SAXS and ASAXS on Dilute Sodium Polyacrylate Chains Decorated with Lead Ions

Bivalent lead ions as representative main group heavy metal cations form specific interactions with the negatively charged COO^– residues of sodium polyacrylate chains in dilute aqueous solution. The interactions eventually lead to aggregation and precipitation of sodium polyacrylate chains partially neutralized with Pb²⁺ cations. The present work outlines a small-angle X-ray (SAXS) and light scattering study of the polyacrylate chains undergoing changes in coil conformation and successive aggregation while approaching and crossing the Pb²⁺-induced precipitation threshold. The study reveals a coil shrinking while approaching the precipitation threshold. Anomalous SAXS (ASAXS) complemented this information with a first insight into the spatial distribution of the Pb²⁺ cations captured by the polyacrylate chains together with a semiquantitative estimation of the amount of Pb²⁺ cations located within the collapsed domains of the shrinking chains. Conformational aspects of the shrinking coils could be established by means of model form factors of hybrid chains formed by a freely jointed chain of rods with spheres located on all or part of the joints. Development and application of the form factors of the pearl-necklace-like hybrid model chains and the use of quantitative analysis of ASAXS data are described and discussed in detail

Coaggregation of Two Anionic Azo Dyestuffs: A Combined Static Light Scattering and Small-Angle X‑ray Scattering Study

The formation of azo dyestuff aggregates in dilute aqueous solution induced by the addition of Mg²⁺, Ca²⁺, Sr²⁺, or Ba²⁺ ions is followed by time-resolved static light scattering (SLS) and time-resolved small-angle X-ray scattering (SAXS). Time-dependent molar mass data of the growing aggregates is interpreted by means of a kinetic model introduced by Lomakin et al. (Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 1125) for the description of β-amyloid aggregation. This interpretation reveals significant trends within the homologous series of alkaline earth cations. The trends refer to the nucleation and the growth rate of the dyestuff fibers. Time-resolved SAXS experiments indicate that these first two stages are followed by a third one during which a network forms by partial lateral alignment of fibers. At high enough dyestuff concentrations, this network formation even leads to a gel-like phase. Anomalous SAXS (ASAXS) on such a gel phase formed upon the addition of Sr²⁺ revealed the extent of neutralization of the dyestuff molecules within the gel by the specifically interacting alkaline earth cations