(67c) Predicting Thermodynamic and Transport Properties of Organophosphates: A Molecular Simulation Substitute for Experiments that You Don't Want to Perform

While the production and stockpiling of organophosphorus chemical warfare agents (CWAs), such as sarin, was banned three decades ago, CWAs have remained a threat. New approaches for decontamination and destruction of CWAs require detailed knowledge of their various physicochemical properties. Organophosphorus CWAs, are colloquially called "nerve gases", however they are stockpiled as liquids and form aerosols when dispersed. To predict the formation and evolution of CWA aerosols one needs to know such properties as vapor pressures, surface tension and viscosity.

Due to the extreme toxicity of sarin, most experimental studies are carried out using its surrogates -- organophosphorus compounds which, while having similar structures, are much less toxic, e.g., dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP). However, not only for sarin, but also for its surrogates, literature data on the surface tension and viscosity are scarce. Therefore, theoretical predictions based on molecular dynamics simulations can complement and even substitute experimental data.

Here we present predictions of the surface tension and viscosity of sarin and its two common surrogates, DMMP and DIMP from the classical molecular dynamics simulations. We utilized the Transferable Potentials for Phase Equilibria (TraPPE) force field, which demonstrated an excellent agreement with the available experimental data for both properties. Our simulations provided the surface tension and viscosity data in the broader range of temperatures than what was measured experimentally. Furthermore, these data allowed us to evaluate suitability of different surrogates. Our results suggest that for applications where liquid viscosity is important, DMMP is a more suitable surrogate for sarin than DIMP. However, it is not the case when surface tension is more important, despite the difference in molecular size, that surface tension of sarin is closer to that of DIMP than to DMMP. The temperature-dependent surface tension and viscosity values for sarin and its surrogates obtained in our study can be used in large scale models predicting the formation and evolution of their aerosols.