Mechanism of chain collapse of strongly charged polyelectrolytes

TL;DR

This study uses molecular dynamics simulations to explore how strongly charged polyelectrolyte chains collapse, revealing two distinct regimes of chain size dependence on electrostatic strength and modifying existing theories accordingly.

Contribution

The paper identifies and characterizes two electrostatic regimes in polyelectrolyte collapse and extends the counterion-fluctuation theory to explain the novel strong electrostatic regime.

Findings

Weak electrostatic regime: $R_g \,\sim\, \ell_B^{-1/2}$

Strong electrostatic regime: $R_g \,\sim\, \ell_B^{-1/5}$

Modified counterion-fluctuation theory explains the strong regime.

Abstract

We perform extensive molecular dynamics simulations of a charged polymer in a good solvent in the regime where the chain is collapsed. We analyze the dependence of the gyration radius $R_g $ on the reduced Bjerrum length $\ell_B$ and find two different regimes. In the first one, called as a weak electrostatic regime, $R_g \sim \ell_B^{-1/2}$, which is consistent only with the predictions of the counterion-fluctuation theory. In the second one, called a strong electrostatic regime, we find $R_g \sim \ell_B^{-1/5}$. To explain the novel regime we modify the counterion-fluctuation theory.