Thermodynamic study on phase transition in adsorbed film of fluoroalkanol at the hexane/water interface. 8. Phase transition and miscibility in the adsorbed film of fluoroalkanol mixture

The interfacial tension gamma of the hexane solution of a mixture of 1,1,2, 2-tetrahydroheptadecafluorodecanol (FC10OH) and 1,1,2,2,-tetrahydrohenicosa fluorododecanol (FC12OH) against water was measured as a function of the to tal molality m and composition of FC12OH X-2 at 298.15 K under atmospheric pressure. The gamma vs m curve has one or two distinct break points dependi ng on X2 By plotting the gamma and m values at the break points and drawing the interfacial pressure Jr vs mean area per adsorbed molecule A curves, i t was suggested that the triple point of adsorbed film, at which the three kinds of states (gaseous, expanded, and condensed states) coexist simultane ously, exists at a middle composition. The phase diagrams of adsorption (PD A) were constructed and the excess Gibbs energy of adsorption g(H,E) was ca lculated to examine the miscibility of FC10OH and FC12OH molecules in the a dsorbed film. It was found that these alcohols mix almost ideally both in t he gaseous and expanded states and nonideally in the condensed state. Furth ermore, the mixing of FC10OH and FC12OH in the condensed film was accompani ed by a positive g(H,E) value. This suggests that the difference in the mag nitude of mutual interaction between the same species affects appreciably t he miscibility of molecules in the condensed state where the adsorbed molec ules are assumed to be closely packed and arranged regularly.