Phase behavior of weakly charged polymer/surfactant/water mixtures

Ternary phase diagrams have been determined for aqueous mixtures of low-sulfated sodium dextran sulfate (DxS) with penta(ethylene oxide) dodecyl ether (C(12)E(5)) containing small amounts of sodium dodecyl sulfate (SDS). A segregative phase separation occurs. A low fraction of charges on either the polymer or the surfactant micelle increases the miscibility considerably, compared to the uncharged reference mixture of Dr and C(12)E(5). Addition of SDS in small steps to DxS/C(12)E(5) (charges on both polyelectrolytes) first leads to a decrease and then to an increase in miscibility. At the minimum in miscibility the effect of charges disappears, and the mixture behaves as an uncharged polymer-surfactant mixture. The trends are interpreted in terms of the entropy of mixing of the dissociated counterions.