Using an improved version of the recently suggested NpT + test particle method, phase equilibria are determined for the Lennard-Jones fluid in the temperature range T* = 0·70 to T* = 1·30 in steps of ΔT* = 0·05. For the final results, gas simulations are necessary only at the two highest temperatures; for T* = 1·20 and downwards the gas phase can be described with sufficient accuracy by the Haar-Shenker-Kohler equation. The resulting vapour pressures, bubble and dew densities can be correlated by the equations , , , with the critical temperature and density being T*c = 1·310 and ρ*c = 0·314, respectively. Then a detailed comparison with other simulation results and perturbation theory results is given. Outstanding findings are that the present data are rather smooth, that they match quite well the early data of Hansen and Verlet (1969, Phys. Rev., 184, 151) and are much closer to perturbation theory than most previous results. Finally, using the vapour pressure equation and directly calculated volume chan...

Vapour liquid equilibria of the Lennard-Jones fluid from the NpT plus test particle method

Molecular models for 25 different pure fluids are presented:  neon, argon, krypton, xenon, methane, oxygen, nitrogen, fluorine, chlorine, bromine, iodine, carbon dioxide, carbon disulfide, ethane, ethene, ethyne, perfluoroethane, perfluoroethene, perchloroethene, propadiene, propyne, sulfurhexafluoride, tetrafluoromethane, tetrachloromethane, and propylene. The models are based on the two-center Lennard-Jones plus pointquadrupole pair potential (2CLJQ). The model parameters were adjusted to experimental vapor−liquid equilibria of the pure fluids using a highly efficient procedure. The application of these models to the calculation of vapor−liquid equilibria and homogeneous fluid state points by molecular simulation shows good agreement with experimental results. Numbers for model parameters correlate reasonably with geometric data of the molecules and experimental quadrupole moments. Due to the compatibility of the presented models, applications to the prediction of vapor−liquid equilibria of mixtures are...

A Set of Molecular Models for Symmetric Quadrupolar Fluids

The adsorption and reaction of low molecular weight alkanes on metallic single crystal surfaces

In order to determine the vapour liquid equilibrium of a pure fluid, the liquid and the vapour branch of the isotherms in the chemical potential μ vs pressure p-diagram, are constructed explicitly. The liquid branch is obtained by molecular dynamics simulations in an NpT-ensemble into which test particles are inserted to calculate the chemical potential. The vapour branch is obtained at lower temperatures by using the second virial coefficient, at higher temperatures it is determined again by simulations. As an example the two-centre Lennard-Jones fluid with elongation L = 0·505 is considered at temperatures ranging from 0·69 to 0·92 of the estimated critical temperature. As expected, the inaccuracies of the liquid chemical potential increase with decreasing temperature as a consequence of the increasing saturated density. The uncertainties in μ/RT range from 0·02 at the highest to 0·10 at the lowest temperature which creates an uncertainty in the reduced vapour pressure Pσ3/ϵ of the order of 0·002. Withi...

Vapour liquid equilibrium of a pure fluid from test particle method in combination with NpT molecular dynamics simulations

A new equation of state (EOS) is proposed for the Helmholtz energyF of the Lennard Jones fluid which represents the thermodynamic properties over a wide range of temperatures and densities. The EOS is written in the form of a generalized van der Waals equation.F =Fu +Fv. WhereFu is a hard body contribution andFA an anttractive dispersion force contribution. The expression forFH is closely related to the hybrid Barker Henderson pertubation theory. The construction ofFA is accomplished with the Setzmann Wagner optimization procedure on the basis of virial coefficients and critically assessed computer simulation data. A comparison with the EOS of Johnson et al. shows improvement in the description of the vapor liquid coexistence properties, thepvT data. and in peculiar, of the calorie properties. A comparison with the EOS of Kolafa and Nezbeda which appeared after the bulk of this work was finished shows still by about 30%.

An accurate Van der Waals-type equation of state for the Lennard-Jones fluid

Model fluids consisting of spherical n/6 molecules and of linear two-centre Lennard-Jones molecules are considered. The orthobaric densities and the vapour pressures are calculated by using perturbation theory at liquid densities and second virial coefficients at gas densities. In order to classify the results the deviations from corresponding states principle are investigated. For spherical molecules, a steeper repulsive potential lowers the vapour pressure considerably but hardly effects the liquid saturation density. For linear molecules, an increasing elongation lowers the vapour pressure and increases the liquid saturation density. Finally, comparison with real Ar, CH4, N2, F2, C2H6, and CO2 is made.

Wolfgang Lemming

Papers

Influence of intermolecular potential parameters on orthobaric properties of fluids consisting of spherical and linear molecules