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. Copyright Â