  We present a Ginzburg-Landau theory of ion-induced nucleation in a gas phase
of polar one-component fluids, where a liquid droplet grows with an ion at its
center. By calculating the density profile around an ion, we show that the
solvation free energy is larger in gas than in liquid at the same temperature
on the coexistence curve. This difference much reduces the nucleation barrier
in a metastable gas.
