Thermodynamic study on phase transition in adsorbed film of fluoroalkanol at the hexane/water interface. 3. Temperature effect on the adsorption of 1,1,2,2-tetrahydroheptadecafluorodecanol

The interfacial tension γ of a hexane solution of 1,1,2,2-tetrahydroheptadecafluorodecanol CF₃(CF₂)₇(CH₂)₂-OH (FC₁₀OH) against water was measured as a function of temperature T and molality m₁ under atmospheric pressure. By the calculation of the interfacial density Γ^H₁ and then drawing the interfacial pressure π area per adsorbed molecule A curves, it was concluded that two types of first-order phase transitions take place from the gaseous to the expanded state and from the expanded to the condensed one in the adsorbed film of FC₁₀OH. The partial molar entropy s̄^H₁ - s^O₁ and energy changes ū^H₁ - u^O₁ associated with the adsorption were estimated and compared with those of 1,1,2,2-tetrahydrohenicosafluorododecanol CF₃(CF₂)₉(CH₂)₂OH (FC₁₂OH). It was found that these values of FC₁₀OH are negative and larger than those of FC₁₂OH. It was suggested that, although both s̄^H₁ and s^O₁ values increase, the increment in s̄^H₁ is larger than that in s̄^H₁ when the fluorocarbon chain is lengthened from FC₁₀OH to FC₁₂OH. Furthermore, it was realized that the energetical stabilization of the FC₁₀OH molecule accompanied by adsorption is less than that of the FC₁₂OH molecule. Furthermore, it was shown that the temperature and pressure dependencies of the π vs A curve are related to the partial molar entropy and volume changes of adsorption, respectively. By estimation of the temperature and pressure coefficients of A at constant π, it is found that a temperature increase of only 1 K is sufficient to keep π and A constant upon a decrease of the pressure by 1 MPa.