Counterion-Mediated Like-Charge Attraction of PMAA at Amorphous Silica Interfaces

Understanding how polyelectrolytes interact with charged solid surfaces is essential for controlling adsorption, dispersion, and colloidal stability in complex formulations. These interactions are particularly intriguing when the polymer and surface carry charges of the same sign, where conventional electrostatic arguments predict repulsion. In this work, we investigate the adsorption of poly(methacrylic acid) (PMAA), an anionic polyelectrolyte, on negatively charged amorphous silica surfaces.

Atomistic molecular dynamics simulations combined with enhanced sampling methods are used to quantify the polymer–surface interaction free energy and examine the effects of polymer tacticity, silica ionization, and electrolyte composition. Despite the like-charged nature of PMAA and silica, the simulations reveal a distinct attractive interaction. Counterintuitively, adsorption becomes stronger as the degree of silica ionization increases, demonstrating that the interfacial behavior cannot be explained by direct polymer–surface electrostatics alone.

The attraction is strongly enhanced in the presence of CaCl₂. Divalent calcium ions coordinate simultaneously with PMAA carboxylate groups and negatively charged silica sites, creating persistent ion-mediated contacts at the interface. These coordination structures alter the adsorption free-energy landscape and overcome the electrostatic repulsion between the polymer and the surface.

The results identify counterion valency and interfacial ion coordination as key parameters governing like-charge polyelectrolyte adsorption. This molecular-level framework contributes to a broader understanding of polymer adsorption at charged interfaces and may guide the design of polymeric dispersants with tunable affinity for colloidal surfaces.