Isotopic Thermal-Diffusion Factor of Argon

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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.