Study of adsorbed monolayers of a cationic surfactant and an anionic polyelectrolyte at the air-water interface

We have investigated the coadsorption at the air-water interface from mixed solutions of a non-surface-active anionic polyelectrolyte (poly(acrylamide sulfonate)) and a cationic surfactant (cetyltrimethylammonium bromide). A combination of various physicochemical methods such as surface tensiometry, excited electrocapillary waves, compression isotherms, X-ray reflectivity, and Brewster angle microscopy (BAM) is used to probe the surface complexes at the air-water interface. A significant synergistic lowering of surface tension is observed in the concentration range where no appreciable complexation of polyelectrolyte and surfactant occurs in the bulk of solution. This decrease is due to the formation of polyelectrolyte-surfactant complexes at the air-water interface. Surface viscoelastic properties, that is, elasticity and viscosity, of mixed monolayers exhibit a maximum at a surfactant concentration much smaller than that for solutions of the pure surfactant. Compression isotherms give evidence for the irreversible character of the adsorption of the surface complexes. A first-order phase transition is seen in the mixed monolayer above a critical surface density. X-ray reflectivity and BAM images reveal thickening of the monolayer with increasing pressure. BAM images suggest reversible multilayer formation when the monolayer is compressed.