MONOLAYER PROPERTIES OF OCTADECYLAMMONIUM PERFLUOROALKANOATES

Surface pressure (pi)- and surface potential (DELTA-V)-area (A) isotherms for monolayers of octadecylammonium perfluoroalkanoates of six different perfluoroalkyl chains on water and on 4.4 M NaCl solutions were measured at various temperatures by the Langmuir method and the ionizing electrode method. Some pi-A curves of these long-hydro-short-fluoro salts showed three phase transition points. Judging from the surface potential and the apparent molar quantity together with our previous data on the transitions, the first was assigned to the transition from the expanded (E) phase to the condensed (C-I) phase, the second from the C-I phase to another condensed (C-l') phase, and the third from the C-I' phase to another condensed (C-II) phase. There exists a triple point on the phase diagram at which three phases (E, C-I, and C-I') coexist. In this paper, negative inclination of transition pressure (C-I --> C-I') against temperature was newly observed over a certain temperature range on the phase diagram. From the surface potential, the apparent molar entropy change, and phase diagram, the orientation of the long-hydro-short-fluoro-chain salts in the monolayer state was also discussed.