Overcharging: The Crucial Role of Excluded Volume

Details Overcharging: The Crucial Role of Excluded Volume Overcharging: The Crucial Role of Excluded Volume

2001-11-19

arxiv.org/abs/cond-mat/0111335v2

Europhysics Letters, VOL 60, pp 383-389 (2002)

Physics

Condensed Matter

Soft Condensed Matter

7 pages, 3 figs (4 EPS files). To appear in Europhysics Letters

Scientific paper

10.1209/epl/i2002-00275-y

In this Letter we investigate the mechanism for 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 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.

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