Experimental and theoretical study of the sign preference in the nucleation of water vapor

Abstract

The nucleation of water vapor on ions in atmospheres of helium and argon was studied using an expansion type cloud chamber. Separation of the positive and negative ions was achieved so that the nucleation could be studied as a function of both the sign of the ionic charge and the supersaturation. A semiphenominological theory was developed as an extension of the classical liquid drop theory to include the effects of the ionic charge on the nucleation process. The theoretical model of the prenucleation embryo was assumed to possess an oriented dipole surface layer with the direction of orientation dependent on the sign of the ionic charge. The theory predicts not only the increase in the nucleation rate compared to that for homogeneous nucleation and a difference in rate between positive and negative ions, the negative ions having the higher nucleation rate, but also predicts a correction term to the classical theory of homogeneous nucleation for polar molecules which exhibit an electrical double layer at the liquid surface. Comparison of the theoretical and experimental results for nucleation on both positive and negative ions yields good agreement and indicates the prenucleation embryo is probably a tightly bonded highly structured cluster possessing an oriented dipole surface layer. © 1971.