Velocity cross correlations in binary mixtures of simple fluids

Abstract

The velocity cross correlation integrals

$$D_{{\text{ab}}}^{\text{J}} = (N/3)\mathop \smallint \limits_{\text{o}}^\infty< {\text{v}}_{{\text{1a}}} ({\text{t}}) \cdot {\text{v}}_{{\text{2b}}} (0) > {\text{dt,}} {\text{a}} {\text{ = }} {\text{1,2;}} {\text{b}} {\text{ = }} {\text{1,2}}$$

can be estimated from the intradiffusion coefficients D ^°₁ and D ^°₂ at each mole fraction x₁ of component 1 on the basis of the exact relations among the Onsager phenomenological coefficients together with an assumed equation relating the joint diffusion coefficients D ^J_ab . The results from several such equations are compared with experimental data and with similar results derived by Hertz in a different way to represent the behavior of f _ab ≡D ^J _ab x _b in ideal reference systems. In some cases the agreement with experimental data for relatively ideal systems is even better than given by Hertz's results. For greater accuracy in predicting the D ^J_ab from D ^dg_a data one would need a prediction of the limiting value of D ^J_aa at x_a=0 for a=1,2. Presently known theory does not give a basis for estimating this limit reliably.