Role of polymer–surfactant interactions in foams: Effects of pH and surfactant head group for cationic polyvinylamine and anionic surfactants

Abstract

In a previous study (Langmuir, 28 (2012) 4996) we showed that the foamability of mixed solutions of the cationic polymer polyvinylamine (PVAm) and the anionic surfactant sodium dodecyl sulphate (SDS) is strongly reduced, whereas the stability of the formed foams is strongly enhanced, as compared to the solutions of SDS alone. Here we study in more detail the foaming properties of mixed solutions of PVAm with anionic surfactants. The effect of surfactant head group is studied by comparing SDS with another anionic surfactant (sodium dodecyl oxyethylene sulphate, SDP1S) which contains an additional ethoxy fragment in the charged head-group. For changing the electrostatic polymer–surfactant interactions, we varied pH between 6 and 10, thus crossing the polymer pKa ≈ 8.6. The foam tests showed that the foamability of all mixed solutions is strongly reduced in the entire range of pH values studied. The negative effect of PVAm on solution foamability is highest at low pH, where the polymer charge density is the highest. Model experiments revealed that the reduced foamability is due to prolonged lag-time for formation of mixed adsorption layer on bubble surfaces. Surprisingly, we found that the stability of SDP1S foams is also reduced strongly by PVAm at pH 6 (contrary to SDS and to conventional understanding). The obtained results indicate that the ethoxy group in SDP1S enhances the surfactant association with the polymer molecules, thus decreasing the concentration of free surfactant monomers, necessary to adsorb on the solution surface for foam stabilization during foaming and immediately after it. These results clearly demonstrate that excessively strong polymer–surfactant interactions could be a problem in the formation and stabilization of foams from mixed solutions. Moreover, the effect of cationic polymers on the foaming properties of anionic surfactants could depend significantly on the specific head group of the surfactant.