Molecular Modeling of Triethyl Phosphate: Extension and Validation of the TraPPE Force Field.

The study of physical properties of organophosphorus chemical warfare agents (CWAs) is essential for detection, protection, decontamination, and destruction. Due to the extreme toxicity of CWAs, other organophosphorus molecules with similar chemical structures are employed in experimental studies as simulants, such as dimethyl methylphosphonate (DMMP) and diisopropyl methylphosphonate (DIMP). Triethyl phosphate (TEP) can also be considered an alternative simulant, especially because it is more accessible due to its various industrial applications. Classical molecular simulations complement the experimental research on organophosphates. Here, we present the extension of the transferable potentials for phase equilibria (TraPPE) force field to TEP. Using this model, TEP can be parametrized based on interaction parameters from similar organophosphates in conjunction with ab initio calculations. The model was validated by performing molecular dynamics (MD) simulations to predict liquid densities and Gibbs ensemble Monte Carlo simulations to model vapor-liquid equilibrium and to predict vapor pressures, which were compared with literature data. MD simulations were performed to predict the enthalpy of vaporization, surface tension, viscosity, and diffusivity. Additionally, we parametrized and tested trimethyl phosphate (TMP) due to its structural similarity to TEP. Our study helps assess TEP and TMP as alternative CWA simulants and provides predictions of their physical properties over a wide range of temperatures, complementing the scarce experimental data. The presented force field can be further utilized for predicting other properties based on molecular simulations, such as adsorption of TEP or TMP.