The Nucleation of Supersaturated Vapors

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Abstract Joseph L. Katz Johns Hopkins University CTS-9626661 The goal of this continuing research project is to provide a better understanding of nucleation processes, and to allow one to make accurate quantitative predictions. The research focuses on three complementary fields of investigation; ion-induced nucleation, homogeneous nucleation of metal vapors, and heterogeneous nucleation of water vapor onto uniform surfaces. Efforts in ion-induced nucleation have resulted in the development of a method for measuring nucleation rates under conditions where all the important variables (supersaturation, temperature, and ion density) are accurately measurable and all can be varied. Initial measurements on n-Nonane have been made. Measurements now will be made on several substances for which ions have a much larger nucleation propensity. These will begin with substances with significant dipole moments (1-Pentanol as well as 1-Butanol and 1-Hexanol), and will be followed by measurements on substances with significant quadrupole moments (Benzene, p-Xylene). A second major effort is the homogeneous nucleation of metal vapors. Metals in their dilute vapor state are non-metallic. Thus there has to be a non-metal to metal transition which is a function of cluster size. Nucleation studies of metal vapors enable the study of this size dependent non-metal to metal transition and also the testing of the validity of nucleation theories when they are applied to a class of substances which are very different from any studied until now. This part of the project involves collaboration with a research group in Marburg, Germany, and to date has focused on the homogeneous nucleation of Cesium vapor. Results obtained thus far suggest that a smooth non-metal to metal transition is being observed. These measurements will be extended to higher temperatures, since critical cluster sizes then will be much larger and thus closer to being fully metallic. The homogeneous nucleation of Rubidium then wil l be investigated and will be followed by measurements on Cadmium and Mercury, substances which are known to have more sharply size-dependent non-metal to metal transitions. The third major effort is the heterogeneous nucleation of water vapor onto the surface of water insoluble liquids. By investigating heterogeneous nucleation onto a liquid substrate one takes advantage of the uniformity of the substrate's surface properties, thus avoiding many of the uncertainties encountered in heterogeneous nucleation onto solid surfaces. Even more important, all surface free energies needed to characterize this process (the interfacial tension between the condensing liquid and the liquid that it is condensing onto, and the surface tension of each liquid) are experimentally measurable. The critical supersaturations required for water nucleation onto Dodecane and Hexadecane will be determined and compared to theory.