Ultrastable Water-in-Water Emulsions by Addition of Polyion Complex Micelles.

We report the use of polyion complex micelles (PICM) obtained by complexation of poly(sodium acrylate) and poly(N-methyl-2-vinylpyridinium iodide-block-ethylene oxide) as novel stabilizers for water-in-water emulsions consisting in poly(ethylene oxide) (PEO) and dextran mixtures in water. PICM concentrations as low as 1 g/L lead to a strong increase of stability for both PEO-in-dextran and dextran-in-PEO emulsions, as they did not display any macroscopic phase separation after several months. Such long term stability is to our knowledge unprecedented in the literature dedicated to water-in-water emulsions. Moreover, increasing the concentration of PICM further induces a decrease in the droplet size, resulting in a strong reduction of the cream-ing or sedimentation rates, and suppression of the coalescence after a few days. The dissociation of PICM at high salt concentration was taken as an opportunity to trigger the destabilization of the water-in-water emulsions by addition of a concentrated salt solution. PEO-in-dextran emulsions displayed more sensitivity to PICM and salt concentrations than dextran-in-PEO emulsions, in terms of both stability and droplet size, indicating that the underlying stabilization mechanisms are different for the two systems.