From Colloidal Aggregates to Layered Nanosized Structures in Polymer−Surfactant Systems. 1. Basic Phenomena

Abstract

In this work, we examine the rich crystallization behavior that occurs in PbII/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, 13C- and 207Pb−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.

Formation of necklace-shaped haematite nanoconstructs through polyethylene glycol sacrificial template technique

“…In this study, PEG acts as a stabiliser both during the formation of haematite and also after by forming a carbonaceous coat around the nanoparticle, which in turn prevents direct precipitation and aggregation of haematite nanoparticles. PEG also helps in connecting the crystals into loose flocs, which can be considered to be similar to the bridging mechanism [24].The assembly of nanoparticles in an oriented manner by PEG is similar to that by surfactants or polymer-surfactant mixtures [25,26]. Once assembled, the residual magnetic moment, particle size and shape and ionic strength could have promoted the formation of string-like structures [27].…”

Formation mechanism of nanotubes comprising layers of PbS nanoparticles in polymer–surfactant solutions

“…A Pb/SDS molar ratio of 1:2 was used most of the time to avoid the possible precipitation of Pb(CH 3 COO)(DS). This compound is formed for Pb/SDS molar ratios higher than 1:1, and is insoluble in water even at temperatures as high as 60 • C [4,5]. The pH value of the solutions was adjusted to neutral with 1 M nitric acid for basic solutions and with 1 M sodium hydroxide for acidic solutions.…”

“…Recently, it was found that the crystallization of PbS in polymer-containing aqueous solutions of the surfactant sodium dodecyl sulfate (SDS) resulted in unusual structures comprising ordered PbS particles. These structures were supposed to originate from polymer-surfactant interactions that exist in these solutions, leading to micellar polymersurfactant structures [4][5][6][7]. It was formerly suggested that the polymer-surfactant interaction is necessary for the stabilization and size and shape modification of the PbS nanoparticles [6].…”

“…over a reaction time from one day to six weeks ( Fig. 1) [5,6]. Remarkably, these structures are among the few examples of spontaneously forming nanolayered materials or nanotubes, the layers of which are composed of individual nanoparticles with a narrow size distribution [8], in contrast to ordinary nanotubes, which are built up by walls of quasi-two-dimensional atomic networks.…”

“…It seemed that the formation of the structures is linked to the existence of sheet-like particles of Pb(DS) 2 , a salt that is formed in the beginning of the reaction, has low solubility in water up to 70 • C, and competes with PbS formation in the initial stages of the crystallization reaction [4][5][6]. However, if polymer-surfactant interactions play an important role in the formation of the uncommon arrangements of PbS nanoparticles, the formation of those structures should clearly depend on the ability of the polymer to interact with SDS.…”