Molecular Dynamics Simulation of the Interfacial Behavior of Short-Chain Polystyrene Sulfonate Aqueous Solutions in Contact with Graphene Surfaces in the Presence of Multivalent Cations

We present a detailed analysis of the behavior of aqueous electrolyte-polyelectrolyte systems in contact with neutral and charged graphene substrates, based on an extensive molecular dynamics simulation effort. Our study involves aqueous systems comprising short chains of lithium polystyrene sulfonate with an explicit atomistic description of water, the chain backbones, and their interactions with all species in solution as well as with the graphene surface. We place special emphasis on the behavior of the axial profiles of species concentrations, local electrostatic charge density, electric field, and corresponding surface-charge screening to provide a full characterization of the inhomogeneous environment at the solid-liquid interface, that is, the electric double layer and the effect of the added salts (BaCl(2) and LaCl(3)) on its structure. To complete the analysis, we assess the tendency toward ion pairing along planes parallel to the graphene surface and estimate, according to the axial distribution profiles, the strength of the adsorption of the polyelectrolyte, counterions, and other species in solution, in order to interpret the degree of surface-charge screening and the occurrence of surface-charge overcompensation and reversal. We present evidence of a recently reported new phenomenon of overcharging and discuss the central role played by the explicit description of the solvent on this occurrence. Moreover, to interpret the conformational behavior of the polyelectrolyte backbones we determine the axial profiles of the perpendicular to- and parallel to-components of the corresponding radius of gyration and end-to-end distance.