On the molecular mechanism of surface charge amplification and related phenomena at aqueous polyelectrolyte-graphene interfaces

TL;DR

This study uses molecular dynamics simulations to explore surface-charge amplification at aqueous polyelectrolyte-graphene interfaces, revealing water's role in charge magnification rather than screening, contrasting with primitive models.

Contribution

It demonstrates that water's explicit molecular modeling shows surface-charge amplification without macroion co-adsorption, differing from previous implicit solvent models.

Findings

Water preferentially adsorbs and orients to induce charge magnification.

Surface-charge amplification occurs within a limited charge density range.

Explicit water models reveal phenomena not seen in primitive models.

Abstract

In this communication we illustrate the occurrence of a recently reported new phenomenon of surface-charge amplification, SCA, (originally dubbed overcharging, OC), [Jimenez-Angeles F. and Lozada-Cassou M., J. Phys. Chem. B, 2004, 108, 7286] by means of molecular dynamics simulation of aqueous electrolytes solutions involving multivalent cations in contact with charged graphene walls and the presence of short-chain lithium polystyrene sulfonates where the solvent water is described explicitly with a realistic molecular model. We show that the occurrence of SCA in these systems, in contrast to that observed in primitive models, involves neither contact co-adsorption of the negatively charged macroions nor divalent cations with a large size and charge asymmetry as required in the case of implicit solvents. In fact the SCA phenomenon hinges around the preferential adsorption of water (over the hydrated ions) with an average dipolar orientation such that the charges of the water's hydrogen and oxygen sites induce magnification rather than screening of the positive-charged graphene surface, within a limited range of surface-charge density.