Capturing the Freeze-Drying Dynamics of NaCl Nanoparticles Using THz Spectroscopy

Sodium chloride (NaCl) aqueous solution becomes NaCl hydrate, NaCl·2H<sub>2</sub>O, 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·2H<sub>2</sub>O during freezing of NaCl solution by using terahertz (THz) spectroscopy. In the freezing process, Na<sup>+</sup> and Cl<sup>–</sup> ions form two types of metastable unit-cell-sized NaCl particles on the pathway to the well-known NaCl·2H<sub>2</sub>O 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.