From beads-on-a-string to colloidal aggregation: Novel crystallization phenomena in the PEO-SDS system

The anionic surfactant sodium dodecyl sulfate (SDS) interacts strongly with poly(ethylene oxide) (PEO) in aqueous solution, forming micellar aggregates attached to the polymer chains above the so-called critical association concentration (cac). By adding lead and sulfide ions to the PEO/SDS solution, we form lead-sulfide-containing crystallites directly on the polymer-bound micelles. We can then, through this indirect staining technique, visualize polymer chains with polymer-bound micelles using TEM. Subsequent aggregation of the particles adsorbed on the polymer chains through a bridging flocculation mechanism leads to the production of novel interesting organic/inorganic nanocomposites, in the form of long (0.5-5 mu m) and thin (100 nm) rodlike colloidal aggregates. XRD analysis of these anisotropic aggregates reveals a well-defined layered structure with a spacing of 31 Angstrom. Combination of elemental analysis, FTIR, XPS, and XRD measurements suggests that the aggregates consist of mixed dodecyl sulfate/acetate layers, which sandwich a layer of lead and sulfide ions. Similar colloidal aggregation phenomena have been observed with other cations as well, suggesting that polymer/surfactant complexes are useful novel templates for the preparation of organic/inorganic nanocomposite materials.