Isotopic Thermal-Diffusion Factor of Argon

Abstract

An extensive investigation of the isotopic thermal-diffusion factor α0 of argon in the mean temperature range 127° to 653°K has been carried out with a four-tube swing separator employed previously in similar work with helium and neon. Because of a linear variation of α0 with T in the low-temperature region, the mean temperature T̄ = (T2—T1)/ln(T2/T1 ) has been used instead of the arithmetic mean of T1 and T2. This temperature assignment has been verified by varying the temperature gradient significantly. The experimentally determined value of α0 increases from 0.068±0.015 at T̄ = 127°K to 0.485±0.035 at T̄ = 653°K, in fair agreement with earlier experiments. Just as for helium and neon, a Lennard-Jones 12:6 potential model fails to account for the observed temperature variation of argon α0 values. The best fit for an exp—six force model occurs for α = 17 and ε/k = 176°K. Using these force parameters, the best fit with experimental viscosity data yields rm = 3.535 Å. These parameters are then used to calculate the diffusion coefficient and second virial coefficient for argon. There is fair agreement with the experimental values of these other transport coefficients.

Analytical potential models and the properties of noble gases

“…This result may indicate the inapplicability of two-parameter correspondingstates theory based on Boyle properties. In order to Paul, et of., 44 Watson, et af., 46 and Saxena,et ctf.42b be sure that the deviations were not due to quantum effects, corrections to Kihara's first approximation to the thermal diffusion factor42 were calculated from the quantum collision integrals for the Lennard-Jones…”

Determining the Thermal Diffusion Factor for 40Ar/36Ar in Air To Aid Paleoreconstruction of Abrupt Climate Change

“…To calculate the extent of isotopic unmixing of air in the firn due to thermal diffusion, the value of a phenomenological coefficient known as the “thermal diffusion factor” 12 is needed. The literature supplies such values for 40 Ar/ 36 Ar in pure argon − rather than in air, which is the relevant mixture for paleoreconstructions. Values of the thermal diffusion factor for an isotopic pair in pure and in mixed gas are often found to be rather different. , The effect of adding a third gas to an isotopic mixture is commonly to reduce the thermal diffusion factor.…”

On the utility of the m-6-8 potential function

“…The four-paranneter m- Data from [39]. We use the same m-6-8 potential as for the other transport potential, found to be satisfactory for the dilute gas properties of argon, with the potential estimated from high energy scattering of beams of neutral argon atoms [41].…”