Site binding of Ca2+ ions to polyacrylates in water:: A molecular dynamics study of coiling and aggregation

The interaction of Ca2+ ions with short polyacrylate chains (NaPA) in water is investigated using molecular dynamics simulations, accelerated with the recently developed metadynamics algorithm. The much discussed "site binding" of calcium ions to these industrially relevant polymers is driven by an entropy gain as water molecules are released into the solution. At high NaPA concentrations, increased Ca2+-PA monomer ratios will not result in strong coiling of PA oligomers. This is due to the local rigidity induced by the binding of many Ca2+ ions to the polymer. Because the uncoiled state of the crowded chain obstructs formation of high Ca2+-COO- coordination numbers, interchain interactions will become favorable, and formation of PA aggregates can be expected. On the other hand, at low NaPA concentrations, introduction of Ca2+ ions to the solution leads to the formation of very stable coiled configurations, with local crystal-like structures, in which the Ca2+ ions cluster together. It is the sharing of carboxylate groups among the metal ions that leads to the formation of these crystal-like conformations, inducing desolvation and precipitation of the chain at threshold Ca2+ concentrations.