Molecular Simulations of Vapor–Liquid Equilibrium of Isocyanates

The wide range of applications of
the isocyanates across multiple
industries sparks the interest in the study of their phase behavior.
A molecular simulation is a powerful tool that can go beyond experimental
investigations relying on a molecular structure of a chemical. The
success of a molecular simulation relies on a description of the system,
namely, force field, and its parameterization on reproducing properties
of interest. 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. 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. Additionally, we performed similar
calculations for methyl, ethyl, and butyl isocyanates based on the
all-atom GAFF-IC force field available in the literature for modeling
isocyanate viscosities. We showed that the developed TraPPE-based
force field generally overperformed the GAFF-IC force field and overall
showed excellent performance in modeling phase behavior of isocyanates.
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. The predictions are of particular
use in the investigation of thermodynamic properties for those isocyanates
lacking experimental vapor pressure data. Results can also be employed
in modeling the phase behavior of isocyanate mixtures to investigate
their sensing and capturing. 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.