Multi-valent Ion Mediated Polyelectrolyte Association and Structure

TL;DR

This study uses molecular dynamics simulations to uncover how multi-valent Ca$^{2+}$ ions mediate attraction between polyelectrolyte chains, revealing mechanisms like ion bridging and solvent effects that influence polyelectrolyte interactions.

Contribution

It provides detailed molecular insights into like-charge attraction mechanisms mediated by multi-valent ions, advancing understanding of polyelectrolyte behavior in solutions.

Findings

Moderate Ca$^{2+}$ ions induce chain attraction via ion bridging.

High Ca$^{2+}$ density reduces bridging but still promotes attraction.

Solvent-mediated interactions contribute to polyelectrolyte association.

Abstract

Polyelectrolytes are commonly used to chelate multi-valent ions in aqueous solutions, playing a critical role in water softening and the prevention of mineralization. At sufficient ionic strength, ion-mediated polyelectrolyte--polyelectrolyte interactions can precipitate polyelectrolyte--ion complexes, a phenomenon known as "like-charge attraction". While the significant influence of small ions on polyelectrolyte solution phase behavior is recognized, the precise molecular mechanisms driving the counterintuitive phenomenon remain largely elusive. In this study, we employ all-atom molecular dynamics simulations to investigate the molecular mechanism of like-charge attraction between two poly(acrylic acid) (PAA) chains in solution. We find that moderate quantities of Ca$^{2+}$ ions induce attraction between PAA chains, facilitated by the formation of PAA--Ca$^{2+}$--PAA bridges and a significant increase in the coordination of Ca$^{2+}$ ions by the PAA chains. At high Ca$^{2+}$ number densities, ion bridges are disfavored due to electrostatic screening, yet the chains are still attracted to each other due to solvent-mediated interactions between the chains and their chelated ions. The insights gleaned from this study not only enrich our understanding of the intricate mechanism of like-charge attraction between polyanions in solution but also illuminate the influence of multi-valent ions on polyelectrolyte interactions.