Coil-Collapse and Coil-Aggregation due to the Interaction of Cu2+ and Ca2+ Ions with Anionic Polyacylate Chains in Dilute Solution

A detailed light scattering investigation is presented on dilute solutions of long chain sodium polyacrylate in the presence of Cu 2+ ions under conditions which are close to the precipitation threshold of the respective Cu 2+ −PA chains. The results are compared with literature data ( Eur. Phys. J E 2001, 5, 117−126) from the corresponding system in the presence of Ca 2+ ions. In all cases the solvent is a 0.1 M NaCl solution in water. The PA coils shrink considerably with increasing Cu 2+ concentration as the conditions approach the precipitation threshold. Yet, the extent of shrinking can not be driven as far as for the respective Ca 2+ −PA system, where fully collapsed sphere-like polymers had been observed at the threshold. Analysis of the aggregation process with time-resolved static light scattering reveals loose coil-like aggregate structures for Cu 2+ −PA aggregates and compact sphere-like aggregates for Ca 2+ −PA in accordance with the limiting shape of the respective shrunken single chains. The onset of Ca 2+ or Cu 2+ induced aggregation of PA chains at the precipitation threshold borders an intramolecular coil shrinking process. The transition of shrinking into aggregation occurs more readily with Cu 2+ −PA as it does with Ca 2+ −PA.