Thermodynamic study on phase transition in adsorbed films of fluoroalkanol at the hexane/water interface. 5. Miscibility in the adsorbed film of an alkanol and fluoroalkanol mixture

The miscibility of 1-icosanol (C₂₀OH) and 1,1,2,2-tetrahydroheptadecafluorodecanol (FC₁₀OH) in the adsorbed film at the hexane/water interface was investigated by evaluating the composition of FC₁₀OH in the adsorbed film X^H₂ from the interfacial tension data and drawing the phase diagram of adsorption (PDA). We found that the PDA changes its shape from the positive azeotrope in the expanded state, where the C₂₀OH and FC₁₀OH molecules are miscible at all proportions, to the heteroazeotrope in the condensed state, where they are completely immiscible with each other, as the interfacial tension γ decreases. To examine the deviation from the ideal mixing in the adsorbed film quantitatively, the activity coefficient of component i, f^H_i, and the excess Gibbs energy, g^H,E, in the expanded state were estimated from the PDA. It was found that the f^H_i value is larger than unity in a whole X^H₂ range, and therefore, the corresponding g^H,E value is positive at all compositions and increases with decreasing γ. This result shows that the C₂₀OH and FC₁₀OH molecules mix nonideally in the adsorbed film and the mutual interaction between the different kinds of alcohol molecules is very weak compared with that between the same kinds. Furthermore, we estimated the excess area in the adsorbed film A^H,E from the dependence of g^H,E on γ. The A^H,E value was positive, and therefore, the mixing of C₂₀OH and FC₁₀OH molecules in the adsorbed film causes an increase in area A. By comparison of this result with the corresponding volume increase associated with the mixed micelle formation of hydrocarbon and fluorocarbon surfactants, it was suggested that the difference in the geometrical structure between adsorbed films and micelle affects appreciably the molecular packing in them and eventually the molecular interaction between hydrocarbon and fluorocarbon molecules.