Ramanathan, M., Mueller, H.J., Vedula, K., Mohwald, H., Krastev, R.
Colloids Surf A Physicochem.
The paper summarizes the results of a study on the basic properties (film thickness, contact angle film/meniscus and gas permeability) of foam films stabilized with the anionic perfluoro octane sulfonate (PFOS) surfactant. The surfactant was used as a tetraethyl ammonium salt. The dependencies on the electrolyte (NH4-CI) and surfactant concentration were studied. Strong dependence of the film thickness and the contact angle on the concentration of NH4CI and PFOS was observed. Well pronounced transition in the film thickness from common black (electrostatically stabilized) films to Newton black (sterically stabilized) films was registered. The critical salt concentration necessary for the formation of NBF was defined to be 0.1 M NH4CI. The contact angles between the films and the meniscus were collected. The gas permeability coefficient (K, cm/s) was measured at various conditions. The permeability coefficient of NBF decreases with increasing surfactant concentration below the cmc and remains constant above it. The gas permeability of the films depends on the salt concentration, respectively film thichkness in a complicated manner which cannot be explained by simple application of the Fick's law of diffusion.