We have examined the polymer/surfactant interaction in mixed aqueous solutions of cationic surfactants and anionic polyelectrolytes combining various techniques: tensiometry, potentiometry with surfactant-selective electrodes, and viscosimetry. We have investigated the role of varying polymer charge density, polymer concentration, surfactant chain length, polymer backbone rigidity, and molecular weight on the critical aggregation concentration (Cac) of mixed polymer/surfactant systems. The Cac of these systems, estimated from tensiometry and potentiometry, is found to be in close agreement. Different Cac variations with polymer charge density and surfactant chain length were observed with polymers having persistence lengths either smaller or larger than surfactant micelle size, which might reflect a different type of molecular organization in the polymer/surfactant complexes. The surfactant concentration at which the viscosity starts to decrease sharply is different from the Cac and probably reflects the polymer chain shrinkage due to surfactant binding.

We have studied aqueous solutions of a polyelectrolyte, carboxymethylcellulose, which is an anionic cellulose derivative, and a cationic surfactant, dodecyltrimethylammonium bromide (DTAB). We have investigated the interactions between the two species, both at the air/water interface and in the bulk, for increasing DTAB concentrations. Mixed surfactant/polymer aggregates are formed at the air/water surface at extremely low surfactant concentrations, whereas bulk aggregates are formed later, above a critical aggregation concentration (cac). A small viscosity maximum at the cac reveals a small degree of bridging of the polymer chains by the surfactant. Above the cac, the viscosity drops, indicating that the polymer chains undergo a rapid collapse. At higher surfactant concentrations, light scattering shows the existence of larger structures, which are surprisingly monodisperse and whose size increases with surfactant concentration. At still higher surfactant concentrations, a classical strong associative phase separation takes place. During the evolution of bulk properties, the surface tension remains constant, suggesting that the surface aggregates remain unchanged.