Adsorption of polyelectrolytes at solid-liquid interfaces plays a crucial role in the formation of membranes and nanoscale layered-films assembled by the layer-by-layer (LBL) technique. Since the inception of this technique a variety of experimental studies have provided some insights into the mechanisms behind the membrane formation, including evidence that the backbone adsorption appears irreversible with no participation of counterions, that the surface charge inversion proceeds via ion pairing between oppositely charged polyions, as well as that the membrane buildup might be tuned by adding salt of multivalent cations, by manipulation of polyion concentration, and by the solvent quality.

Although the body of experimental evidence is significant, the theoretical analysis of the deposition process and the success of the corresponding modeling effort are still limited. At least a couple of reasons lie behind this situation, namely, the intrinsic difficulties of interpreting and translating the experimental observations into working hypotheses, and then, the need for unambiguous connections between the microscopic details (embedded into the hypotheses) of the system and their macroscopic manifestation, i.e., the formulation of the theory.

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. This phenomenon was revealed by integral equation calculations of three-component inhomogeneous primitive models of macroion solutions in contact with a charged surface, and represents an adsorption of “an effective charge onto a like-charged” surface. In fact, the authors indicated that “such an effect defines a new phenomenon, hereafter referred to as overcharging (OC), i.e., at the wall's neighborhood we find the accumulation of an effective additional charge with the same sign of the wall. This effect is due to 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 pf macroions and/or little particles and/or large macroion size or little ion size”

In this communication we argue that the OC can be viewed as a more general solvation phenomenon and illustrate its occurrence by means of molecular dynamics simulation of atomistic model aqueous-polyelectrolytes solutions in contact with positively charged graphene walls, with and without added salts of multivalent cations, including the explicit description of water. In fact, we show that the occurrence of OC does not necessarily require the co-adsorption of negatively charged macroions and divalent counterions with a large size and charge asymmetry, but rather the preferential adsorption of the hydrogen and oxygen water-sites resulting in specific orientational water structure at the solid surface 2. Finally, we discuss the microscopic mechanism underlying overcharging, charge reversal and charge inversion, as well as the modeling consequences.

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. Submitted 2008.