Overcharging: The crucial role of excluded volume

¹ Max-Planck-Institut für Polymerforschung Ackermannweg 10, 55128 Mainz, Germany
² Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa Apartado Postal 55-534, 09340 D.F., México
³ Instituto de Física, Universidad Autónoma de San Luis Potosí Álvara Obregón 64, 78000 San Luis Potosí, México
⁴ Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo Lázaro Cardenas 152, 07730 D.F., México

Corresponding authors: messina@mpip-mainz.mpg.de, henry@dec1.ifisica.uaslp.mx, marcelo@www.imp.mx, holm@mpip-mainz.mpg.de

Received: 31 May 2002
Accepted: 22 August 2002

Abstract

In this letter we investigate the mechanism for the overcharging of a single spherical colloid in the presence of aqueous salts within the framework of the primitive model by molecular dynamics (MD) simulations as well as integral-equation theory. We find that the occurrence and strength of overcharging strongly depends on the salt-ion size, and the available volume in the fluid. To understand the role of the excluded volume of the microions, we first consider an uncharged system. For a fixed bulk concentration we find that upon increasing the fluid particle size one strongly increases the local concentration nearby the colloidal surface and that the particles become laterally ordered. For a charged system the first surface layer is built up predominantly by strongly correlated counterions. We argue that this is a key mechanism to produce overcharging with a low electrostatic coupling, and as a more practical consequence, to account for charge inversion with monovalent aqueous salt ions.