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Journal Article

Overcharging: The crucial role of excluded volume


Messina,  R.
MPI for Polymer Research, Max Planck Society;

Holm,  Christian
MPI for Polymer Research, Max Planck Society;

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Messina, R., Gonzalez-Tovar, E., Lozada-Cassou, M., & Holm, C. (2002). Overcharging: The crucial role of excluded volume. Europhysics Letters, 60(3), 383-389.

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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.