(660a) Molecular Simulations of Vapor–Liquid Equilibrium of Isocyanates

The wide range of applications of the isocyanates across multiple industries sparks interest in the study of their phase behavior. Due to the high toxicity of several members of the isocyanates family and risk of human exposure, the development of effective sensing and protective nanomaterials is required, which in many ways rely on the knowledge of thermodynamic properties of the pure isocyanates and their mixtures. A molecular simulation is a powerful tool that can go beyond experimental investigations relying on a molecular structure of a chemical, and its results strongly depend on the force field employed. Whereas there are generic force fields that could be used to model isocyanates, there has been no force field developed for modeling their vapor-liquid equilibrium properties specifically.

In this work, we propose a united-atom force field based on the transferable potentials for phase equilibria (TraPPE) to model the vapor–liquid phase behavior of isocyanates [1]. With Monte Carlo and molecular dynamics simulation methods and the introduced force field, we modeled vapor-liquid equilibrium for a family of linear mono-isocyanates, from methyl isocyanate to hexyl isocyanate, and hexamethylene diisocyanate. We showed that the developed TraPPE-based force field generally overperformed the all-atom GAFF-IC force field (developed recently to model isocyanates viscosities), and overall showed excellent agreement with experimentally measured vapor pressures and boiling points. Based on the simulated vapor pressures for the considered compounds, we estimated the Antoine equation parameters to calculate the vapor pressure in a range of temperatures. Furthermore, from the vapor–liquid equilibrium binodals, we predicted the critical properties of isocyanates which can be used in thermodynamic models based on an equation of state. The predictions are of particular use in the investigation of thermodynamic properties for those isocyanates lacking experimental vapor pressure data, as well as experimentalists working on the investigation of pure isocyanates and their mixtures.

[1] Emelianova, A., & Gor, G. Y. (2021). Molecular Simulations of Vapor–Liquid Equilibrium of Isocyanates. The Journal of Physical Chemistry B, 125(45), 12528-12538.