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 aggregate s attached to the polymer chains above the so-called critical association c oncentration (cac). By adding lead and sulfide ions to the PEO/SDS solution , we form lead-sulfide-containing crystallites directly on the polymer-boun d micelles. We can then, through this indirect staining technique, visualiz e polymer chains with polymer-bound micelles using TEM. Subsequent aggregat ion of the particles adsorbed on the polymer chains through a bridging floc culation mechanism leads to the production of novel interesting organic/ino rganic nanocomposites, in the form of long (0.5-5 mu m) and thin (100 nm) r odlike colloidal aggregates. XRD analysis of these anisotropic aggregates r eveals a well-defined layered structure with a spacing of 31 Angstrom. Comb ination of elemental analysis, FTIR, XPS, and XRD measurements suggests tha t the aggregates consist of mixed dodecyl sulfate/acetate layers, which san dwich a layer of lead and sulfide ions. Similar colloidal aggregation pheno mena have been observed with other cations as well, suggesting that polymer /surfactant complexes are useful novel templates for the preparation of org anic/inorganic nanocomposite materials.