Transport Properties of Polar Gases

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Abstract
A model is proposed for the calculation of viscosity, diffusion, thermal diffusion, and the translational part of the heat conductivity of dilute polar gases. It is assumed that the molecular-collision trajectories are negligibly distorted by transfer of internal rotational energy, and that the relative orientation of two colliding dipoles remains fixed throughout the significant portion of the collision trajectory around the distance of closest approach. For this model, the Chapman-Enskog theory retains its usual form, but the collision integrals which appear must be averaged over all possible relative orientations occurring in collisions. Collision integrals have been calculated for the Stockmayer (12–6–3) potential, [open phi](r)=4ε0[(σ0/r)12−(σ0/r)6+δ(σ0/r)3],for kT/ε0 from zero to 100 and for δ from —2.5 to +2.5, and averaged over all orientations (assumed equally probable). Sufficient collision integrals are tabulated that the convergence error of the Chapman-Enskog theoretical expressions is not a problem. Experimental viscosities and dipole moments of a number of polar gases have been used to determine the potential parameters ε0, σ0, and δ, which were then used to calculate other properties for comparison with experiment. The over-all agreement between experiment and this model for polar gases is comparable to that of the Lennard-Jones (12–6) model for nonpolar gases.