Effect of the molecule shape on the diffusion in molecular fluids. Molecular dynamics calculation results for SF6–C6H6 and a related model solution

Abstract

Solutions of molecules of planar and globular shape have been investigated by molecular dynamics (MD) calculations. Six‐center Lennard‐Jones (LJ) potential functions have been used to describe the molecular interactions for both types of molecules. An extended density range between liquid and dense gas was considered, and the concentration of the flat molecules was kept low. Two sets of potential parameters and ‘‘bond length’’ parameters have been chosen for the calculations. The first set reproduced, approximately, the thermodynamic and diffusive properties of SF₆–C₆H₆ at high dilution of C₆H₆ as recently determined experimentally. The second one ensured that the planar molecule had the same ‘‘molecular diameter’’ as the globular one. For both mixture kinds, we compared the diffusion coefficient of the highly diluted component with the self‐diffusion coefficient of the solvent. In the case of the SF₆–C₆H₆ model, the self‐diffusion coefficient of SF₆ appeared to be always larger than that of highly diluted benzene. This agrees well with recent measurements. For the ‘‘model mixture,’’ the self‐diffusion coefficient of the solute was however found to be larger than that of the solvent, but with a tendency to decrease more rapidly with decreasing density. This change of the relative mobility of the flat molecules depending on density is explained by the rotational properties of these molecules. Determination of the first two reorientational correlation functions confirmed these conclusions.