Inter- and Intra-Chain Attractions in Solutions of Flexible Polyelectrolytes at Nonzero Concentration

TL;DR

This study uses molecular dynamics simulations to explore how flexible polyelectrolyte chains behave at various concentrations, revealing counterion condensation, chain shrinking at high Bjerrum lengths, and interchain attractions that lead to clustering.

Contribution

It provides new insights into the interplay of electrostatic interactions and chain conformations in flexible polyelectrolyte solutions, highlighting the presence of inter- and intra-chain attractions.

Findings

Counterion condensation increases with concentration

Chains become compact at large Bjerrum lengths

Interchain attractions lead to chain clustering

Abstract

Constant temperature molecular dynamics simulations were used to study solutions of flexible polyelectrolyte chains at nonzero concentrations with explicit counterions and unscreened coulombic interactions. Counterion condensation, measured via the self-diffusion coefficient of the counterions, is found to increase with polymer concentration, but contrary to the prediction of Manning theory, the renormalized charge fraction on the chains decreases with increasing Bjerrum length without showing any saturation. Scaling analysis of the radius of gyration shows that the chains are extended at low polymer concentrations and small Bjerrum lengths, while at sufficiently large Bjerrum lengths, the chains shrink to produce compact structures with exponents smaller than a gaussian chain, suggesting the presence of attractive intrachain interactions. A careful study of the radial distribution function of the center-of-mass of the polyelectrolyte chains shows clear evidence that effective interchain attractive interactions also exist in solutions of flexible polyelectrolytes, similar to what has been found for rodlike polyelectrolytes. Our results suggest that the broad maximum observed in scattering experiments is due to clustering of chains.