Adsorption of ammonium perfluorooctanoate at the air–water interface

The effect of solution conditions on the structure and composition of ammonium perfluorooctanoate (APFO) layers at the air–water interface has been examined using the technique of neutron reflection. The surface excess was found to decrease with increasing temperature, but the overall thickness of the layer showed an opposite trend and increased. This observation indicates that with the increase of thermal motion, the layer becomes thicker but more loosely packed. Addition of salt resulted in an increase in the surface excess and under this condition, the layer also became thicker. Both temperature rise and salt addition reduced the surface tension, resulting in the roughening of surface layers. If the effect of thermal capillary waves is removed, the thickness of the layers is always close to 13 Å which is comparable to the fully extended length of the APFO molecule. The relative location between the layer and water was determined using the more direct approach of the kinematic approximation involving reflectivity measurements using different ratios of H2O and D2O. The cross-distances between the centers of APFO layers and those of water were found to be 4.5 Å under the different solution conditions. These cross-distances would suggest a substantial mixing between the APFO layers and water, but after the removal of capillary wave roughness, the extent of mixing was close to 30% which is similar to that found for hydrocarbon surfactants.