Theoretical and computational analysis of the electrophoretic polymer mobility inversion induced by charge correlations

TL;DR

This paper develops a theoretical and computational framework to understand electrophoretic mobility inversion in charged polymers caused by charge correlations, revealing how salt concentration and dielectric properties influence this phenomenon.

Contribution

It introduces an analytical mobility formula incorporating charge correlations and validates it with molecular dynamics simulations, advancing understanding of mobility inversion mechanisms.

Findings

Charge correlations induce mobility inversion at dilute salt concentrations.

Increasing salt concentration suppresses the mobility inversion effect.

Simulation results align qualitatively with experimental observations.

Abstract

Electrophoretic (EP) mobility reversal is commonly observed for strongly charged macromolecules in multivalent salt solutions. This curious effect takes place, e.g., when a charged polymer, such as DNA, adsorbs excess counterions so that the counterion-dressed surface charge reverses its sign, leading to the inversion of the polymer drift driven by an external electric field. In order to characterize this seemingly counterintuitive phenomenon that cannot be captured by electrostatic mean-field theories, we adapt here a previously developed strong-coupling-dressed Poisson-Boltzmann approach to the cylindrical geometry of the polyelectrolyte-salt system. Within the framework of this formalism, we derive an analytical polymer mobility formula dressed by charge correlations. In qualitative agreement with polymer transport experiments, this mobility formula predicts that the increment of the monovalent salt, the decrease of the multivalent counterion valency, and the increase of the dielectric permittivity of the background solvent, suppress charge correlations and increase the multivalent bulk counterion concentration required for EP mobility reversal. These results are corroborated by coarse-grained molecular dynamics simulations showing how multivalent counterions induce mobility inversion at dilute concentrations and suppress the inversion effect at large concentrations. This re-entrant behavior, previously observed in the aggregation of like-charged polymer solutions, calls for verification by polymer transport experiments.