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 P b-II/S-II/poly(ethylene oxide) (PEO)/ sodium dodecyl sulfate (SDS) systems, in which the anionic surfactant interacts strongly with the polymer molecu les, forming micellar aggregates attached to the polymer chains above the c ritical association concentration. Lead sulfide crystallites are formed in the vicinity of polymer-bound micelles by adding lead and sulfide ions to t he polymer-surfactant solution. Surfactant stabilized inorganic particles a dsorbed on the polymer chains combine through a polymer-mediated bridging f locculation mechanism to produce characteristic rodlike colloidal aggregate s. Under certain conditions, these evolve into a range of metastable struct ures, composed of lead sulfide, PbS, and lead dodecyl sulfate, Pb(DS)(2). X RD analysis of the metastable reaction products allows us to follow the slo w 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 the se superstructures. At other pH values and system compositions, the product ion of pure PbS or pure Pb(DS)2 is favored, by appropriate tuning of the co ncentrations of Pb2+ and S2- ions. The resulting unexpectedly rich crystall ization behavior illustrates the complexity of colloidal aggregation phenom ena in polymer-surfactant solutions and the significance of coupling colloi dal aggregation to ionic equilibria.