Soft effective interactions between weakly charged polyelectrolyte chains

TL;DR

This study combines Molecular Dynamics simulations and analytical models to explore how weakly charged, flexible polyelectrolyte chains interact and behave in salt-free environments, revealing how charge fraction influences chain conformation and interactions.

Contribution

It introduces a combined simulation and analytical approach to understand effective interactions and conformations of weakly charged polyelectrolyte chains, highlighting the impact of charge fraction.

Findings

Good agreement between simulations and theory below 10% charge fraction.

Chain conformation transitions from coil-like to rod-like near 20% charge.

Effective interactions are well described by Gaussian charge profiles at low charge fractions.

Abstract

We apply extensive Molecular Dynamics simulations and analytical considerations in order to study the conformations and the effective interactions between weakly charged, flexible polyelectrolyte chains in salt-free conditions. We focus on charging fractions lying below 20%, for which case there is no Manning condensation of counterions and the latter can be thus partitioned in two states: those that are trapped within the region of the flexible chain and the ones that are free in the solution. We examine the partition of counterions in these two states, the chain sizes and the monomer distributions for various chain lengths, finding that the monomer density follows a Gaussian shape. We calculate the effective interaction between the centers of mass of two interacting chains, under the assumption that the chains can be modeled as two overlapping Gaussian charge profiles. The analytical calculations are compared with measurements from Molecular Dynamics simulations. Good quantitative agreement is found for charging fractions below 10%, where the chains assume coil-like configurations, whereas deviations develop for charge fraction of 20%, in which case a conformational transition of the chain towards a rodlike configuration starts to take place.