Inhibition of solute crystallisation in aqueous H(+)-NH(4)(+)-SO4(2-)-H2O droplets.
Inhibition of solute crystallisation in aqueous H(+)-NH(4)(+)-SO4(2-)-H2O droplets.
复制标题
抑制 H( )-NH(4)( )-SO4(2-)-H2O 水溶液中溶质结晶。
DOI：
10.1039/b802216j
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发表时间：
2008
期刊：
影响因子：
--
通讯作者：
A. Bertram
中科院分区：
文献类型：
--
作者：
B. Murray;A. Bertram
地球对流层上层的冰云可以通过水滴中冰的均匀成核形成。在这项研究中，我们调查的结晶，或缺乏结晶，溶质相和冰在水溶液（NH（4））（3）H（SO（4））（2）/H（2）O和NH（4）HSO（4）/H（2）O液滴。这是使用安装在冷台上的乳化溶液液滴的原位X射线衍射来完成的。从衍射图中，我们能够识别在微米大小的液滴中均匀冷冻后形成的结晶溶质和冰的相。这项研究的一个重要发现是，结晶的溶质并不总是发生，即使结晶是强烈的结晶倾向。溶质相晶体的成核和生长受到抑制，因为盐水溶液的粘度很可能随着盐水浓度的增加和温度的降低而急剧增加。如果冰成核低于阈值冻结温度，盐水似乎迅速变得如此粘稠，溶质结晶被抑制。在（NH（4））（3）H（SO（4））（2）和NH（4）HSO（4）中，该阈值温度分别为192 K和180 K。我们还推测，立方冰的形成在一个高粘度的盐水块溶剂介导的立方相到六方相的转变，从而稳定的亚稳立方冰在最集中的溶液液滴。
Ice clouds in the Earth's upper troposphere can form via homogeneous nucleation of ice in aqueous droplets. In this study we investigate the crystallisation, or lack of crystallisation, of the solute phase and ice in aqueous (NH(4))(3)H(SO(4))(2)/H(2)O and NH(4)HSO(4)/H(2)O droplets. This is done using in situ X-ray diffraction of emulsified solution droplets mounted on a cold stage. From the diffraction patterns we are able to identify the phases of crystalline solute and ice that form after homogeneous freezing in micrometer sized droplets. An important finding from this study is that crystallisation of the solute does not always occur, even when crystallisation is strongly thermodynamically favoured. The nucleation and growth of solute phase crystals becomes inhibited since the viscosity of the aqueous brine most likely increases dramatically as the brine concentration increases and temperature decreases. If ice nucleates below a threshold freezing temperature, the brine appears to rapidly become so viscous that solute crystallisation is inhibited. This threshold temperature is 192 K and 180 K, in (NH(4))(3)H(SO(4))(2) and NH(4)HSO(4), respectively. We also speculate that the formation of cubic ice within a highly viscous brine blocks the solvent mediated cubic to hexagonal phase transformation, thus stabilising the metastable cubic ice in the most concentrated solution droplets.