(700d) Molecular-Based Mechanism of Surface Overcharging On Graphene/Aqueous Poly/Electrolyte Interfaces

The conventional view on the behavior of electrolyte solutions in contact with charged-surfaces hinges around the concept of surface-charge screening by the ions in solution, which ultimately render the entire system electro-neutral. This screening might take the form of adsorption, i.e., by alternating layers of opposite charged ions giving rise to an oscillatory behavior for the total axial charge density represented by charge reversal followed by charge inversion phenomena.

Jimenez-Angeles and Lozada-Cassou 1 have recently reported a new phenomenon that occurs during the adsorption of macroions onto a positively-charged planar surface and unveiled through by integral equation calculations of three-component inhomogeneous primitive models of macroion solutions. In principle, these results appear at odd with the conventional wisdom, in that, they represent an accumulation of additional effective charge with the same sign as that of the surface; they have referred to as overcharging (OC) and provided an explanation in terms of ?the strong electrostatic attraction between macroions and the divalent cations. However, for this effect to be present, a high particle's excluded volume is needed, i.e., a high concentration of macroions and/or little particles and/or large macroion size or little ion size?

The goal of this presentation is threefold: (a) to illustrate by molecular dynamics simulation the occurrence of OC in model aqueous-polyelectrolytes solutions in contact with positively-charged graphene walls in the presence of multivalent cations and the explicit atomistic description of the solvent water; (b) to show that the occurrence of OC does not necessarily require the co-adsorption of the negatively-charged macroions and the divalent cations with a large size and charge asymmetry, but rather the preferential adsorption of the solvent water through the direct interactions of its hydrogen and oxygen sites with the surface2; and (c) to address the plausible role played by the poly-anion site-connectivity as well as the counterion condensation and ion-pair formation in the appearance of this phenomenon3. Finally, we discuss some theoretical and practical implications of the observed behavior.

REFERENCES:

1. Jimenez-Angeles, F.; Lozoda-Cassou, M., ?A model macroion solution next to a charged wall: Overcharging, charge reversal, and charge inversion by macroions? Journal of Physical Chemistry B 2004, 108, (22), 7286-7296.

2. Chialvo, A. A.; Simonson, J. M., ?Interfacial behavior of aqueous polyelectrolyte solutions in contact with graphene surfaces in the presence of multivalent cations? Journal of Physical Chemistry C 2008, 112, (49), 19521-19529.

3. Chialvo, A. A.; Simonson, J. M., ?On the Molecular Mechanism of Surface Overcharging At Aqueous Polyelectrolyte-Graphene Interfaces? Manuscript in preparation.