APPARENT ANOMALIES IN SURFACE EXCESSES DETERMINED FROM NEUTRON REFLECTION AND THE GIBBS EQUATION IN ANIONIC SURFACTANTS WITH PARTICULAR REFERENCE TO PERFLUOROOCTANOATES AT THE AIR WATER INTERFACE/

The surface tension and neutron reflectivity of several perfluorooctan oic acid salts with monovalent metal cations have been investigated ov er a wide range of surfactant, counterion, and co-ion concentrations. Previous interpretations of similar surface tension data have indicate d that there is significant dissociation of the counterions from the s urfactant layer, leading to a prefactor less than 2 in the Gibbs equat ion for the surfactant in the absence of electrolyte, which could be i nterpreted in terms of a depletion layer of surfactant in the region u nderneath the surfactant ion monolayer. We show that this result is an artifact resulting from the presence of small amounts of divalent cat ion impurity. Once the divalent ions are removed, we obtain complete c onsistency between the neutron and surface tension results using a Gib bs prefactor of 2 with no significant subsurface depletion. It is conc luded that contamination of anionic surfactants by divalent ions may b e the most common cause of inaccurate determination of adsorption isot herms of these materials at the air/solution interface.