Intermolecular Forces from Diffusion and Thermal Diffusion Measurements

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Abstract
Experimental values of the diffusion coefficient and the thermal diffusion factor for the systems He–Ar, He–CO2, and H2–CO2, over a temperature range of about −78° to 325°C, are analyzed for the purpose of testing the Lennard‐Jones (12−6) and the exp‐6 intermolecular potentials, as well as the combination rules usually used in conjunction with these potentials. The exp‐6 potential gives excellent results for He–Ar, but the 12−6 potential is not satisfactory. This system affords no opportunity to choose between the various combination rules. The 12−6 potential works fairly well for He–CO2 and H2CO2. On the whole, the combination rules are surprisingly good, being no worse than the potentials themselves. The importance of considering the higher theoretical approximations for the thermal diffusion factor is emphasized. An expression interrelating the transport properties and independent of any particular intermolecular potential is proposed, which can be used to test how well a mixture conforms to the basic assumptions of the Chapman‐Enskog theory or to indicate the consistency of the experimental measurements.