Mobility of O₂^- ions in supercritical Ar: Experiment and Molecular Dynamics Simulations

A new analysis and new Molecular Dynamics (MD) simulations of the measurements of the mobility $\mu_{i}$ of O$_{2}^{-} $ ions in dense supercritical Ar gas are reported. $\mu_{i}$ shows a marked dependence on the distance from the critical temperature $T_{c}.$ A mobility defect appears as a function of the gas density and its maximum value occurs below the critical density. The locus of points of maximum mobility defect in the $P-T$ plane appears on the extrapolation of the coexistence curve into the single-phase region. MD simulations quantitatively reproduce the mobility defect near $T_{c}.$