How to concentrate an aqueous polyelectrolyte/surfactant mixture by adding water

Aqueous systems containing sodium polyacrylate (NaPA) and the surfactant cetyltrimethylammonium bromide (CTABr) can separate into a concentrated phase and a dilute phase. The phase separation can be induced by addition of water. Further addition of water causes the concentrated phase to become still more concentrated. A structural analysis of the concentrated phase shows that the surfactant ions (CTA(+)) are always aggregated into elongated micelles. These micelles are packed with short-range order (concentrated micellar phase) or long-range order (hexagonal phase). The ionic compositions of the concentrated and the dilute phase have been determined. They indicate that the association results from an ion-exchange process, where the polyacrylate ions displace some bromide counterions of the surfactant aggregates. The separation of a dilute aqueous phase and the deswelling of the concentrated phase are then explained by the gain in entropy of the simple salt (Na+ and Br-) that is released in the dilute phase.