ADSORPTION OF AMMONIUM PERFLUOROOCTANOATE AND AMMONIUM DECANOATE AT THE AIR SOLUTION INTERFACE

The composition and structure of layers of ammonium decanoate (AMDEC) and ammonium perfluorooctanoate (APFO) adsorbed at the air/liquid interface have been studied by a combination of surface tension measurements and neutron reflectivity. At the critical micelle concentration (c.m.c.) and in solutions of pH = 8.8 and ionic strength 0.1, the areas occupied per surfactant anion were found to be 41 +/- 2.5 angstrom2 for APFO and 35 +/- 3 angstrom2 for AMDEC. In each case it was necessary to include fugacity corrections and to fit the surface pressure-ln c curve by least squares for there to be agreement with the surface excess as measured by neutron reflection. The structure of the surfactant layers was determined at the c.m.c. using neutron reflectivity measurements on three different isotopic combinations of water and surfactant. The thickness of the APFO layer was found to be 20 +/- 2 angstrom, ca. 30% larger than the fully extended chain length and the thickness of the AMDEC layer was 17 +/- 2 angstrom, only ca. 15% greater than the fully extended chain length. Both model fitting and a more direct method of analysing the separation between surfactant chains and water suggest that the chains are ca. 25-30% immersed in the aqueous phase, with the APFO being slightly more immersed than the AMDEC.