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Soft Matter and Biological Physics


Eur. Phys. J. E 7, 371-379 (2002)
DOI: 10.1140/epje/i2001-10102-y

Electrophoresis of a charge-inverted macroion complex: Molecular-dynamics study

Motohiko Tanaka1 and A.Yu. Grosberg2

1  National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
2  Department of Physics, University of Minnesota, Minneapolis, MN 55455, USA

mtanaka@nifs.ac.jp

(Received 5 December 2001 and Received in final form 10 April 2002)

Abstract
We have performed molecular-dynamics simulations to study the effect of an external electric field on a macroion in the solution of multivalent Z:1 salt. To obtain plausible hydrodynamics of the medium, we explicitly make the simulation of many neutral particles along with ions. In a weak electric field, the macroion drifts together with the strongly adsorbed multivalent counterions along the electric field, in the direction proving inversion of the charge sign. The reversed mobility of the macroion is insensitive to the external field, and increases with salt ionic strength. The reversed mobility takes a maximal value at intermediate counterion valence. The motion of the macroion complex does not induce any flow of the neutral solvent away from the macroion, which reveals screening of hydrodynamic interactions at short distances in electrolyte solutions. A very large electric field, comparable to the macroion unscreened field, disrupts charge inversion by stripping the adsorbed counterions off the macroion.

PACS
61.25.Hq - Macromolecular and polymer solutions; polymer melts; swelling.
82.45.-h - Electrochemistry and electrophoresis.
82.20.Wt - Computational modeling; simulation.


© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2002