Conformal solution theory for viscosity and thermal conductivity of mixtures

Abstract

The first-order terms are derived in a conformal solution theory expansion for the viscosity and thermal conductivity of a fluid mixture. Provided that pairwise additivity of the intermolecular potentials is assumed, these terms can be made to vanish exactly by a suitable choice of the potential parameters and molecular mass for the reference system. A generalization of Enskog's dense gas theory to molecules with a general intermolecular potential law is used to suggest suitable combinations of potential parameters and reduced masses as expansion variables. Several sets of mixing rules are thus suggested. These are first tested for dilute gases by comparison with the Chapman-Enskog results, and the best mixing rules for gases are determined. This is followed by a comparison with experimental data for simple liquid mixtures. The general behaviour of the excess viscosity and thermal conductivity for binary mixtures as a function of the potential and mass parameters is investigated using the theory. It is shown that whereas the excess viscosity is nearly always negative or zero, the excess thermal conductivity may be positive or negative.