Finite-Thickness and Charge Relaxation in Double-Layer Interactions

TL;DR

This paper extends the classical Gouy-Chapman model to include finite colloid thickness, deriving a closed-form interaction force and analyzing charge redistribution effects on colloidal interactions at the mean field level.

Contribution

It introduces a finite-thickness extension of the Gouy-Chapman model, providing a closed-form solution for the interaction force and examining charge relaxation effects.

Findings

Finite thickness modifies double-layer interactions.

Derived a closed-form interaction force depending on geometry.

Charge redistribution influences colloidal stability.

Abstract

We extend the classical Gouy-Chapman model of two planar parallel interacting double-layers, which is used as a first approximation to describe the force between colloidal particles, by considering the finite-thickness of the colloids. The formation of two additional double layers due to this finite thickness, modifies the interaction force compared to the Gouy-Chapman case, in which the colloids are semi-infinite objects. In this paper we calculate this interaction force and some other size-dependent properties using a mean field level of description, based on the Poisson-Boltzmann (PB) equation. We show that in the case of finite-size colloids, this equation can be set in a closed form depending on the geometrical parameters and on their surface charge. The corresponding linear (Debye-Huckel) theory and the well-known results for semi-infinite colloids are recovered from this formal solution after taking appropriate limits. We use a density functional corresponding to the PB level of description to show how in the case when the total colloidal charge is fixed, it redistribute itself on their surfaces to minimize the energy of the system depending on the afore mentioned parameters. We study how this charge relaxation affects the colloidal interactions.