Effects of density and temperature on electron transport in hydrocarbon fluids

Abstract

Electron mobilities μ were determined in the liquid and vapor of the three pentane isomers. The density n was varied by two orders of magnitude, from the dilute gas through the critical region to the normal liquid. In the dilute gases the quantities μn and E/n represent μ and the electric field strength E normalized for differences in n. The magnitudes of the temperature and field co-efficients, d(μn)/dT and d(μn)/d(E/n), changed in the same direction from one isomer to another: neopentane > isopentane > n-pentane. However, the values of μn at a given T and E/n fell in the opposite order: neopentane < isopentane < n-pentane. This behavior can be interpreted in terms of either a Ramsauer-Townsend effect of a contribution of transient negative ion states to electron scattering, with the resonance energies depending on molecular shape. In the high density gases near the vapor/liquid coexistence curve electrons may become quasilocalized by large density fluctuations. Quasilocalization is characterized by large negative values of ΔH and ΔS, and a small negative value of ΔG. The relative magnitudes of the mobilities in the liquid phase isomers is the reverse of those in the low density gases. The cross-over occurs in the gas phase at n ≈ 0.7 n c. Electrons reside mainly in a conduction band in dense neopentane, and mainly in localized states in dense iso- and n-pentane.