Polyelectrolyte-Surfactant Phase Behavior and Mechanisms of Interaction in Multi-Component Systems

Polymer-Surfactant (P-S) systems and their implications both in industry and academia have been studied for the last fifty years. Despite the fact that the majority of synthetic and biological systems have a multicomponent nature, most previous studies focused on molecular interactions of individual polymers and surfactants in aqueous solutions, at the air/water interface or at the solid/water interface. P-S interactions in multicomponent systems have not received sufficient attention. Among the important issues that remain unresolved, the following are of particular significance: a) lack of information on P-S systems phase behavior in the presence of a third component which is widely used in industrial applications and of academic interest, b) insufficient quantitative information on competitive interactions of all species in the system and uncertainty on how molecules conform upon interaction and, c) lack of mathematical models to describe P-S interactions in a multicomponent environment. To address the unresolved issues, a study focusing on the mechanisms of interaction of an oppositely charged polyelectrolyte-surfactant (PE-S) pair in the presence of fatty acid (FA) was carried out. Experimental results using surface tensiometry, light scattering, zeta potential and, nuclear magnetic resonance (NMR) showed that the FA solubility in the system determined the overall phase behavior of the multicomponent mixture. In a system containing FA at a concentration above the solubilization limit of the surfactant, FAs form solid aggregates. Such aggregates were found to possess a considerable negative surface charge, consequently, preferential binding of the polycation to FA aggregates was observed and the usual tendency of PE-S pair to form a complex was inhibited. These results were useful to understand the polyelectrolyte-induced flocculation disruption by FA aggregates in emulsions systems. In a system containing FA at a concentration below the solubilization limit of the surfactant, the FA was found to be mostly solubilized forming mixed micelles with the surfactant. NMR revealed detailed information on the competitive binding behavior of both the fatty acid and the surfactant, including the morphology of aggregates at the polymer chain. Furthermore, solid state NMR showed how the structure and composition of the actual PE-S complex changed in the presence of FA. Finally, the law of mass action model is successfully employed to describe the PE-S surface tension profile in the presence of FAs.