The surface activity of a surfactant or protein is best documented by the so-called adsorption isotherm. This isotherm represents the relationship between the bulk concentration c and the surface excess Γ (adsorption) at the interface. The most simple relationship is the linear Henry isotherm:
The most frequently used isotherm, however, is the Langmuir isotherm, which contains two physical parameters, a characteristic concentration a at which half of the interface is covered by the surfactant, and the maximum amount that can be adsorbed Γ∞:
The following schematic shows how Γ is changing with increasing concentration c in the solution bulk.
