| Summary: | 博士 === 國立成功大學 === 化學工程學系 === 89 === Condensation of supersaturated water vapor on monodisperse submicrometer particles of insoluble SiO2, TiO2 and soluble glucose and monosodium glutamate is investigated in a flow cloud chamber (FCC). The dependence of the critical supersaturation Scr on particle size in the range of 15 to 120 nm and on temperature in the range of 10 to 50 oC are determined experimentally.
For unsoluble particles, SiO2 and TiO2, the results show that the experimental Scr decreases with increasing particle size, at a rate in reasonable agreement with that predicted by Fletcher's verson of Volmer's theory of heterogeneous, but decreases with increaseing temperature at a rate higher than that predicted by Fletcher's theory. The experimental Scr is smaller than the theoretical prediction even with the line tensionand surface diffusion taken into account; the particles induce heterogeneous nucleation better than theory dridicts even for perfectly wetted particles. The discrepancy cannot be fully accounted for by the effects of line tension, surface diffusion, or for the curvature dependence of the physical properties.
For soluble particles, glucose and monosodium glutamate, a heterogeneous nucleation theory is developed to incorperate the effect of dissolution. The results show that (1) the Scr evaluated by the modified theory are found to be very close to that predicted by Kohler theory, and (2) the experimental Scr decreases with increasing particle size, qualitatively in agreement with those predicted by both theories. The experimental Scr is smaller than the theoretical prediction, i.e., the particles induced heterogeneous nucleation better than theory predicts even for perfectly wetted and completely dissolved particles. The theoretical Scr well agree with the experimental Scr if the surface tension of solution is reduced to 40-50 dyne/cm for glucose particle and 50-60 dyne/cm for monosodium glutamate particle. The discrepancy cannot be fully accounted for by the bulk solution properties. The dependence of Scr on temperature is qualitatively in agreement with the theoretical prediction, i.e., the lower temperature is, the higher Scr is.
The condensation of supersaturated vapor on singly-positively-charged particles, soluble and unsoluble, with diameters of 30, 60 and 90 nm is also examined. No obvious effect of charge on Scr is observed, in agreement with the theoretical prediction.
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