Adsorption Isotherm and Mechanism of Ca Binding to Polyelectrolyte.

Polyelectrolytes, such as poly(acrylic acid) (PAA), can effectively mitigate CaCO scale formation. Despite their success as antiscalants, the underlying mechanism of binding of Ca to polyelectrolyte chains remains unresolved. Through all-atom molecular dynamics simulations, we constructed an adsorption isotherm of Ca binding to sodium polyacrylate (NaPAA) and investigated the associated binding mechanism. We find that the number of calcium ions adsorbed [Ca] to the polymer saturates at moderately high concentrations of free calcium ions [Ca] in the solution. This saturation value is intricately connected with the binding modes accessible to Ca ions when they bind to the polyelectrolyte chain. We identify two dominant binding modes: the first involves binding to at most two carboxylate oxygens on a polyacrylate chain, and the second, termed the high binding mode, involves binding to four or more carboxylate oxygens. As the concentration of free calcium ions [Ca] increases from low to moderate levels, the polyelectrolyte chain undergoes a conformational transition from an extended coil to a hairpin-like structure, enhancing the accessibility to the high binding mode. At moderate concentrations of [Ca], the high binding mode accounts for at least one-third of all binding events. The chain's conformational change and its consequent access to the high binding mode are found to increase the overall Ca ion binding capacity of the polyelectrolyte chain.