Softness expansion of gaseous transport properites. II. Moderately dense gases

The first density corrections to gaseous transport properties for the inverse power potential f = f0(s/r)n are evaluated according to the theory of Curtiss, Snider, and co-workers. A linear softness expansion predicts that the first density corrections are positive for large n but negative for small n, and that the changes of sign occur at n = 32.75 and n = 8.42 for the viscosity and thermal conductivity, respectively. Direct numerical integration yields n = 29.60 and n = 7.07, respectively. These results explain the opposite signs of density corrections to the two transport properties seen in high temperature experimental data, where, effectively, n = 12. Predictions of the modified Enskog theory are compared and contrasted with the above results. It is shown that some of the Snider–Curtiss (collisional transfer) collision integrals may be simplified, one of them being the usual dilute gas omega integral with Smith’s collision lifetime included in the integrand.