  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.
