Thermodynamic study of the adsorption of sodium perfluorooctanoate at the water/hexane interface

The interfacial tension of aqueous sodium perfluorooctanoate (SPFO) solution against hexane was measured as a function of temperature at various concentrations around the critical micelle concentration (c.m.c.). The interfacial density and the entropy and energy of adsorption were evaluated by applying the thermodynamic relations developed previously. It was observed that the surface pressure versus area curves of SPFO, dodecylammonium chloride (DAC), and sodium dodecyl sulfate (SDS) are in the order of the cross-sectional area of the polar head group at high pressure. Further, the entropy and energy of adsorption of SPFO were found to be larger than those of DAC and SDS. These results were attributed to a weak mutual interaction between the fluorocarbon chain and the hexane molecule as well as the strong hydrophobicity of SPFO. It was demonstrated that the micelle formation of SPFO can also be treated approximately the same as a macroscopic bulk phase in the range of concentration near the c.m.c.