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

Area of Science:

• Physical Chemistry
• Materials Science
• Crystallography

Background:

• Sodium chloride (NaCl) solutions form NaCl hydrate (NaCl·2H₂O) at low temperatures, serving as a model for freeze-drying studies in food and pharmaceuticals.
• Understanding NaCl crystallization during freezing is crucial for optimizing freeze-drying processes.

Purpose of the Study:

• To investigate the initial stages of NaCl crystallization during the freezing of aqueous solutions.
• To identify precursor substances to NaCl hydrate formation using advanced spectroscopic techniques.

Main Methods:

• Terahertz (THz) spectroscopy was employed to detect and analyze particles during the freezing of NaCl solutions.
• Analysis focused on identifying unit-cell-sized particles and their spectral signatures.

Main Results:

• Unit-cell-sized NaCl particles, not previously documented in phase diagrams, were detected in ice during NaCl solution freezing.
• Two types of metastable unit-cell-sized NaCl particles were identified as precursors to NaCl·2H₂O formation.
• These precursor particles exhibited distinct absorption peaks in the terahertz spectra.

Conclusions:

• The existence of unit-cell-sized NaCl particles highlights the complexity of NaCl crystallization during freezing.
• This discovery provides new insights into the freeze-drying process.
• It opens avenues for developing novel freeze-drying technologies for producing nanometer-sized particles, such as ultrafine pharmaceutical powders with enhanced solubility.