ON THE SIGNIFICANCE OF MOLECULAR-SURFACES AND THERMODYNAMIC INTERACTIONS FOR THE EXCESS VISCOSITIES OF LIQUID-MIXTURES

The viscosities eta of homogeneous binary mixtures of liquids are usually approximated as ln eta = phi(1) ln eta(1) +phi(2) ln eta(2) where phi i and eta i are the volume fractions and the viscosities, reap., of the i-th pure substance; the behavior of real systems is then discussed in terms of Delta ln eta, the deviations from the above reference behavior. Here a semi-empirical approach is presented according to which volume fractions are replaced by the surface fractions Omega(i) to create a more realistic reference state, and the thermodynamic interaction parameter g is taken into account. The new equation reads (expressing it for practical purposes still in the terms of phi(1)) Delta ln eta = gamma delta/1+gamma phi(2) phi(2)(1-phi(2))+2g (1+gamma)/(1+gamma phi(2))(2) phi(2) (1-phi(2)); gamma is a geometric factor, measuring the difference between Omega(i) and phi(i), and delta = ln (eta(2)/eta(1)). The comparison of calculations according to the above equation with experimental data demonstrates that the first term is decisive in the majority of cases and only with systems of large interaction does the second become important. For all nine systems under investigation quantitative agreement between experiment and theory can be achieved using a theoretical value for either gamma or g, and adjusting the other within physically reasonable limits. Possible explanations for the necessity to fit one of the parameters are given.