Repulsive and attractive depletion forces mediated by nonadsorbing polyelectrolytes in the Donnan limit

TL;DR

This paper develops a theoretical model to describe how nonadsorbing polyelectrolytes influence the forces between colloidal surfaces, revealing a crossover from attraction to repulsion based on separation distance.

Contribution

The authors derive a zero-field theory for disjoining pressure in colloid-polyelectrolyte mixtures, incorporating the Donnan potential and conformational free energy effects.

Findings

Disjoining pressure exhibits a crossover from attraction to repulsion at chain size scale.

Model predictions quantitatively match self-consistent field computations.

Donnan potential influences polyelectrolyte accumulation and force behavior.

Abstract

In mixtures of colloids and nonadsorbing polyelectrolytes, a Donnan potential arises across the region between surfaces that are depleted of polyelectrolyte and the rest of the system. This Donnan potential tends to shift the polyelectrolyte density profile towards the colloidal surface and leads to local accumulation of polyelectrolytes. We derive a zero-field theory for the disjoining pressure between two parallel flat plates. Polyelectrolyte is allowed to enter the confined interplate region at the cost of a conformational free energy penalty. The resulting disjoining pressure shows a crossover to a repulsive regime when the interplate separation gets smaller than the size of the polyelectrolyte chain, followed by an attractive part. We find a quantitative match between the model and self-consistent field computations that take into account the full Poisson-Boltzmann electrostatics.