Ion Entropy in Phase-Separated Aqueous Mixtures of Polyelectrolyte and Neutral Polymer

The macroscopic phase separation of aqueous mixtures of a neutral polymer and a polyelectrolyte is well described by a modified blob model, taking into account the entropy of ideal ions under the restriction of macroscopic charge neutrality. This is demonstrated by detailed measurements on aqueous mixtures of a neutral polymer (dextran) and a polymer whose charge is adjustable via the pH (nongelling fish gelatin). The critical point of the phase diagram of demixing, the asymmetric distribution of the solvent, and the interfacial electric potential difference all depend on polyelectrolyte charge and background salt concentration in a manner that is consistent with a dominant role for ion entropy.