Competitive adsorption of polymer and surfactant at a liquid droplet interface and its effect on flocculation of emulsion

We investigate the competitive adsorption of polymer and surfactant at oil–water interface by measuring the hydrodynamic diameter, zeta potential, microstructure and rheology. The polymer used in our experiment is a statistical copolymer of polyvinyl alcohol and vinyl acetate copolymer (PVA–Vac) and the emulsion is oil-in-water system with an average droplet diameter of 200 nm. At low surfactant concentrations, the hydrodynamic diameter remains unchanged but above a critical aggregation concentration, it increases dramatically. The phase contrast optical microscopic images of emulsion droplets preadsorbed with polymers of higher molecular weight show a systematic increase in the floc size on increasing surfactant concentration due to biding of polymer–surfactant complexes on several droplets. The dramatic increase in zeta potential of the droplets on increasing ionic surfactant concentration clearly indicates a preferential adsorption of surfactant at the oil–water interface. The enhanced viscosity upon addition of ionic surfactant into polymers confirms the strong interaction between them. Our studies show that lower molecular weight polymers with suitable ionic surfactants can synergistically enhance the stability of formulations, while longer chain polymers induce bridging flocculation. Our results are useful for preparing oil-in-water formulations with long term stability.

At low surfactant concentrations the monomers and micelles associate with polymer coils, leading to stretching of the tail regions. At surfactant concentration ≫CAC, the surfactant molecules preferentially adsorb at the droplet interface by desorbing the polymers and at very high surfactant concentration, the polymer–surfactant complexes binds several drops together forming flocs.Figure optionsDownload high-quality image (203 K)Download as PowerPoint slideHighlights
► Studies on competitive adsorption of polymer and surfactant at oil–water interface show some interesting results.
► Our results indicate that surfactant has a tendency to preferentially adsorb at the oil–water interface.
► Polymer–surfactant association can lead to a synergistic enhancement of emulsion stability or bridging flocculation.
► Our results are useful for preparing oil-in-water formulations with long term stability.