Nucleation on Gaseous Ions

Abstract

The homogeneous nucleation theory of Lothe and Pound is extended to condensation on ions. A rate equation is derived that gives the frequency of droplet formation on an attenuated concentration of gaseous ions in supersaturated unary vapors. This treatment predicts little catalytic effect of ions in contrast to the classical theories of Volmer and of Becker and Doering, which predict condensation on ions at supersaturations substantially below those needed for homogeneous nucleation. The predictions of classical theory and of the present treatment are compared with Scharrer's cloud‐chamber measurements of homogeneous, positive‐ion, and negative‐ion rain limits in a number of fluids. Neither theory predicts the different catalytic potencies of positive and negative ions. The homogeneous rain limits in CHCl₃, C₆H₆, CCl₄, and C₆H₅Cl are in good agreement with the Lothe–Pound theory, and the catalytic effect of ions is small or nil, in agreement with the present treatment. Classical theory correctly predicts only the homogeneous rain limit in H₂O and homogeneous and positive‐ion limits in C₂H₅OH.