Interfacial Rheology and Foam Stability in Poloxamer-Sodium Dodecyl Sulfate Mixtures

The interaction between nonionic block copolymers and ionic surfactants plays a key role in interfacial behavior and foam stability, yet remains poorly understood in concentrated systems. This work examines the self-assembly and interfacial rheology of Pluronic F68 (PF68) mixed with sodium dodecyl sulfate (SDS) across concentrations spanning below and above the critical micelle concentration (CMC) and its relation to foam stability. Dilatational interfacial rheology reveals that increasing SDS reorganizes the PF68 interfacial layer, shifting the air-liquid interface from viscoelastic to predominantly elastic above the CMC. Our analysis showed that the Gibbs stability criterion is more effective at predicting foam stability in mixed surfactant systems compared to simple viscoelasticity. Additionally, the addition of the cosurfactant leads to more polydisperse foams up to 25 CMC of the cosurfactant. The peak stability was observed around 25 CMC, while at 50 CMC, the increased osmotic pressure indicated a possible structurally induced capillary effect, which may be further examined through disjoining pressure measurements. These findings advance the understanding of polymer-surfactant interactions and guide the design of foams with tunable properties.