Structure of polyelectrolyte and multi-valent ion complexes

Experiments have established the ability of aqueous polyelectrolytes to passivate and delay the crystallization of limescale (CaCO₃). Polyelectrolytes are expected to influence the CaCO₃ crystallization by chelating Ca²⁺ ions from the solution and modifying the crystal growth by preferentially adsorbing to certain crystal surfaces. These processes may potentially delay the onset of nucleation and crystal growth, respectively. However, the mechanism through which polyelectrolytes operate is currently unknown. The binding of a Ca²⁺ to a charged residue on the polymer backbone is strongly dependent on the charge state of a polymer and its conformation, which are affected by the pH, concentration of Ca²⁺, and the overall ionic strength of the solution. The ability of a polyelectrolyte to sequester free Ca²⁺ depends on the polyelectrolyte-Ca²⁺ binding energy. We conduct molecular dynamics simulations with enhanced sampling techniques to study the interaction of Ca²⁺ with various polyanions in an aqueous suspension. We will present our findings on the polyelectrolyte-Ca²⁺ binding energetics, comment on the preferential Ca²⁺ binding sites on a polyelectrolyte backbone, and discuss their impact on the backbone conformations.