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.

Ion Mobilities in Supercritical Ethane, Xenon, and Carbon Dioxide

“…Our emphasis here is on positive ions and in determining the size of the mobile ion clusters in SC fluids using our compressible continuum model 4,5 to determine density and viscosity in regions surrounding the ion. Previous studies of ion mobilities in ethane, CO 2 , and Xe in the density region near critical densities were done either at lower temperatures ( T ≤ T c ) or at lower pressures ( P ≤ P c ). Here we report information about ion transport for T > T c , where the pressure has been varied isothermally.…”

Electron–ion recombination in radiation tracks in liquid argon: a computer simulation study

“…1). Since the mobility of the positive charge in LAr (l þ ¼ 1:52 Â 10 À3 cm 2 /Vs) [24] is much lower than the zero-field electron mobility (l 0 ¼ 490 cm 2 /Vs) [6], we assume that the cations are immobile in the simulation. However, this assumption can be relaxed in future versions of our method.…”

Molecular dynamics simulations of the O<inf>2</inf>− ion mobility in dense Ne gas at low temperature: Influence of the repulsive part of the ion-neutral interaction potential

“…Every 24 ps the velocities are simply rescaled so as to keep the temperature constant within 1 K [19]. This method has been tested by reproducing the well-known diffusion coefficient of pure Ar whose interaction potential is well known [20,21].…”