Comparison of transition from low to high density transport behavior for ions and neutral molecules in simple fluids

Abstract

For cation mobility μ in several simple fluids, the transition from normal gas to liquid type behavior occurs in the density region 0.5≲n/nc≲1.9, which corresponds to the viscosity region 0.6≲η/ηc≲2.5. The subscript c refers to the critical fluids and the other fluids are the coexistence vapor and liquid. The fluids range from monatomic to polyatomic molecules: Ar, Xe, N2, CH4. The semihydrodynamic equation provides a reasonable interpretation of the variation of ημ, or the equivalent ηDe/kT for neutral molecules, with η only in the normal gas regime, where n/nc<0.4 and η/ηc<0.5. Under these circumstances, nμ or nDe/kT is nearly constant. In calculating the polarization interaction contribution to the ion scattering cross section, the largest axis of polarizability of the molecule should be used. As n (or η) increases in the region 0.6<n/nc<1.6 (or 0.7<η/ηc<1.9) the value of nμ (or ημ) increases by ∼60%, whereas nDe/kT (or ηDe/kT) for neutral molecules decreases by ∼20%. The ratio (De/kTμ)n has values 5–6 in the dilute gases; the polarization interaction and clustering make the ion scattering cross sections greater than those of neutral molecules. At higher densities the ratio (De/kTμ)n decreases, due to the destructive interference of polarization interactions of an ion with more than one molecule at a time, and to a decrease of the special importance of the molecules clustered about the ion, compared to about a neutral molecule, in the dense fluid. In the dense liquid (De/kTμ)n = 2.5 in methane and xenon, and 1.7 in argon.