The Law of Corresponding States has been demonstrated for a number of pure substances and binary mixtures, and provides evidence that the transport properties viscosity and diffusion can be determined from a molecular shape function, often taken to be a Lennard-Jones 12-6 potential, that requires two scaling parameters: a well depth ε ij and a collision diameter σ ij , both of which depend on the interacting species i and j. We obtain estimates for ε ij and σ ij of interacting species by finding the values that provide the best fit to viscosity data for binary mixtures, and compare these to calculated parameters using several "combining rules" that have been suggested for determining parameter values for binary collisions from parameter values that describe collisions of like molecules. Different combining rules give different values for σ ij and ε ij and for some mixtures the differences between these values and the best-fit parameter values are rather large.There is a curve in (ε ij, σ ij ) space such that parameter values on the curve generate a calculated viscosity in good agreement with measurements for a pure gas or a binary mixture. The various combining rules produce couples of parameters ε ij , σ ij that lie close to the curve and therefore generate predicted mixture viscosities in satisfactory agreement with experiment. Although the combining rules were found to underpredict the viscosity in most of the cases, Kong's rule was found to work better than the others, but none of the combining rules consistently yields parameter values near the best-fit values, suggesting that improved rules could be developed.Keywords: viscosity, transport properties, combining rules, intermolecular potential parameters 3
I -IntroductionMany important phenomena that occur in gas mixtures depend on molecular transport processes including viscosity, diffusion, and thermal conductivity. Transport properties are often critically important in engineering applications and for understanding phenomena like combustion processes, hypersonic flows, and chemical vapor deposition. A recent trend in engineering design is to use modeling to reduce design times, and combustion is an area benefitting from this approach. Modeling combustion processes requires accurate values of transport properties over a wide range of temperatures and pressures. It is not possible to measure all the requisite transport properties, so we must have models to calculate them. This requires adequate information about the intermolecular potential and the underlying dynamics. The shape function, φ, is frequently taken to be a Lennard-Jones 12-6 potential, where the energy scaling parameter ε ij is the well depth, and σ ij is the length scaling parameter that defines the intermolecular separation at which the potential is zero. Viscosity measurements of a single chemical species allow direct estimation of the parameters that describe the interaction between two molecules of the same species, ε ii and σ ii . The situation is more complicated for even the si...