The Nucleation of Supersaturated Vapors

摘要 Joseph L. Katz Johns霍普金斯大学CTS-9626661 这个持续研究项目的目标是提供一个更好的了解成核过程，并允许一个准确的定量预测。 该研究集中在三个互补的调查领域;离子诱导成核，金属蒸汽的均匀成核，以及水蒸汽在均匀表面上的非均匀成核。 在离子诱导成核方面的努力已经导致了在所有重要变量（过饱和度、温度和离子密度）都可以精确测量并且都可以变化的条件下测量成核速率的方法的发展。 对正壬烷进行了初步测量。 现在将对离子具有大得多的成核倾向的几种物质进行测量。 这些测试将从具有显著偶极矩的物质（1-戊醇以及1-丁醇和1-己醇）开始开始，随后将测量具有显著四极矩的物质（苯、对二甲苯）。 第二个主要的努力是金属蒸气的均匀成核。 处于稀蒸汽状态的金属是非金属的。 因此，必须存在非金属到金属的转变，这是团簇大小的函数。 金属蒸气的成核研究使得能够研究这种尺寸依赖的非金属到金属的转变，并且当成核理论应用于与迄今为止研究的任何物质都非常不同的一类物质时，也能够测试成核理论的有效性。 该项目的这一部分涉及与德国马尔堡的一个研究小组的合作，迄今为止，重点是铯蒸气的均匀成核。 迄今为止获得的结果表明，正在观察到非金属到金属的平稳过渡。 这些测量将扩展到更高的温度，因为临界团簇尺寸将大得多，因此更接近于完全金属化。 然后将研究铷的均匀成核，并随后对镉和汞进行测量，已知这些物质具有更明显的尺寸依赖性非金属到金属的转变。 第三个主要的努力是水蒸气在不溶于水的液体表面上的非均质成核。 通过研究在液体基质上的异质成核，人们利用了基质表面性质的均匀性，从而避免了在固体表面上的异质成核中遇到的许多不确定性。 更重要的是，表征该过程所需的所有表面自由能（冷凝液体与其冷凝的液体之间的界面张力，以及每种液体的表面张力）都是实验可测量的。 将确定水在十二烷和十六烷上成核所需的临界过饱和度，并与理论进行比较。

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.