Surface Adsorption of Oppositely Charged SDS:C12TAB Mixtures and the Relation to Foam Film Formation and Stability

The complexation, surface adsorption, and foam film stabiliztation of the oppositely charged surfactants, sodium dodecyl sulfate (SDS) and dodecyl trimethylammonium bromide (C(12)TAB), is analyzed. The SDS:C(12)TAB mixing ratio is systematically varied to investigate whether the adsorption of equimolar or irregular catanionic surfactant complexes, and thus a variation in surface charge (i.e., surface excess of either SDS or C(12)TAB), governs foam film properties. Surface tension measurements indicate that SDS and C(12)TAB interact electrostatically in order to form stoichometric catanionic surfactant complexes and enhance surface adsorption. On the other hand it can be demonstrated that the SDS:C(12)TAB mixing ratio and, thus, a change in surface charge and composition plays a decisive role in foam film stabilization. The present study demonstrates that varying the mixing ratio between SDS and C(12)TAB offers a tool for tailoring surface composition and foam film properties, which are therefore not exclusively mediated by the presence of equimolar catanionic surfactant complexes. The SDS:C(12)TAB net amount and mixing ratio determine the type, stability, and thinning behavior of the corresponding foam film. These observations indicate the formation of a mixed surface layer, composed of the catanionic surfactant species surrounded by either free SDS or C(12)TAB molecules in excess. Furthermore, a systematic variation in CBF-NBF transition kinetics is rationalized on the basis of a microscopic phase transition within the foam films. Fundamental knowlegde gained from this research gives insight into the surface adsorption and foam film formation of catanionic surfactant mixtures. The study helps researchers to understand basic mechanisms of foam film stabilization and to use resources more efficiently.