Mobility of O-2(-) ions in near critical Ar gas

We have measured the drift mobility mu of O-2(-) in Ar gas in the temperature range (151.5 T-c but for N < N-c. We suggest that this reduction is due to the effect of the long-range ion-atom polarization interaction and of the divergence of the gas compressibility. A satisfactory agreement with the data can be obtained by suitably modifying the Stokes's hydrodynamic expression for mu(0)