Role of interactions between cationic polymers and surfactants for foam properties

• Effect of cationic polymers on foam properties of SLES + CAPB ± MAc was studied.
• Lupasol polymers bind strongly the MAc molecules in the bulk solution.
• Lupasol polymers reduce MAc activity and its adsorption on the bubble surfaces.
• Merquat 740 affects strongly the foam yield stress, foam-wall friction, behavior of BS + MAc films.
• Merquat 100 with very high charge density has week effect on foam properties.

We study the effect of two classes of cationic polymers, Lupasols (based on polyethylenimine, PEI) and Merquats (based on diallyldimethyl ammonium chloride, DADMAC), on the surface and rheological properties of foams, stabilized with the mixture of anionic (SLES) and zwiterionic (CAPB) surfactants, with and without myristic acid (MAc) present as a cosurfactant. The effects of several polymers from each class, with different molecular mass and charge density, are compared. Polymers from Lupasol series were found to suppress the cosurfactant effect of MAc on all surface and foam properties studied—surface rheological properties, inside foam friction, foam-wall friction, and mean bubble size. The obtained results show that Lupasol polymers bind strongly the MAc molecules in the bulk solution, thus reducing MAc surface activity and adsorption. The cosurfactant effect of MAc could be restored at neutral pH, after increasing MAc concentration to reach a certain weight ratio of MAc/Lupasol. Merquat 740, which is a co-polymer of DADMAC and acrylamide with low charge density, affects strongly the dimensionless foam yield stress, foam-wall friction, and thinning behavior of BS + MAc films. This effect is explained with the strong interactions between polymer molecules and MAc in the mixed adsorption layer at the air–water interface—a relatively thick polymer adsorption layer is formed, able to bridge neighbouring surfaces. In contrast, Merquat 100 with very high charge density (homopolymer of DADMAC) has week effect on foam properties, due to the formation of relatively thin adsorption layers. Thus we observe a variety of possible scenario, depending on (1) the competition between the binding of surfactant/cosurfactant to the polymer molecules in the bulk solution with the polymer adsorption on the bubble surface, and (2) on the ability of the polymer to form thick adsorption layers, bridging the surfaces of neighbouring bubbles.

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