  We report the results of the experiment aimed at measuring the mobility of
O2- ions in dense argon gas in the temperature range 180 K < T < 300 K. We show
that an adequate theoretical description of the data is obtained by using the
thermodynamic Free Volume (FV) model, originally developed to describe the
electron bubble mobility in superfluid helium and successfully exploited for
describing the O2- mobility in near critical neon gas. The model goal is to
thermodynamically predict the free space available for ion motion. By
implementing the FV model with the Millikan-Cunningham (MC) slip correction
factor, we can describe the ion mobility in the crossover region bridging the
dilute gas kinetic regime to the high-density hydrodynamic regime of ion
transport. These results confirm the validity of the model and the universality
of some of its features.
