(180t) Characterization of the Interactions of Polyelectrolyte Membranes with Organophosphorus Agents and Their Simulants: A Molecular Dynamics Simulations Study

Nanostructured polyelectrolyte membranes (PEMs) are widely used as perm-selective diffusion barriers in fuel cell technologies and electrochemical processing. PEMs are also suitable for protection against chemical warfare agents. PEMs are made of nanostructured polymers that contain hydrophilic and hydrophobic fragments. For example, in Nafion polymer hydrophilic sidechains terminated by sulfonate groups are attached to a hydrophobic perfluorocarbon backbone. Upon hydration, PEM segregates into hydrophobic and hydrophilic subphases. The former is made by the hydrophobic organic backbone. The latter includes water, counterions, and ionic groups of the polymer, forming a dynamic network of channels in the hydrophobic organic matrix. With respect to transport and adsorption of chemicals, solvated PEM represents a nanoporous material. Water diffuses easily through the network of pores comprised of hydrophilic channels, while more hydrophobic chemical agents are supposedly trapped in the essentially immobile hydrophobic subphase.

We report atomistic molecular dynamics simulations of diffusion of nerve agent sarin and its simulant dimethylmethylphosphonate (DMMP) in perfluorosulfonic acid PEMs such as Nafion and sulfonated polystyrene membranes (sPS) at different agent concentrations. Both sarin and DMMP have hydrophilic and hydrophobic groups and are well-soluble in water. We found that in hydrated Nafion, sarin concentrates at the interface between the hydrophilic and hydrophobic subphases acting as a surfactant. This non-trivial observation is consistent with published NMR studies on DMMP distribution. The solvation of sPS in water-DMMP binary mixture was found to differ substantially from Nafion. In sPS with divalent calcium counterion, DMMP and water compete for the solvation of sulfonate groups. At high water and DMMP content, the diffusion of DMMP turned out to be rather fast with the diffusion coefficient of ca. 30% of that of water. At the same time, water diffusion slows down as the DMMP concentration increases. This observation suggests that the two solvents have different pathways through the system.

The solvation and diffusion mechanism of sarin is slightly different than DMMP. The diffusion coefficient of sarin is about 50% lower than that of DMMP for the same system. Sarin molecules are trapped by hydrophobic subphase, and sarin mobility through the membrane is limited. This can be explained by the fact that sarin is less hydrophilic than DMMP, which is consistent to our previous computational and experimental studies of solubility these chemicals with water.