Theory and Experiment of Binary Diffusion Coefficient of n-Alkanes in Dilute Gases
Changran Liu, William S. McGivern, Jeffrey A. Manion, Hai Wang
Binary diffusion coefficients were measured for n-pentane, n-hexane and n-octane in helium and of n-pentane in nitrogen over the temperature range of (300 - 600) K, using reversed-flow gas chromatography. A generalized, analytical theory was proposed for the binary diffusion coefficients of long-chain molecules in simple diluent gases, taking advantage of a recently developed gas-kinetic theory of the transport properties of nano-slender bodies in dilute free-molecular flows. The theory addresses the long-standing question about the applicability of the Chapman-Enskog theory in describing the transport properties of non-spherical molecular structures, or equivalently, the use of isotropic potentials of interaction for a roughly cylindrical molecular structure such as large normal alkanes. An approximate potential energy function is proposed for the intermolecular interaction of long- chain n-alkane with typical bath gases. Using this potential and the analytical theory for nano slender bodies, we show that the diffusion coefficients of n-alkanes in typical bath gases can be treated by the resulting analytical model accurately, especially for compounds larger than n-butane.