Macroscopic phase separation in a polyelectrolyte gel interacting with oppositely charged surfactant: correlation between anomalous deswelling and microstructure

The volume transitions in cross-linked sodium polyacrylate gels following the absorption of cetyltrimethylammonium bromide from aqueous solutions is studied. The microstructure of the gels is investigated by means of time-resolved fluorescence quenching, small-angle X-ray scattering, and optical anisotropy. In general, the volume of the pre-swelled gels decreases linearly with the number of absorbed surfactant molecules. The shrinking is due to a collapse of the exterior parts of the gels when essentially all the surfactant is located. The surface phase is very dense and is made up from collapsed micelle/polyelectrolyte complexes arranged on a cubic lattice. The surfactant aggregation number is between 110 and 170. At sufficiently high surfactant-to-polymer ratios the entire gel is in a collapsed single phase state with the same microstructure as the surface phase. It is observed that during the transition from the swollen to the fully collapsed state the gels may be trapped in a semi-swollen state, where the gels have a balloon-like shape due to a pressure across the surface phase. There is a strong correlation between this anomalous behavior and the growth of the micelles from globular to long cylinders and a transition from cubic to hexagonal packing. The observations are compared with the macroscopic surface phases observed during phase transitions in related systems.