Transport coefficients of model simple liquids. A molecular-dynamics study and effective hard-sphere analysis

Abstract

The techniques of equilibrium and non-equilibrium molecular dynamics have been used, for the first time, to obtain simultaneously the self-diffusion coefficient, shear viscosity and thermal conductivity of the Lennard-Jones and soft-sphere (SS) fluids over a comprehensive region of their phase diagrams. Temperature-dependent effective hard-sphere diameters are attached to these soft particles and then the transport coefficients predicted from those of the hard-sphere fluid, using a molecular-dynamics-modified Enskog prescription. Agreement is excellent for the viscosity over the entire fluid range above the critical density. The thermal conductivity is similarly in good agreement over the same range, except close to the solid phase boundary. The self-diffusion coefficient gives poor overall agreement with the predictions based on the hard-sphere model, except on the solid phase boundary. Simple expressions for the shear viscosity activation energies at constant volume and pressure are derived in terms of the close-packed volume and isobaric expansivity of the LJ and SS fluids. A similar expression is derived for the pressure viscosity coefficient in terms of the isothermal compressibility. The behaviour of these is investigated. These relationships are applied with some success to other more complicated real molecular fluids.