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

The interfacial tension γ of hexane solution of 1,1,2,2-tetrahydroheptadecafluorodecanol (FC₁₀OH) against water was measured as a function of pressure p and molality m₁ at 298.15 K. The break points were observed on the γ vs p and γ vs m₁ curves. The interfacial density Γ₁^H increases and the volume change associated with the adsorption Δν decreases with increasing m₁, and they change discontinuously at concentrations of the break points. By drawing the interfacial pressure π vs area A curve, it is concluded that two types of 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. By examining our results by the Clapeyron type equations, the order of phase transition was considered first. The partial molar volume change associated with the adsorption of FC₁₀OH was estimated and compared with that of 1-octadecanol (C₁₈OH). It was found that FC₁₀OH molecules were accompanied with the larger decrease in partial molar volume inherent in the interface than C₁₈OH molecules when the phase transition takes place from the expanded to the condensed state in the adsorbed film. This suggests that the microscopic circumstance in the adsorbed film affects appreciably the volume change of adsorption.