New experiments have succeeded in measuring actual rates of nucleation and are revealing the shortcomings of classical nucleation theory, which assumes that the molecular-scale regions of the new phase may be treated using bulk thermodynamics and planar surface free energies. In response to these developments, new theories have been developed that incorporate information about molecular interactions in a more realistic fashion. This article reviews recent experimental and theoretical advances in the study of nucleation of liquids from the vapor and of crystals from the melt, with particular emphasis on phenomena that relate to particle formation in the atmosphere.

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The scaling properties of the critical nucleus, as proposed recently by McGraw and Laaksonen, are analyzed in the context of density functional results for simple Lennard-Jones systems. A simple phenomenological approach with no adjustable parameters is introduced to test the validity of these scaling relations in describing the nucleation behavior of nonpolar and weakly polar vapors.

We present a semiempirical approach to the density functional theory of gas-liquid nucleation, in which the same experimental properties used in classical nucleation theory ͑equilibrium vapor pressure, liquid density, and surface tension͒ are used to fit three adjustable parameters in the intermolecular potential. This approach allows direct comparison of nucleation rates with experimental data. Agreement with results on nonane from three different experimental groups is reasonable, although the comparison clearly reveals the scatter in those results and suggests that further experimental measurements by different groups on the same systems would be very valuable. For water and the n-alcohols, this version of density functional theory gives results quite close to classical nucleation theory, implying that it is not a good approximation to describe polar fluids by effective spherically symmetric potentials.