Electron and ion transport in gaseous methanol and water: density and temperature effects

Abstract

Electron and cation mobilities in methanol and water vapor measured at 293 ≤ T (K) ≤ 617 were used to estimate the corresponding momentum transfer cross sections. The electron cross sections correlate with the square of the dipole moment. Unlike the nonpolar gases, where the average cross section for electrons σavc.c is only 0.01–0.1 times that for cations σavc.+, methanol has σav.c ≈ 0.5σavc.+ and water has σavc.c ≈ σavc.+. The thermal electrons exchange energy with the molecules mainly through molecular rotational modes, as opposed to elastic modes.The onset of electron quasilocalization in the saturated vapor occurs at nql ≈ 5 × 1025 molecules/m3 (nql/nc = 0.010) in methanol and 3 × 1025 (nql/nc ≈ 0.003) in water. These are about 10-fold lower densities than in hydrocarbons, where nql/nc ≈ 0.1. The loffe–Regel limit for quasifree states gives nql ≈ 0.36 × (Tnμc)ql, which holds quite well when the gas molecules are relatively rigid. However, for flexible molecules such as the n-alkanes larger than propane, the loffe–Regal limit predicts too large values for nql.The density normalized mobility of cations nμ+ in saturated methanol vapor is constant at 1.02 × 1021 molecules/m V s up to 67 × 1025 molecules/m3 (25 Amagats). In saturated water vapor nμ+ = 1.87 × 1021 molecules/m V s up to 30 × 1025 molecules/m3 (11 Amagats).