The mixed adsorbed film of 1H,1H-perfluorooctanol (DFCsOH) 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 DFCsOH and TFC10OH mix almost ideally in the gaseous and expanded states. On the contrary, the excess Gibbs energy of adsorption gH,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 gH,E value. (2) Since the interchange energy concerning the interaction between dipoles with different dipóle moments is negative, the mixing of these alcohols reduces the repulsive force between hydrophilic groups and thus leads to a decrease in the gH,E value. In the gaseous and expanded states, both of above two factors are not effective. On the other hand, the positive gH,E value in the condensed state is attributable to more effective dispersion interaction than the dipóle-dipóle interaction in short molecular distance, and so factor 1 becomes dominant. Comparison of the gH,E value of the present system with that of the homologous TFC10OH-TFC12OH mixture leads us to a conclusion that the hydrogénation 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.
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