Pressure effect on the adsorption of 1,1,2,2-tetrahydrotridecafluorooctanol at the hexane/water interface

The interfacial tension of the hexane solution of 1,1,2,2-tetrahydrotridecafluorooctanol CF₃(CF₂)₅(CH₂)₂OH (FC₈OH) against water was measured as a function of pressure at various concentrations m₁ and 298.15 K. There are three series of the break points, two of which result from a phase transition in the adsorbed film at the hexane/water interface and the other one results from a solubility limit of FC₈OH in hexane. The curve of the interfacial pressure vs area per adsorbed molecule of FC₈OH was drawn and compared with those of the 1,1,2,2-tetrahydroheptadecafluorodecanol CF₃(CF₂)₇(CH₂)₂OH (FC₁₀OH) and 1,1,2,2-tetrahydrohenicosafluorododecanol CF₃(CF₂)₉(CH₂)₂OH (FC₁₂OH) systems previously reported. It was concluded that the adsorbed film of FC₈OH reveals the first-order phase transitions between a gaseous and an expanded state and that between an expanded and a condensed state. The value of the volume change Δv associated with the adsorption in the gaseous and expanded state decreases rapidly with an increase in m₁, while that in the condensed state decreases very slowly. It was described from the curves of Δv vs m₁ for FC₈OH, FC₁₀OH, and FC₁₂OH at 80 MPa that the expanded state region in the interfacial film structure strongly decreases with increasing the chain length and disappears at the FC₁₂OH film and that the Δv value in the condensed state rapidly decreases with an increase in the chain length. We can explain that the pressure dependence of Δv in the condensed state is produced by the pressure dependence of the partial molar volume of FC₈OH in the hexane solution. We concluded that the deposition is not a pure liquid but FC₈OH-rich phase containing small quantity of hexane.