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

The adsorption behavior of 1,1,2,2-tetrahydrohenicosafluorododecanol, CF₃(CF₂MCH₂)₂OH (FC₁₂OH), at the hexane/water interface was investigated in both the absence and presence of the deposit by measuring the interfacial tension γ of its hexane solution against water as a function of pressure p and molality m₁ at 298.15 K. It was concluded that a breakpoint on the γ vs p and γ vs m₁ curves in a very low concentration region shows a first-order phase transition from the gaseous to the condensed state in the adsorbed film. It was found that the volume change of adsorption per mole of FC₁₂OH is larger than that of 1,1,2,2-tetrahydroheptadecafluorodecanol, CF₃(CF₂)₇(CH₂)₂OH (FC₁₀OH). Then it was suggested that the increase in the partial molar volume of alcohol with increasing fluorocarbon chain length is larger in the solution than in the interface because of the weak mutual interaction of the fluorocarbon chain with hexane molecules. Furthermore, another breakpoint due to the solubility of FC₁₂OH in hexane was observed on the γ vs p and γ vs m₁ curves in a high concentration and pressure region. A small value of partial molar volume change accompanied by the adsorption from the solid substantiates our view that the condensed state of the adsorbed film resembles the solid state.