Eur. Phys. J. E (2023) 46: 92
https://doi.org/10.1140/epje/s10189-023-00342-2

Regular Article – Soft Matter

Coarse-grained explicit-solvent molecular dynamics simulations of semidilute unentangled polyelectrolyte solutions

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, 37831, Oak Ridge, TN, USA

^a carrillojy@ornl.gov
^c kumarr@ornl.gov
^d sumpterbg@ornl.gov

Received: 22 May 2023
Accepted: 28 August 2023
Published online: 5 October 2023

Abstract

We present results from explicit-solvent coarse-grained molecular dynamics (MD) simulations of fully charged, salt-free, and unentangled polyelectrolytes in semidilute solutions. The inclusion of a polar solvent in the model allows for a more physical representation of these solutions at concentrations, where the assumptions of a continuum dielectric medium and screened hydrodynamics break down. The collective dynamic structure factor of polyelectrolytes, S(q, t), showed that at $q>q^{\ast }$, where $q^{\ast }=2\pi /\xi$ is the polyelectrolyte peak in the structure factor S(q) and $\xi$ is the correlation length, the relaxation time obtained from fits to stretched exponential was ${\tau }_{\text{KWW}}\sim q^{-3}$, which describes unscreened Zimm-like dynamics. This is in contrast to implicit-solvent simulations using a Langevin thermostat where ${\tau }_{\text{KWW}}\sim q^{-2}$. At $q<q^{\ast }$, a crossover region was observed that eventually transitions to another inflection point ${\tau }_{\text{KWW}}\sim q^{-2}$ at length scales larger than $\xi$ for both implicit- and explicit-solvent simulations. The simulation results were also compared to scaling predictions for correlation length, $\xi \sim c_p^{-1/2}$, specific viscosity, ${\eta }_{\mathit{sp}}\sim c_p^{1/2}$, and diffusion coefficient, $D\sim c_p^0$, where $c_p$ is the polyelectrolyte concentration. The scaling prediction for $\xi$ holds; however, deviations from the predictions for ${\eta }_{\mathit{sp}}$ and D were observed for systems at higher $c_p$, which are in qualitative agreements with recent experimental results. This study highlights the importance of explicit-solvent effects in molecular dynamics simulations, particularly in semidilute solutions, for a better understanding of polyelectrolyte solution behavior.