Ion size effects on the electric double layer of a spherical particle in a realistic salt-free concentrated suspension

TL;DR

This paper develops a modified Poisson-Boltzmann model that incorporates ion size effects and realistic salt-free conditions, providing more accurate predictions of electric double layers around particles in concentrated suspensions.

Contribution

It introduces a new finite-size ion model for salt-free suspensions, extending previous theories to include real-world ionic effects and environmental contaminants.

Findings

Realistic models differ significantly from classical predictions.

Ion size effects are crucial for accurate electrokinetic modeling.

The model improves agreement with experimental observations.

Abstract

A new modified Poisson-Boltzmann equation accounting for the finite size of the ions valid for realistic salt-free concentrated suspensions has been derived, extending the formalism developed for pure salt-free suspensions [Roa et al., Phys. Chem. Chem. Phys., 2011, 13, 3960-3968] to real experimental conditions. These realistic suspensions include water dissociation ions and those generated by atmospheric carbon dioxide contamination, in addition to the added counterions released by the particles to the solution. The electric potential at the particle surface will be calculated for different ion sizes and compared with classical Poisson-Boltzmann predictions for point-like ions, as a function of particle charge and volume fraction. The realistic predictions turn out to be essential to achieve a closer picture of real salt-free suspensions, and even more important when ionic size effects are incorporated to the electric double layer description. We think that both corrections have to be taken into account when developing new realistic electrokinetic models, and surely will help in the comparison with experiments for low-salt or realistic salt-free systems.