The adsorption and complexation of polystyrene sulfonate (a highly charged anionic polyelectrolyte) and dodecyltrimethylammonium bromide (a cationic surfactant) at the air-water interface can lead to interfacial gels that strongly influence foam-film drainage and stability. The formation and characteristics of these gels have been studied by combining surface tension, ellipsometry, and foam-film drainage experiments. Simultaneously, the solution electromotive force is measured and used to track the polymer-surfactant interactions in the bulk solution. We find that surface gelation occurs above the critical aggregation concentration in solution but before bulk precipitation of the polymer-surfactant complexes. Furthermore, we reveal that strong readsorption of polymer-surfactant complexes occurs during the resolubilization of the precipitated complexes at high surfactant concentrations (i.e., >>critical micelle concentration). Seemingly overlooked in the past, this readsorption significantly influences the surface rheological properties and foam-film drainage of these systems.

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Using the oscillating drop method as well as the interfacial stress rheometer (ISR), we measure the surface dilational and shear properties of adsorbed layers of oppositely charged polymer-surfactant complexes at the air-water interface. These data are compared to foam volume data measured for the corresponding solutions. We show that the shear surface moduli G′ and G′′ exhibit a maximum at a molar ratio of polymer monomers and surfactant in solution equal to 1. This maximum is strongly correlated to surface ellipticity measurements that indicate a strong adsorption of polymer-surfactant complexes at the air-water interface. These hydrophobic interfacial complexes generate a viscoelastic layer due to the polymer chain entanglements and hydrophobic interactions between bound surfactant. The viscous behavior of the surface layers is further characterized via creep, frequency sweep measurements, and surface aging experiments. Over a 10 h period we find that G′ and G′′ increase logarithmically with time, reminiscent of so-called jammed systems. Curiously, the slow evolution of G′ and G′′ is not reflected by significant changes in the surface tension over time, and parallel measurements for the same systems reveal that the surface complexes do not effect the surface dilational moduli, E′ and E′′. Finally, we observe that the foam volumes measured for the corresponding solutions also exhibit a maximum at a molar ratio of 1, indicating that the gellike layer at the interface inhibits foam drainage and bubble coalescence.