Vapor-Liquid Equilibrium Simulations of Hydrocarbons Using Molecular Dynamics with Long-Range Lennard-Jones Interactions

Because of exposure of injected fuel to extreme temperatures and pressures in modern diesel and jet engines, there is much interest in understanding vapor liquid equilibrium (VLE), interfacial behavior, and critical properties of hydrocarbon mixtures. Molecular dynamics (MD) simulations with long-range dispersion interactions can be used to study liquid-vapor interfacial properties. In this work, the accuracy of three force fields (CHARMM, OPLS-AA, and TraPPE-UA) is assessed by using MD simulations with the Lennard-Jones particle-mesh Ewald method to examine the VLE behavior of 12 pure hydrocarbons. While the TraPPE-UA potential yields significantly better critical temperature predictions, all three force fields predict critical densities and pressures with about the same accuracy. Surface tensions calculated using the TraPPE-UA force field are too high compared to experiment; values using the CHARMM and OPLS-AA potentials tend to be more accurate at lower temperatures and then decrease too quickly as temperature increases. Simulations of binary mixtures show that both the CHARMM and TraPPE-UA potentials predict the same liquid- and vapor-phase compositions as a function of overall mole fraction. Not surprisingly, the composition of the interfacial region is correlated with the surface tension values of the pure components. Pressure versus mole fraction phase diagrams were quantitatively reproduced by TraPPE-UA and only qualitatively reproduced by the CHARMM force field. Given the lack of phase equilibrium data used in the parameterization of the CHARMM potential, its ability to reproduce many aspects of VLE behavior is impressive. This work demonstrates the power of MD simulations for examining the phase behavior of hydrocarbons under conditions that are difficult to achieve experimentally.