Rheological properties of oppositely charged polyelectrolyte-surfactant mixtures: Effect of polymer molecular weight and surfactant architecture

We report on the gel formation and resulting rheological properties in aqueous solutions of cationic cellulose ether polymers of "high" (JR30M) and "low" (JR400) molecular weight (MW), upon the addition of anionic surfactants of different architecture but with the same charge (sodium dodecyl sulfate, SDS, sodium dodecylbenzenesulfonate, SDBS, and sodium bis(2-ethylhexyl) sulfosuccinate, AOT). From the comparison of systems with the same charge density and ratio but different (polymer) MW and (surfactant) architecture, we assess the nature of the interactions leading to gel formation as well as structural properties of the polymer-surfactant aggregates. In the high-MW polyelectrolyte-surfactant systems, polymer-dependent cross-links and chain entanglements are observed, while the surfactant architecture plays a secondary role. For example, the slope of the double-logarithmic plot of the increase in viscosity with surfactant concentration is the same with any of the three surfactants. Surfactant-dependent cross-links are obtained for the low-MW polyelectrolyte-surfactant complexes, and the surfactant architecture and the hydrophobic effect are responsible for the cooperativity of the crosslinks. The slope of the viscosity vs surfactant concentration data is much higher in the case of low-MW polymer. The similarities and the differences in the viscoelastic properties for a polymer or a surfactant-controlled cross-linking mechanisms are discussed.