From colloidal aggregates to layered nanosized structures in polymer-surfactant systems. 1. Basic phenomena

In this work, we examine the rich crystallization behavior that occurs in Pb-II/S-II/poly(ethylene oxide) (PEO)/ sodium dodecyl sulfate (SDS) systems, in which the anionic surfactant interacts strongly with the polymer molecules, forming micellar aggregates attached to the polymer chains above the critical association concentration. Lead sulfide crystallites are formed in the vicinity of polymer-bound micelles by adding lead and sulfide ions to the polymer-surfactant solution. Surfactant stabilized inorganic particles adsorbed on the polymer chains combine through a polymer-mediated bridging flocculation mechanism to produce characteristic rodlike colloidal aggregates. Under certain conditions, these evolve into a range of metastable structures, composed of lead sulfide, PbS, and lead dodecyl sulfate, Pb(DS)(2). XRD analysis of the metastable reaction products allows us to follow the slow kinetics of their formation and reveals a well-defined layered structure, based on lead dodecyl sulfate, the thickness of which is determined by the length of the surfactant chains. Elemental analysis, C-13- and Pb-207-NMR spectroscopy FTIR spectroscopy, XPS, and HRTEM are used to characterize these superstructures. At other pH values and system compositions, the production of pure PbS or pure Pb(DS)2 is favored, by appropriate tuning of the concentrations of Pb2+ and S2- ions. The resulting unexpectedly rich crystallization behavior illustrates the complexity of colloidal aggregation phenomena in polymer-surfactant solutions and the significance of coupling colloidal aggregation to ionic equilibria.