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 semi-quantitative 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.