are explained by considering the following two factors:  (1) The mixing of binary alcohols is accompanied by the loss of dispersion interaction energy due to the difference in extent of fluorination of hydrophobic chains and in their chain length and increases the g H,E value. (2) Since the interchange energy concerning the interaction between dipoles with different dipole moments is negative, the mixing of these alcohols reduces the repulsive force between hydrophilic groups and thus leads to a decrease in the g H,E value. In the gaseous and expanded states, both of above two factors are not effective. On the other hand, the positive g H,E value in the condensed state is attributable to more effective dispersion interaction than the dipole−dipole interaction in short molecular distance, and so factor 1 becomes dominant. Comparison of the g H,E value of the present system with that of the homologous TFC10OH−TFC12OH mixture leads us to a conclusion that the hydrogenation on β-carbons in hydrophobic chains affects appreciably the balance of interactions between hydrophilic and hydrophobic groups which governs the mixing of molecules in adsorbed films.