Volume 60, Number 3, November I 2002
|Page(s)||383 - 389|
|Section||Condensed matter: structural, mechanical and thermal properties|
|Published online||01 October 2002|
Overcharging: The crucial role of excluded volume
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
Accepted: 22 August 2002
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.
PACS: 61.20.Qg – Structure of associated liquids: electrolytes, molten salts, etc / 82.70.Dd – Colloids / 87.15.Aa – Theory and modeling; computer simulation
© EDP Sciences, 2002
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