Doubly pH Responsive Emulsions by Exploiting Aggregation of Oppositely Charged Nanoparticles and Polyelectrolytes.

We present a simple and modular approach to realize highly stable pH responsive Pickering emulsion from mixtures of commercially available oppositely charged nanoparticle and polyelectrolyte. While highly charged nanoparticles and polyelectrolytes when used solely do not stabilize emulsions, we show that the electrostatic attraction between oppositely charged nanoparticles and polyelectrolytes can be exploited to formulate emulsions with long-term stability of up to 8 months. The Ludox CL nanoparticles and poly(4-styrenesulfonate) sodium salt (PSS) when dispersed in aqueous solution at pH 2-11 form particle polyelectrolyte complexes (PPCs) due to heteroaggregation. These complexes are effective in stabilizing oil-in-water Pickering emulsions. We demonstrate that this is due to the formation of weakly charged complexes that are surface active and hence readily adsorbed to the oil-water interface created during emulsification. We show that the composition of nanoparticles and polyelectrolytes in the mixture as well as the pH can be tuned to control the average diameter of the emulsions droplets. Immediate destabilization and doubled responsiveness of the emulsions stabilized by particle polyelectrolyte complexes are illustrated by changing the pH of the stable emulsions formed at intermediate pH to either 1 or 13. The aggregation behavior of nanoparticle-polyelectrolyte mixtures and the effect of various parameters such as mixing fraction, pH, and energy input on the formation of Pickering emulsions is discussed. Furthermore, we show that the formation of near charge neutral aggregates that exhibit optimal wetting conditions is a requirement to accomplish emulsion formation. The visualization of particle polyelectrolyte complexes around the emulsion droplets, their morphology prior to emulsification, and their wetting properties are also investigated to elucidate the mechanism of emulsification.