Ion Sizes near the Liquid-Gas Transition in Helium

The sizes of positive and negative ions near the critical point are examined in the light of existing models using an idealized equation of state. It is found that close to the critical point, both signs of ion experience increases in radius, whether the critical point is approached from the liquid or gas phase. Specifically, for ${\mathrm{He}}^3$ it is found that radius variations account in part for the anomalies seen in the ionic mobility. It is concluded that the viscosity of ${\mathrm{He}}^3$ near the critical point exhibits an increase of at most 20%.