William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA.
Phys Chem Chem Phys. 2011 Apr 7;13(13):5855-71. doi: 10.1039/c0cp02485f. Epub 2011 Feb 18.
We have combined static pressure, spectroscopic temperature, Fourier transform infrared spectroscopy (FTIR), and small angle X-ray scattering (SAXS) measurements to develop a detailed picture of methanol condensing from a dilute vapor-carrier gas mixture under the highly supersaturated conditions present in a supersonic nozzle. In our experiments, methanol condensation can be divided into three stages as the gas mixture expands in the nozzle. In the first stage, as the temperature decreases rapidly, small methanol n-mers (clusters) form, increase in concentration, and evolve in size. In the second stage, the temperature decreases more slowly, and the n-mer concentrations continue to rise. Thermodynamic and FTIR experiments cannot, however, definitively establish if the average cluster size is constant or if it continues to increase. Finally, when the vapor becomes supersaturated enough, liquid droplets form via nucleation and growth, consuming more monomer and reducing the concentration of clusters. At the point where liquid first appears, cluster formation has already consumed up to 30% of the monomer. This is significantly more than is predicted by a model that describes the vapor phase as an equilibrium mixture of methanol monomer, dimer, and tetramer. An energy balance suggests that a significant fraction of the cluster population is larger than the tetramer, while preliminary SAXS measurements suggest that these clusters contain, on average, 6 monomers.
我们结合静压、光谱学温度、傅里叶变换红外光谱（FTIR）和小角 X 射线散射（SAXS）测量，深入研究了在超声速喷管中高度过饱和条件下，甲醇从稀蒸气-载体气体混合物中冷凝的情况。在我们的实验中，甲醇冷凝可以分为三个阶段，随着混合物在喷管中膨胀。在第一阶段，随着温度迅速下降，形成小甲醇 n-聚体（团簇），浓度增加，尺寸增大。在第二阶段，温度下降较慢，n-聚体浓度继续上升。然而，热力学和 FTIR 实验不能明确确定平均簇大小是否保持不变，或者它是否继续增大。最后，当蒸气变得足够过饱和时，通过成核和生长形成液滴，消耗更多的单体并降低聚集体的浓度。在首次出现液体的点，聚集体的形成已经消耗了多达 30%的单体。这明显高于描述蒸气相为甲醇单体、二聚体和三聚体平衡混合物的模型所预测的量。能量平衡表明，聚集体的一部分明显大于三聚体，而初步的 SAXS 测量表明，这些聚集体平均含有 6 个单体。
