(428c) Interfacial Behavior Of Aqueous Polyelectrolyte Solutions In Contact With Graphene Surfaces In The Presence Of Multivalent Cations

Molecular-based modeling of the self-assembly of aqueous polyelectrolytes over solid substrates relies on the microscopic understanding of the relevant interactions between species in solutions and the substrates. Because adsorption of charged macromolecules at solid surfaces plays a leading role in many disciplines associated with materials science and separation processes, this understanding is crucial for the design of novel systems with potential for improved separation performance, and a prerequisite to link this performance to the molecular architecture of the raw polyelectrolyte, the nature of the substrate, as well as the microstructure of the interface. The resulting interfacial behavior of aqueous polyelectrolyte solutions obviously depends on the nature of the counterions, their electrostatic charges, their short-range interactions with the binding sites, as well as their interactions with the solid substrate, and will develop from a delicate balance between short-range (solvation) interactions characterizing the local environment, and long-range (though partially screened by the presence of ions) electrostatic interactions leading to intrinsic as well as extrinsic charge compensation. Moreover, the local environment around the charged species will depend strongly on the solvent's properties, the ionic strength, as well as the state conditions, and will be significantly different from that characterized by a solvent described as a continuum dielectric. These factors highlight the need for a more detailed understanding, beyond the macroscopic treatments, of all relevant interactions with and within an explicit description of the solvent and the substrates. We present a detailed analysis of the behavior of aqueous aqueous electrolyte-polyelectrolyte systems in contact with neutral and charged graphene substrates, based on an extensive molecular dynamics simulation effort. Our study comprises aqueous short-chain lithium-polystyrene sulfonate systems involving the explicit atomistic description of water, the chain backbones, and their interactions with all species in solution (1), and addresses the impact of added salts (BaCl2 and LaCl3) on the behavior of the electric double layer, the degree of surface charge screening, and the extent of the ion-pair formation between the sulfonate groups and the counterions. Special emphasis is placed on the analysis of the behavior of the axial profiles of all species concentrations, local electrostatic charge density, electric field and corresponding electric potential, to provide a full characterization of the inhomogeneous environment at the solid/liquid interface, i.e., the electric double layer and the effect of the added salts on its structure. To complete the characterization, and based on the corresponding axial profiles, we also estimate the strength of the adsorption of the polyelectrolyte, counterions, and other ions, in order to assess the effect of counterion condensation, and its interplay with ion pairing (2), on the net interaction of the polyelectrolyte with the graphene surface.

REFERENCES: (1) Chialvo, A. A. and J. M. Simonson (2007). "Ion Pairing and Counterion Condensation in Aqueous Electrolyte and Poly-electrolyte Solutions: Insights from Molecular Simulation." Journal of Molecular Liquids 134: 15-22 (2) Chialvo, A. A. and J. M. Simonson (2005). "Solvation Behavior of Short-Chain Polystyrene Sulfonate in Aqueous Electrolyte Solutions: A Molecular Dynamics Study." Journal of Physical Chemistry B 109: 23031-23042.