Formation of polyelectrolyte–surfactant complexes on surfaces

The interfacial behavior of polyelectrolytes, mainly cationic with varying content of amphiphilic groups, and their complexes with oppositely charged surfactant are discussed. Both the kinetics and the reversibility aspect of the adsorption are considered. The structure of adsorbed layer formed was found to be dependent not only on the bulk solution phase behavior, but also on the pre-applied conditions, i.e., the path used to obtain a particular solution condition (e.g., by changing pH and concentration of salt, surfactant or polymer). Polyelectrolyte adsorption appears only partly reversible, due to its high affinity to the surface, which slows down the adsorption process. In general, relaxation occurs more easily if the direction of the process is from low to high surface coverage. Association of the surfactant with the polymer, which depends on the surfactant concentration, can completely alter the interfacial behavior. Maximum adsorption occurs generally at a surfactant concentration just before the expected phase separation region, while the complex in some cases could desorb from the surface at high enough surfactant concentration (above the cmc). Different results were obtained for coadsorption of amphiphilic polyelectrolytes when surfactant was added to the preadsorbed polymer layers and when complexes were pre-formed in the solution prior to exposing the surface to the polymer–surfactant solution.