Effect of film free energy on the gas permeability of foam films

The dependence of the gas permeability of foam films on the adsorption dens ity of the stabilizing surfactant at the film surfaces is studied. Recently , it has been found that in black foam films the permeability of the adsorb ed surfactant monolayers governs the total permeability of the film and the permeability of the very thin Newton black films is; smaller than that of the thicker common black films. Here, we propose a model which explains thi s behavior by taking into account the effect of the interactions between th e film surfaces on the adsorption density. Variation of salt concentration in the film-forming solution results first in increasing surfactant adsorpt ion due to the influence of the salt on the chemical potential of the surfa ctant and second in the modification of the interaction forces in the film. When the electrical double-layer forces are screened with an increasing co ncentration of counterions, the thickness of the film decreases. On a furth er increase in the electrolyte concentration, the free energy of formation of the film becomes negative and very thin films are formed. In this range of film states we find an additional increase in the adsorption density by using a recently developed theory of enhanced colloidal interaction. From t he adsorption density the accessible area for the transfer of gas molecules is determined. We show that the accessible area resulting from the combine d effect of salt concentration and free energy of film formation on the ads orption density correlates well with the permeability of the foam film.