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-tetrahydrotrideca fluorooctanol CF3(CF2)(5)(CH2)(2)OH (FC8OH) against water was measured as a function of pressure at various concentrations m(1) and 298.15 K. There ar e three series of the break points, two of which result from a phase transi tion in the adsorbed film at the hexane/water interface and the other one r esults from a solubility limit of FC8OH in hexane. The curve of the interfa cial pressure vs area per adsorbed molecule ofFCsOK was drawn and compared with those of the 1,1,2,2-tetrahydroheptadecafluorodecanol CF3(CF2)(7)(CH2) (2)OH (FC10OH) and 1,1,2,2-tetrahydrohenicosafluorododecanol CF3(CF2)(9)(CH 2)(2)OH (FC12OH) systems previously reported. It was concluded that the ads orbed film of FC8OH reveals the first-order phase transitions between a gas eous and an expanded state and that between an expanded and a condensed sta te. The value of the volume change Delta nu associated with the adsorption in the gaseous and expanded state decreases rapidly with an increase in mi, while that in the condensed state decreases very slowly. It was described from the curves of Delta nu vs m(1) 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 fil m and that the by value in the condensed state rapidly decreases with an in crease in the chain length. We can explain that the pressure dependence of by in the condensed state is produced by the pressure dependence of the par tial molar volume of FC8OH in the hexane solution. We concluded that the de position is not a pure liquid but FC8OH-rich phase containing small quantit y of hexane.