Molecular dynamics simulations of liquid binary mixtures: Partial properties of mixing and transport coefficients

Abstract

Equilibrium molecular dynamics (MD) computer simulations have been used to determine the transport coefficients of model Ar–Kr, Ar–CH4, and CH4–N2 mixtures at a large number of liquid and dense fluid state points for which experimental data are available. Both species in each mixture are represented by single-site Lennard-Jones pair potentials with Lorentz–Berthelot mixing rules for the unlike interactions. Green–Kubo formulas and mean-square displacements are used to calculate the self-diffusion coefficients for each species and mutual-diffusion coefficients. The shear and bulk moduli and viscosities, thermal conductivity and the thermal diffusion coefficient are determined by Green–Kubo in the [NVE] and [NVT] ensembles. The thermotransport coefficients employ a rigorous definition for the heat flux, which includes the partial enthalpy of the two species, used for the first time to compute these transport coefficients. The partial volumes and enthalpies, and chemical potentials for each species, were obtained from separate computations carried out at constant pressure in the [NPT] ensemble. The simulated density at fixed pressure, shear viscosity, and thermal conductivity of the Ar–Kr mixtures are in excellent agreement with experiment. However, the bulk viscosity shows a significant qualitative difference in the composition and temperature dependence (the latter even in the single component fluids). Agreement with experiment deteriorates as the quasispherical molecules progressively depart from spherical shape. For Ar–CH4 the density (obtained using [NPT] MD) is in good agreement with experiment, whereas the shear viscosity is in progressively poorer agreement with increasing methane content. This is caused by an overestimation of the methane viscosity (∼50% higher than experiment for pure methane). For CH4–N2 there are substantial differences between the simulated quantities and experiment. The average simulated densities are ∼5% higher than experiment over a wide temperature and composition range. The simulated shear viscosity is typically ∼10%–50% higher than experiment. The enthalpy, chemical potential, and shear modulus of the model mixtures are reproduced well using a van der Waals one-fluid model. In this, there is little to distinguish between Enskog’s mixing rules for the model from the simpler mole fraction weighted (‘‘Kay’s’’) mixing rules. The shear viscosity and thermal conductivity are also predicted reasonably well with either model, recognizing the greater statistical uncertainty in the transport coefficients when compared with the thermodynamic averages, even for the pure Lennard-Jones fluid.