(676a) Interactions of G-Agents and Their Simulants with Water and the Components of Permselective Polyelectrolyte Membranes

Development of the barrier material is one of the major focuses in current defense technology for individual protection from contaminants. Nafion and various block copolymers including sulfonated polystyrene and polyolefin blocks were tested for the purpose of creating breathable material capable of protecting against chemical warfare agents (CWA). In particular, we considered two common G-agents sarin (GB) and soman (GD), simulated by dimethyl methylphosphonate (DMMP) and diisopropyl fluorophosphate (DIFP).

Our work pursues three main targets (1) quantitatively characterize interactions of G-agents with water and fragments of perfluorinated PEM and block-copolymer PEM based on sulfonated polystyrene (2) understand to what extend the behavior of G-agents can be mimicked by typical simulant compounds (3) establish the models and techniques that can be reliably applied to modeling of CWA and simulants inside PEM. We employed ab-initio modeling, molecular dynamics (MD), and semiempirical modeling using COSMO-RS that allows calculation of thermodynamic properties of reference solutions based on ab-initio optimization results.

Overall, we conclude that DMMP and DIFP mimic the behavior of G-agents reasonably well. Our ab-initio minimization and the analysis of infra-red spectra of small complexi of these phosphororganic chemical with water showed that methylphosphonates are more hydrophilic, and fluorophosphates are more hydrophobic compared to the G-agent molecules of simlar size. Based on the ab-initio optimization, we constructed molecular models of DIFP, and performed molecular dynamics simulations of G-agents, simulants and their aqueous solutions. We obtained a reasonable agreement with the excess mixing volume of for DMMP-water solution that we measured experimentally. We found that the aqueous solutions of the organophosphorous compounds display non-trivial molecular scale structures and dynamic properties, which are explained by a competition of interactions between strongly hydrophobic and hydrophilic groups.

The ab-initio optimizations were linked to thermodynamic properties of reference solutions using COSMO-RS approach. First, we tested COSMO-RS against the published and our own experimental data for DMMP-water, soman-water and sarin-water mixtures and obtained semiquantitative agreement with the experiments. Then, we explored the interactions of G-agents and simulants with the components of select PEM based on thermodynamic properties of model solutions of small molecules that contain the same fragments as PEM. The results indicate different mechanisms of solvation of the hydrated PEM fragments by simulants and G-agents, most visible in case of Nafion PEM. In particular, benzenesulfonate ion appeared to interact unfavorably (compared to the triflate ion) with G-agents and simulants, especially at hight hydration levels.

Our theoretical and experimental studies have advanced a fundamental understanding of interactions of phosphor-organic nerve agents and their simulants with water and PEM. This work was supported by DTRA grant HDTRA1-08-1-0042.