Thermodynamics of Charge Regulation Near Surface Neutrality

TL;DR

This paper investigates the thermodynamics of charge regulation near surface neutrality, revealing that surface entropy governs the process and deriving universal laws for ion adsorption free energy.

Contribution

It introduces a new thermodynamic framework for charge regulation near neutrality, including explicit expressions and a universal limiting law for ion adsorption free energy.

Findings

Charge regulation is governed by surface entropy near neutrality.

Derived a universal free energy law proportional to k_B T.

Applicable to oxides near zero charge and proteins near isoelectric point.

Abstract

The interaction between two adjacent charged surfaces immersed in aqueous solution is known to be affected by charge regulation - the modulation of surface charge as two charged surfaces approach each other. This phenomenon is particularly important near surface neutrality where the stability of objects such as colloids or biomolecules is jeopardized. Focusing on this ubiquitous case, we elucidate the underlying thermodynamics and show that charge regulation is governed in this case by surface entropy. We derive explicit expressions for charge regulation and formulate a new universal limiting law for the free energy of ion adsorption to the surfaces. The latter turns out to be proportional to $k_{B}T$, and independent of the association energy of ions to surface groups. These new results are applied to the analysis of unipolar as well as amphoteric surfaces such as oxides near their point of zero charge or proteins near their isoelectric point.