Diffusion and thermal diffusion coefficients of a binary mixture in the van der Waals model

Model expressions for the concentration diffusion coefficient and thermal diffusion factor of a binary mixture obeying the van der Waals equation of state are obtained. Model calculations at elevated pressure (density) require the equation of state parameters and thermal diffusion factor of the mixture in a low-density ideal gas state. The behavior of model relations is studied depending on pressure along isotherms. Near the critical state, the model diffusion coefficient has a minimum, and the thermal diffusion coefficient has a maximum. The diffusion and thermal diffusion models at elevated pressure are compared with experimental data for a number of mixtures. It is shown that they are in qualitative and generally quantitative agreement at different temperatures and concentrations of components. The van der Waals equation is not quite suitable for describing the experiment on compressibility and phase equilibrium at high pressure. This requires more complex and flexible modifications of the equation. Generalized expressions for model characteristics of the diffusion of the binary mixture in the case of such equations of state are given.