Simulation of 1-alkene and n-alkane binary vapour-liquid equilibrium using different united-atom transferable force fields

The configurational-bias Monte Carlo method in the Gibbs ensemble, using the NERD, TraPPE and Spyriouni et al. 1-alkene force fields, was used to simulate phase equilibrium data for the ethane + propene, 1-hexene + n-octane, n-dodecane + 1-octadecene, propene + 1-butene and 1-butene + 1-hexene binary mixtures. The NERD force field yields P-x-y data that are in better agreement with measured experimental data than those obtained from the TraPPE force field for alkane + 1-alkene and 1-alkene + 1-alkene mixtures that are comprised of short chain molecules. Both of these force fields yield results that agree with experimental data for binary mixtures consisting of long chain molecules. The Spyriouni et al. 1-alkene force field also yields P-x-y data for the 1-alkene + 1-alkene mixtures that agrees well with experiment data, but the n-alkane parameters from the SA1 force field yield inaccurate coexistence densities for n-alkanes shorter than n-dodecane and hence are not transferable to these shorter n-alkanes. All force fields yield x-y diagrams that are in very good agreement with experimental data for the mixtures studied. It was found that assuming ideal solution behaviour and linear additivity of the species molar volumes provided a means to reasonably estimate the starting conditions for the simulations. It is also shown how the results of a Gibbs ensemble simulation and a reaction Gibbs ensemble simulation may be easily interchanged by using a simple calculation procedure without the need for a full calculation, and that the reaction Gibbs ensemble in general compromises the true molar composition predicted by a force field to achieve a shift in the phase diagram.