Comparison between direct experimental measures of surface excess concentration in surfactant solutions with estimates based on the Gibbs Adsorption Isotherm

Recently, Menger et al [J. Am. Chem. Soc. 131 (30): 10380 (2009)] have suggested that the Gibbs Isotherm Method (GIM) is not the best way to estimate surfactant surface excess concentration from surface-tension measurements at the air-water interface. Important experimental techniques, such as Neutron Reflectivity and the radiotracer method, have arisen in recent years helping to clarify the controversy. This is because they are able to measure the surface excess concentration without the use of the GIM. We review the best available surface-tension data for some ionic and nonionic surfactants, and apply the GIM to them in order to directly compare with Neutron Reflectivity and radiotracer data [Adv. Coll. Int. Sci. 247: 178 (2017)]. As the thermodynamic theory behind the GIM predicts the agreement is good for concentrations smaller than the CMC, both for ionic and nonionic surfactants. In the case of nonionic surfactants, we found also a good agreement for concentrations larger than the CMC.