Comparison of transition from low to high density transport behavior for ions and neutral molecules in simple fluids

摘要

For cation mobility mgr; in several simple fluids, the transition from normal gas to liquid type behavior occurs in the density region 0.5lsim;n/nclsim;1.9, which corresponds to the viscosity region 0.6lsim;eegr;/eegr;clsim;2.5. The subscriptcrefers 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 eegr;mgr;, or the equivalent eegr;De/kTfor neutral molecules, with eegr; only in the normal gas regime, wheren/nc0.4 and eegr;/eegr;c0.5. Under these circumstances,nmgr; ornDe/kTis 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. Asn(or eegr;) increases in the region 0.6n/nc1.6 (or 0.7eegr;/eegr;c1.9) the value ofnmgr; (or eegr;mgr;) increases by sim;60percnt;, whereasnDe/kT(or eegr;De/kT) for neutral molecules decreases by sim;20percnt;. The ratio (De/kTmgr;)nhas values 5ndash;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/kTmgr;)ndecreases, 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/kTmgr;)n= 2.5 in methane and xenon, and 1.7 in argon.