Modeling Ion-Specific Effects in Polyelectrolyte Brushes: A Modified Poisson-Nernst-Planck Model

TL;DR

This paper develops a modified Poisson-Nernst-Planck model to better understand ion-specific effects in polyelectrolyte brushes, incorporating non-Coulombic forces and ion binding affinities for more accurate ion distribution predictions.

Contribution

It introduces a novel continuum model that includes ion-specific binding and Born forces, extending traditional electrostatic models for polyelectrolyte brushes.

Findings

The model predicts distinct ion profiles based on ion-specific properties.

Inclusion of Born radii and binding affinities alters ion distribution predictions.

The approach improves understanding of ion behavior in biological and synthetic polyelectrolyte systems.

Abstract

Polyelectrolyte brushes consist of a set of charged linear macromolecules each tethered at one end to a surface. An example is the glycocalyx which refers to hair-like negatively charged sugar molecules that coat the outside membrane of all cells. We consider the transport and equilibrium distribution of ions, and the resulting electrical potential, when such a brush is immersed in a salt buffer containing monovalent cations (sodium and/or potassium). The Gouy-Chapman model for ion screening at a charged surface captures the effects of the Coulombic force that drives ion electrophoresis and diffusion, but neglects non-Coulombic forces and ion pairing. By including the distinct binding affinities of these counter-ions with the brush, and their so-called Born radii, which account for Born forces acting on them when the permittivity is non-uniform, we propose modified Poisson-Nernst-Planck continuum models that show the distinct profiles that may result depending on those ion-specific properties.