Effects of molecular properties on electron transport in hydrocarbon fluids

The electron momentum transfer cross sections σm of each of the hydrocarbon gases possess a minimum at the energy ε(eV) noted: propene (0.16), cyclopropane (0.10), propane (0.13), n-butane (0.14), and i-butane (0.19). The minimum is much shallower for cyclopropane than for the other molecules. When σm∝ε−p with p≳0.5 over the energy distribution of the electrons in the sample, the electron mobility μ increases with increasing gas temperature T, and with increasing applied electric field strength above the threshold (E/n)th. However, upon increasing the gas density n to where an electron can interact with more than one molecule at a time, and at temperatures near the coexistence curve, the temperature dependence of μ increases and the field dependence decreases; these are symptoms of quasilocalization of electrons by density fluctuations in the fluid. The Arrhenius temperature coefficient of mobility Eμ in the constant density vapors, over the range 0.1<n/nc<1.0 at temperatures a few degrees above the vapor–liquid coexistence curve, increases superlinearly with density. The subscript c refers to the critical point. Near the critical region Eμ≊1×102 kJ/mol. The extent of quasilocalization is similar at the same n/nc and T/Tc in vapors as diverse as xenon, ethane, the pentane isomers, and dimethyl ether. Molecular orientational disorder makes a much smaller contribution to the (shallow) traps in the dense vapor than to the (deeper) traps in the normal liquids. At n≳nc the value of Eμ decreases and reaches values <20 kJ/mol in the normal liquids at n/nc≳2.6; electron traps in the dense liquids are less sensitive to temperature than are those in the dense vapors near the coexistence curve. Electron behavior in liquid hydrocarbons is dominated by the degree of molecular sphericity. A higher degree of sphericity is related to an enhanced mobility in the liquid. This is opposite to the effect in the low density gas where a higher degree of sphericity is related to a lower mobility. The density induced transition between these two effects of sphericity is examined.