Gibbs Adsorption Isotherm: The Relevance of the Solvent in the Adsorption Layer

The thermodynamic basics of the Gibbs adsorption isotherm are restated to compare the Gibbs zero solvent assumption to a formulation including the solvent in the surface region, as given by Guggenheim. A comparison of the two isotherms shows close values of the surfactant surface densities from experiments in many cases. The Guggenheim isotherm also provides an account of the solvent in the surface region in the form of necessary inequality relations in the surfactant and solvent adsorption densities. The role of the solvent component of the surface region is examined in the context of the growing evidence of ionic double layers in the surface regions of water and non-ionic surfactant solutions.