Counterion Condensation in Strong, Flexible Polyelectrolytes

TL;DR

This study uses molecular dynamics simulations to explore how added salt influences the structure of strong, flexible polyelectrolyte chains, revealing effects that challenge classical Manning condensation theory.

Contribution

It provides the first detailed simulation-based analysis of salt effects on polyelectrolyte structure across different densities, including regimes above and below Manning's limit.

Findings

Chain contraction due to salt is weaker than simple screening predicts.

Ion density near the chain depends on added salt, contrary to classical theory.

Chains contract significantly above the condensation limit with added salt.

Abstract

We present results of molecular dynamics simulations of strong, flexible polyelectrolyte chains in solution with added salt. The effect of added salt on the polyelectrolyte chain structure is fully treated for the first time as a function of polymer density. Systems above and below the Manning condensation limit are studied. The chain contraction due to added salt is weaker than expected from simple screening arguments. The chain structure is intimately tied to the ion density near the chain even for chains in the counterion condensation regime. In contradiction to Manning counterion condensation theory, the ion density near the polymer chain depends on the amount of added salt, and above the condensation limit the chains significantly contract due to added salt.