Theory and Experiment of Binary Diffusion Coefficient of <i>n</i>‑Alkanes in Dilute Gases

Binary diffusion coefficients were measured for <i>n</i>-pentane, <i>n</i>-hexane, and <i>n</i>-octane in helium and of <i>n</i>-pentane in nitrogen over the temperature range of 300 to 600 K, using reversed-flow gas chromatography. A generalized, analytical theory is 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 nanoslender 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 nonspherical 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 <i>n</i>-alkane with typical bath gases. Using this potential and the analytical theory for nanoslender bodies, we show that the diffusion coefficients of <i>n</i>-alkanes in typical bath gases can be treated by the resulting analytical model accurately, especially for compounds larger than <i>n</i>-butane