Molecular shape effect on electron transport in fluids: Density and temperature studies in mixtures of n-pentane and neopentane

Abstract

Electron mobility μ0 in nonpolar and slightly polar (D≤0.5 D) hydrocarbon fluids is strongly affected by the external shape of the molecules. The mobility is large in a liquid of spherelike molecules such as neopentane. Addition of rodlike molecules such as n-pentane decreases μ0, but has little effect on the value of the threshold field for electron heating. A liquid of spherelike molecules forms a conduction band that has relatively small undulations in its lower potential surface. Addition of rodlike molecules provides orientational disorder, anisotropic polarizability, and restricted rotation, which in combination produce potential fluctuations that are durable enough to localized electrons. The maximum in μ0 that occurs in pure liquid neopentane at 3.5×1027 molecule/m3 progressively decreases in magnitude and shifts to lower densities as n-pentane is added; it occurs at the critical density in n-pentane. A plot of log μ0 against mol % n-pentane for liquid mixtures of n- and neo-pentane at constant T/Tc is slightly S shaped. A plot of the Arrhenius temperature coefficient Eμ against mole % n-pentane is S shaped in the opposite direction. At T/Tc=0.999 the log μ0 plot is not S shape and the slope is much smaller than at T/Tc≤0.92; the molecular shape effect is minimal in the critical fluids and dense gases, 1.0≥n/nc≥0.3. Quasilocalization occurs to a similar extent in all gases at the same n/nc and T/Tc. In the low density gases the molecular shape effect is the opposite of that in dense liquids; at electron energies <0.10 eV the spherelike neopentane has a larger scattering cross section than does its nonspherelike isomer n-pentane. In an equimolar mixture the stronger scatterer dominates the electron behavior, which means n-pentane in the liquid and neopentane in the normal gas.