Sodium chloride (NaCl) aqueous solution becomes NaCl hydrate, NaCl·2H2O, at low temperature, which is different from potassium chloride and is a typical complex model for studying the freeze-drying process in foods and pharmaceuticals. Here, we detected unit-cell-sized NaCl particles in ice as precursor substances of NaCl·2H2O during freezing of NaCl solution by using terahertz (THz) spectroscopy. In the freezing process, Na+ and Cl- ions form two types of metastable unit-cell-sized NaCl particles on the pathway to the well-known NaCl·2H2O crystal production, which are not listed in the phase diagram of freezing of NaCl solution but have absorption peaks in THz spectra. This finding of single unit-cell-sized particles signifies the importance of studying the freeze-drying process in-depth and offers a new possibility for the development of freeze-drying technology for the manufacture of nanometer-sized particles such as ultrafine pharmaceutical powders, which more readily dissolve in water.

中文翻译：

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使用太赫兹光谱捕获 NaCl 纳米颗粒的冷冻干燥动力学

氯化钠（NaCl）水溶液在低温下变成NaCl水合物NaCl·2H2O，不同于氯化钾，是研究食品和药品冷冻干燥过程的典型复杂模型。在这里，我们使用太赫兹 (THz) 光谱检测了冰中晶胞大小的 NaCl 颗粒作为 NaCl·2H2O 的前体物质，在 NaCl 溶液冷冻过程中。在冷冻过程中，Na+ 和 Cl- 离子在众所周知的 NaCl·2H2O 晶体生成途径中形成两种亚稳态的晶胞大小的 NaCl 颗粒，未列在 NaCl 溶液冷冻相图中，但在太赫兹光谱中有吸收峰。