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Siegel der Universität
Department für Chemie - Arbeitsgruppe Prof. Strey

Homogene Keimbildung von H2O und D2O

Judith Wölk, May 2001

Unary and binary homogeneous nucleation of light (H2O) and heavy (D2O) water has been measured under identical conditions using a nucleation pulse chamber and a supersonic nozzle. Comparing the nucleation rates for H2O and D2O at the same respective vapor pressure pv and temperature T the rates differ by a factor of 3000. However, the individual data points superimpose, if compared at the same supersaturation. Predictions by the classical nucleation theory - using the most recent expressions for temperature-dependent vapor pressure, surface tension and density - are compared to the experimental data. While the predictions correctly yield the slope of the experimental nucleation rate curves and for temperatures around T @ 240 K even the correct absolute rate, the temperature dependence is experimentally shown to be weaker. In contrast, the self-consistent theory by Girshick and Chiu provides an improved temperature dependence, but predicts rates which are off by a factor of 7000. The number of molecules in the critical cluster, which are obtained from the slopes [d ln J /d ln S ]T of the nucleation rate curves are nearly the same for both isotopes, and allow a direct test of the Gibbs-Thomson equation and support its validity. An ideal behavior of the binary H2O-D2O system was found with both, the nucleation pulse technique and the nozzle. While the nucleation pulse measurements show that the number of particles in the critical cluster are nearly the same for the examined mixtures, a combination of the nozzle experiment with SANS provides information on the number density N, the mean radius <r> and the polydispersity pd of the generated aerosol. On the basis of the combined experiments an empirical relation is developed allowing to predict for temperatures (200 < T / K < 270) and supersaturations (5 < S > 200) nucleation rates for water over an extended range (105 < J / cm^(-3s-1) < 1017).</r>