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 % mater. In the studied range the. aggregation number estimated from TRFQ increased from 54 to 101, which resembled the behavior of the dodecyltrimethylammonium chloride (C(12)TAC)/water system. The gel volume was found to decrease close to linearly with beta (surfactant to polyelectrolyte molar ratio in the gel) until beta 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 C(12)TAC/water system.