Charge regulation: a generalized boundary condition?

TL;DR

This paper investigates whether charge regulation acts as a truly generalized boundary condition in charged colloidal systems, revealing it is distinct from constant charge and potential conditions with unique pressure behaviors.

Contribution

The study demonstrates that charge regulation is not a simple limit of other boundary conditions but a separate boundary condition with distinct pressure scaling.

Findings

Charge regulation boundary condition yields a different pressure scaling.

Disjoining pressure at small separations depends on dissociation/association details.

Charge regulation is not reducible to constant charge or potential conditions.

Abstract

The three most commonly-used boundary conditions for charged colloidal systems are constant charge (insulator), constant potential (conducting electrode) and charge regulation (ionizable groups at the surface). It is usually believed that the charge regulation is a generalized boundary condition that reduces in some specific limits to either constant charge or constant potential boundary conditions. By computing the disjoining pressure between two symmetric planes for these three boundary conditions, both numerically (for all inter-plate separations) and analytically (for small inter-plate separations), we show that this is not, in general, the case. In fact, the limit of charge regulation is a separate boundary condition, yielding a disjoining pressure with a different characteristic separation-scaling. Our findings are supported by several examples demonstrating that the disjoining pressure at small separations for the charge regulation boundary-condition depends on the details of the dissociation/association process.