Adsorption and interaction forces of commercial Poly(naphthalene sulfonate) (PNS) and Poly(carboxylate ether) (PCE) polyelectrolytes with negatively charged surfaces in monovalent and divalent electrolytes

Adsorption of polyelectrolytes on colloidal particles alters the interaction forces and rheological properties of colloidal suspensions. Poly(naphthalene sulfonate) (PNS) and poly(carboxylate ether) (PCE) polyelectrolytes are commonly added to improve the flowability of hydrated cement paste in low-water conditions. The interactions of commercial PNS and PCE admixtures with negatively charged mica in aqueous electrolytes were directly studied with a surface forces apparatus in this work. In 0.1 M K₂SO₄, robust adsorption of PNS induced a strong steric repulsion. A slightly thicker adsorption layer formed in low ionic strength 5 mM Ca(NO₃)₂ exhibiting long-range electrostatic repulsion and short-range steric repulsion. Increasing the Ca(NO₃)₂ concentration to 0.1 M doubled the adsorption layer thickness and strongly screened any electrostatic repulsion. Contact of the adsorbed PNS layers caused weak physical cohesion under both low and high calcium concentrations presumably due to Ca²⁺ ion bridging of the opposing PNS layers. These findings indicate that (i) divalent Ca²⁺ possesses a much stronger ability than monovalent K⁺ to assemble PNS molecules resulting in significantly thicker adsorption and (ii) the interaction force between PNS adsorption layers in high ionic strength conditions is dominated by steric repulsion. In comparison, PCE did not appreciably adsorb from 0.1 M Ca(NO₃)₂. This finding challenges the well-accepted divalent ion bridging adsorption mechanism of PCE onto negatively charged surfaces and warrants further study.