Structure of black foam films

Abstract

Radiotracer measurements have shown that black foam films possess a sandwich structure, consisting of two monolayers of surface-active agent enclosing an aqueous core. The reflection coefficient and the angle at which reflected light becomes fully plane-polarized have been used together with the infra-red transmission at an absorption maximum for liquid water to calculate the total thickness of the film and the thickness of the surface monolayers. Total film thicknesses obtained using this three-layer optical model are in good agreement with those obtained by low-angle X-ray scattering. The aqueous core of a thin black film has a limiting thickness of ∼20 Å. Radiotracer methods have shown that the surface monolayers adsorb inorganic ions, and conductance measurements have indicated that these ions are mobile. It is suggested that the relative stability of these films is determined by the free energy required to dehydrate the hydrophilic head groups of the surface-active ions and the inorganic electrolyte in the film.