Surfactant Self-Assembly in Polyelectrolyte Gels: Aggregation Numbers and Their Relation to the Gel Collapse and the Appearance of Ordered Structures in the NaPA/C₁₂TAB System

Structural studies of a slightly cross-linked sodium polyacrylate gel interacting with the cationic surfactant dodecyltrimethylammonium bromide have been made using time-resolved fluorescence quenching (TRFQ) and small-angle X-ray scattering (SAXS). The surfactant aggregation number and the arrangement of the micelles were investigated during the transition of the gels from the highly water-swelled state at low surfactant contents to the collapsed state containing only about 50 wt % water. In the studied range the aggregation number estimated from TRFQ increased from 54 to 101, which resembled the behavior of the dodecyltrimethylammonium chloride (C12TAC)/water system. The gel volume was found to decrease close to linearly with β (surfactant to polyelectrolyte molar ratio in the gel) until β was 0.7−0.8, where a minimum volume was reached. The onset of this minimum was found to coincide with an ordering of the micelles in the gel. The reflections from SAXS indicated a cubic structure. The effect of the polyion as counterion on the micellar morphology and the micelle−micelle interaction is discussed in relation to the C12TAC/water system.