When Bulk Matters: Disentanglement of the Role of Polyelectrolyte/ Surfactant Complexes at Surfaces and in the Bulk of Foam Films

Foam films display exciting systems as on one hand they dictate the performance of macroscopic foams and on the other hand they allow studies of surface forces. With regard to surface forces, we attempt to disentangle the effect of the foam film surfaces and the foam film bulk. For that, we study the influence of salt (LiBr) on foam films formed by mixtures of oppositely charged polyelectrolyte and surfactant: anionic monosulfonated polyphenylene sulfone (sPSO2-220) and cationic tetradecyltrimethylammonium bromide (C14TAB). Adding a small amount of salt (<= 10-3 M) decreases the foam film stability due to a weakened electrostatic net repulsion. In contrast, a large amount of salt (10-2 M) unexpectedly increases the foam film stability. Disjoining pressure isotherms reveal that the increased stability is due to an additional steric stabilization, which is attributed to sPSO2-220/C14TAB complexes in the film bulk. These bulk complexes also contribute to the measured apparent surface potential between the two air/water interfaces. We find, for the first time, the formation of Newton black films for mixtures of anionic polyelectrolytes and cationic surfactants.

Automated summary

Foam films play a crucial role in determining the performance of macroscopic foams and can be used to study surface forces. This study investigates the effects of salt on the stability of foam films formed by mixtures of oppositely charged polyelectrolyte and surfactant. It is found that a small amount of salt weakens the electrostatic repulsion and decreases the film stability, while a large amount of salt unexpectedly increases the film stability due to additional steric stabilization provided by the polyelectrolyte/surfactant complexes in the film bulk.