AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Classical Nucleation Theory Does Not Correctly Predict the Dependence of Nucleation Rate on Supersaturation
STEVEN GIRSHICK, University of Minnesota
Abstract Number: 461 Working Group: Aerosol Physics
Abstract While it is well known that classical nucleation theory (CNT) does not correctly predict the temperature dependence of the stationary rate of homogeneous nucleation, it is commonly asserted that CNT does at least correctly predict the supersaturation dependence. In this work we reexamine this claim in light of a number of experimental studies in which homogeneous nucleation rates of various substances (water, several 1-alcohols, argon and nitrogen) were measured over ranges of temperature and saturation ratio, and the measured supersaturation dependence of nucleation rate was used to infer critical cluster sizes by means of the Nucleation Theorem. In general, the inferred critical sizes agree poorly with those predicted by CNT, implying that the supersaturation dependence of nucleation rate is poorly predicted, contrary to what is commonly believed.
If one considers the expression for the stationary nucleation rate in summation form, before any assumption is made regarding the form of the free energy change (delta-G) for formation of a cluster from the monomer vapor, one finds that every term in the summation has a supersaturation dependence that arises purely from well-established thermodynamics (the dependence of Gibbs free energy on pressure) and kinetics (the dependence of molecular collision rates on number densities of the collision partners). Superficially, it thus seems that CNT, or any other theory that accepts the summation expression for the nucleation rate as valid, must correctly predict the supersaturation dependence. However, the model used to predict delta-G does affect the value of the critical size, and, since the summation is dominated by terms close to the critical size, the model for delta-G in turn affects the dependence of nucleation rate on saturation ratio. Therefore, we can conclude that CNT's incorrect model for delta-G for small clusters leads to an incorrect prediction of the supersaturation dependence of nucleation rate.