Specific Interactions of Ag+ Ions with Anionic Polyacrylate Chains in Dilute Solution

A detailed light scattering investigation is presented on dilute solutions of two long-chain sodium polyacrylates (NaPA) in the presence of monovalent Ag+ ions in 0.01 M NaNO3 aqueous solution at a pH of 9. The relevance for the investigation is based on two features: (i) despite its monovalency, Ag+ is expected to exhibit a much more complex interaction pattern with NaPA than salts based on alkaline cations do; (ii) aqueous solutions of AgNO3 nucleate Agnanoparticle formation in the presence of NaPA under UV light, which may be modulated by the solution behavior. Our study revealed the following results. Addition of a low amount of Ag+ ions leads to an aggregation of PA-chains without considerable coil shrinking. At a ratio of silver ions per monomer of 0.001 <= [Ag+]/[COO ] <= 0.05, combined static and dynamic light scattering (SLS/DLS) suggests formation of homogeneous low density aggregates. Further increase of the Ag+ ion concentration results in a formation of dense unstable aggregates. At ratios of [Ag+]/[COO ] > 0.3 with variable NaPA concentrations aggregation occurs within a few seconds, resulting in small but stable aggregates with a high density of Ag(+)PA aggregates. A structure-sensitive factor rho close to 0.5 obtained from combined SLS/DLS identifies a corona of dangling chains enclosing the stable dense Ag(+)PA aggregates which have a size of 2550 nm. Finally, a 3-fold excess of Ag+ to COO in the systems induces precipitation of Ag(+)PA.