Charge-driven co-assembly of polyelectrolytes across oil–water interfaces

We report a simple strategy to co-assemble oppositely charged polyelectrolytes across oil–water interfaces; this allows the accumulation of an electrostatic complex at the interface of species that are not surface active by themselves. To this end, we use a new, oil-soluble anionic polymer, poly-(fluorene-co-benzothiadiazole-co-benzoic acid), in combination with a cationic polyelectrolyte that is dissolved in the aqueous phase. When only one of the two charged components is present, no positive adsorption is observed in interfacial tension measurements; by contrast, when both polyelectrolytes are present, in the oil and water phases respectively, a rapid decrease of the interfacial tension is observed, indicating co-adsorption of the cationic and anionic polyelectrolytes. The complexation strength can be tuned through changes in both ionic strength and pH. Confocal microscopy and co-localization analysis further verifies the presence of both polyelectrolytes at the interface. With this approach, emulsions can be stabilized for several weeks; moreover, using the sensitivity of the complex to changes in pH, we are able to reversibly break and make the emulsions on demand.