Binary Nucleation Along Thermodynamically Unfavored Paths

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Barbara Wyslouzil is supported by a CAREER grant from the Theoretical and Computational Chemistry Program to perform experimental studies of binary nucleation along thermodynamically unfavored paths. The thermodynamically favored path for nucleation crosses the free energy surface through the saddle region with the lowest free energy. However, the combined effect of the monomer impingement rates and the shape of the free energy surface can drive the flux over a ridge in a miscible binary system. The flux can also be driven through the saddle with the higher free energy in a partially miscible system. In both cases, the analytical theories based on a saddle crossing will not predict the correct nucleation rate behavior. The proposed work has two goals: 1) to identify real chemical systems and conditions where binary nucleation proceeds primarily along a thermodynamically unfavored path; and 2) to design and conduct experiments that clearly demonstrate binary nucleation can occur in this manner. The experimental part of this project will use a two pulse expansion cloud chamber to conduct nucleation rate measurements. Careful modeling of particle formation processes in the chamber, together with knowledge of the composition of the critical nucleus, should make it possible to distinguish whether nucleation has followed the thermodynamically favored path. The deliberate enhancement or suppression of the formation of a thermodynamically favored phase is important in many fields of engineering. In material science for example, processes which produce submicron ceramic particles by vapor phase nucleation from a volatile precursor can lead to materials with lower sintering temperatures over conventionally produced ceramic powders. In the chemical processing industry the rates of nucleation, growth and recrystallization affect the purity, quality, and sales value of bulk products such as common as sugar and salt as well as specialty products such as zeolites. In metallurgy, different and potentially useful forms of metals may be produced if large undercoolings are achieved without nucleation of a crystal phase. A better molecular level understanding of the nucleation could have a potentially large impact on chemical processing in a large number of manufacturing areas.