Donnan potentials in aqueous phase-separated polymer mixtures.

A promising approach to texturize water is by the addition of mutually incompatible polymers, leading to phase separation. Here, we demonstrate that the phase stability of aqueous polymer solutions is affected not only by chemical differences between the polymers but also by their electric charge. Direct electrochemical measurements are performed of the electric potential difference between two coexisting phases in aqueous solutions of the charged protein fish gelatin (nongelling) and the uncharged polysaccharide dextran. Charge counteracts demixing because of the entropic cost of confining the counterions to one phase, resulting in a strong shift of the critical point upon an increase of the charge on one of the polymers. Upon phase separation, the charged polymer is spatially confined, and due to the Donnan effect, an interfacial electric potential is developed. A direct proportionality is found between this Donnan potential and the difference in gelatin concentration in the two phases, for which we propose a theoretical explanation. The electrostatics may provide a new handle in the development of stable water-in-water emulsions.