Abstract
In this communication we illustrate the occurrence of a recently reported new
phenomenon of surface-charge amplification, SCA, (originally dubbed
overcharging, OC), 1 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.