Polyelectrolyte multilayer membranes (PEM) can be assembled efficiently by the layer-by-layer (LBL) method, i.e., by alternating exposure of a surface-charged substrate to aqueous solutions of polycations and polyanions. Molecular-level modeling of the self-assembly of charged macromolecules onto solid substrates relies on the microscopic understanding of the relevant interactions between species in solutions and binding sites of the surface. 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, as well as the system microstructure and dynamics.

Molecular-based simulation can potentially provide a direct route from the microscopic details of precisely defined interaction models to the macroscopic (equilibrium and time dependent) behavior of the systems, and consequently, becomes a powerful means to interpret the mechanisms of polyelectrolyte adsorption onto substrates from aqueous solutions, with simultaneous ion pair formation and counterion condensation in the presence of salts. In solution, the interplay of these two association phenomena gives rise to complex behavior such as the appearance of like-charge inter-chain attractive interactions (Chialvo and Simonson 2005; 2006).

Several factors appear to affect interfacial aqueous polyelectrolyte solutions, including the nature of the counterions, their electrostatic charges, their short-range interactions with the binding sites, the location and distribution of the binding sites along the backbones, as well as interactions of polyions and counterions with the solid substrate. The binding between polyanions and polycations in the presence of counterions during deposition will result from a dynamic balance between short-range (solvation) interactions characterizing the local environment, and longer-range solvent-screened electrostatic interactions providing intrinsic and extrinsic charge compensation. Thus, the resulting local environment around these charged species will depend strongly on the solvent's properties, the ionic strength, and the state conditions, and may differ significantly from that approximated by a solvent as a continuum dielectric. These factors highlight the need for a more detailed understanding, beyond the macroscopic treatments, of relevant interactions including an explicit description of the solvent and the substrates.

In this presentation we provide a detailed analysis of the behavior of aqueous short-chain (octamers) lithium poly(styrene sulfonate) in contact with a charged graphene substrate, based on molecular dynamics simulation including fully atomistic descriptions of the polyelectrolyte, counterions, solvent, and the carbon wall. We investigate the effect of the degree of sulfonation and its distribution along the chain backbone on the resulting conformation as well as solvation structure. Further, we assess the impact of added salts (BaCl2 and LaCl3) on the net charge of the chain backbone, placing emphasis on the valence of the counterion, and the extent of the ion-pair formation between the sulfonate group and the counterions.

We focus our attention on the behavior of the axial profiles of the species concentrations, local charge, electric field and corresponding electric potential, to provide a full characterization of the solid/liquid interface. Special emphasis is placed in the analysis of 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 degree of adsorption of the polyelectrolyte, counterions, and other ions, in order to assess the effect of counterion condensation, and its interplay with ion pairing, on the net interaction of the polyelectrolyte with the graphene surface.

REFERENCES: Chialvo, A. A. and J. M. Simonson (2006). "Ion Pairing and Counterion Condensation in Aqueous Electrolyte and Poly-electrolyte Solutions: Insights from Molecular Simulation." Journal of Molecular Liquids Accepted for publication (Special issue about the 2005 EMLG Conference (Prague)).

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.