The transport properties of fluid mixtures near the vapor-liquid critical line

We apply the mode-coupling theory for the dynamics of critical fluctuations to binary fluid mixtures near the vapor-liquid critical line and derive a set of coupled equations for the mode-coupling contributions to the diffusion coefficients of the relevant hydrodynamic modes. An approximate solution of the mode-coupling integrals yields representative equations for the effects of the critical fluctuations on the thermal conductivity, the viscosity, the mutual diffusivity and the thermal-diffusion coefficient, which are valid not only in the asymptotic critical region but also describe the crossover to regular behavior far away from the critical line. The ''crossover'' functions depend on the thermodynamic properties of the mixtures, background values of all the transport coefficients and two concentration-dependent cutoff wave numbers. We compare the proposed crossover model with experimental thermal-conductivity data for mixtures of carbon dioxide and ethane in the critical region and find good agreement between theory and experiment. (C) 1996 American Institute of Physics.