Dynamic interfacial properties and foamability of polyelectrolyte-surfactant mixtures

Polymer/surfactant interaction and complexes can strongly influence foam formation and stability, however, a clear mechanism is still unknown and under investigation. Critical Aggregation Concentration (CAC) is introduced as an important parameter in recent investigations, however, CAC is mostly characterized as an equilibrium parameter, while foam formation and consequently the stability mechanism, is significantly influenced by the dynamics of adsorption. In this research the dynamic surface tension and viscoelasticity mixtures of Polystyrene Sulfonate (PSS) as an anionic polyelectrolyte and Cetyl Trimethyl Ammonium Bromide (CTAB) as a cationic surfactant, at the air-water interface are investigated by using Profile Analysis Tensiometry (PAT). The measured dynamic surface tension values illustrate three different regions of the surfactant/polymer ratio. It is demonstrated that a minimum number of surfactants per polymer molecule is needed in order that the polymer-surfactant complexes become sufficiently surface active and adsorb significantly at the water-air surface. For high surfactant concentrations, the large amounts of excess CTAB compete with polymer-surfactant complexes at the interfaces and remove the complexes into the bulk. The measured elasticity values confirm this observation. At intermediate surfactant concentration, when the complexes have their maximum surface activity, a fast dynamics of adsorption is observed and the measured elasticity values go through a maximum. The results of foam formation and stability also show a meaningful correlation with these results using Ross-Mill test and gas sparging method.