A rough-hard-sphere theory for diffusion in supercritical carbon dioxide

Abstract

A rough-hard-sphere (RHS) theory is found to successfully account for the effects of molecular rotation and dynamically correlated molecular collisions on self-diffusion in carbon dioxide at temperatures from 35 to 100°C and pressures from 70 to 246 atm. The Mathur-Thodos empirical equation finds its molecular basis within the framework of the RHS theory. The theory is also applicable to tracer diffusion of benzene in carbon dioxide at 35–55°C and 91–158 atm, where densities are greater than 0.500 g/cm 3. Its failure at lower densities suggests that the molecular dynamics computer result of self-diffusion is inadequate for describing the effect of collective molecular motion encountered in tracer diffusion across the entire density range. Further computer work is recommended which will refine the prediction of tracer diffusion in supercritical fluids using the RHS theory.