High temperature gaseous diffusion experiments and intermolecular potential energy functions

Abstract

A new method of measuring molecular diffusion coefficients of gases which makes use of precise sampling and gas analysis downstream of an essentially point source of trace gas in a uniform carrier gas has been recently described in several publications. The technique is especially adapted for determinations at elevated temperatures. The present paper is a discussion of binary diffusion data over the temperature range 300°–1150° K for all of the gas pairs studied at APL to date. These include HeN 2, CO 2N 2, HeA, CO 2O 2, CH 4O 2, H 2O 2, COO 2 and H 2OO 2. The data cover a wide enough range in temperature and are of sufficient precision (∼±2 per cent) so that it is possible to examine them in terms of the kinetic theory of dilute gases and the intermolecular potential energy functions characterizing binary collision processes. The three functions used for which the necessary collision integrals had already been evaluated are the inverse power point center of repulsion potential, the Lennard-Jones (12ndash;6) potential, and the modified Buckingham (Exp-6) potential. In addition, the collision integrals for the exponential point center of repulsion potential were evaluated by numerical means. The details of this calculation, which was performed on the Univac 1103 Digital Computer, will be published elsewhere. It is shown that while all four potentials can be fitted to the data about equally well, the actual potentials obtained in this way may be quite different. Conversely, however, the high temperature diffusion coefficients predicted from the four potentials fitted at lower temperatures agree rather well.