A new freeze-drying approach has been devised that provides greater efficiency and sustainability compared to the conventional freeze-drying process to manufacture superfine powder or nanoparticles.
Study: Freezing method: a fast green technology for the production of nanoparticles and ultrafine powder. Image Credit: petrmalinak/Shutterstock.com
In research published in the journal ACS Sustainable Chemistry & Engineeringsphere-shaped ice particles formed in an aqueous mixture of NH4H2Purchase order4 or NaHCO3 produce their respective nanoparticles.
What is the freeze-drying method?
Due to their large specific areas and high reactivity, nanomaterials and ultrafine powders are gaining popularity in areas such as sustainable and environmental applications.
Nanoparticles (NPs) and superfine powders are often produced using freeze-drying techniques. The initial step in the freeze-drying technique is a cryogenic procedure that freezes the target particles or molecules in an aqueous mixture.
In the aqueous mixture, the water molecules rapidly solidify via the quick-freezing step, generating a crystallized ice pattern. This step is also called ice molding or freeze molding. The crystallized ice framework forces targeted dissolved molecules or components to produce nanoscale scaffold architecture, resulting in substances with nanoscale or microscale pores.
The freezing step defines the architecture of the scaffold and the ice model, as well as the crystal architecture of the targeted substances inside the ice models or the scaffolds, depending on the freezing parameters.
The second phase is a drying procedure that uses the process of sublimation to separate water as ice patterns. The ice melts throughout the drying phase, but the targeted substances, particles or molecules remain in the ice. From inside the ice, frozen NPs or porous substances of identical architecture and characteristics can be recovered.
Schematic diagram of the experimental setup for the freeze-drying method (top) and the freeze-drying method (bottom). © Yu, Q., Wang, Y., Luo, J. and Yang, H. (2022).
Limits of freeze-drying
Due to the cooler temperatures used in the drying phase, sublimation rates are slow and batch drying times for common pharmacological items can take up to several days. Production speeds of these batch-based technologies are limited by slow freeze-drying rates and extended cycle run times.
Some disadvantages can be mitigated by purchasing a larger freeze dryer. Unfortunately, it takes much longer to establish perfect vacuum settings, and temperature and pressure are less constant throughout the vessel, which can influence output quality. Due to the cold temperatures and the vacuum arrangement, the drying phase consumes a lot of energy.
Why is the freeze-dissolve method better?
The initial step of freeze-drying is the same as that of freeze-drying, i.e. freeze-drying to create ice containing the target components inside and build an ice scaffold target architecture.
The ice is then dissolved at a cold temperature, such as a subzero temperature in an additional solvent with a low freezing point in the next phase of the freeze-dissolve process. This additional solvent, like ethanol, acts as an anti-solvent for the targeted components while being miscible with water.
As a result, the ice scaffold will quickly dissipate into the additional solvent, leaving only the solid-state targeted components in the mix, and the architecture of the targeted components produced in the ice will be retained.
Fire extinguishing chemicals, baking soda, ammonium dihydrogen phosphate (NH4H2Purchase order4), and sodium bicarbonate (NaHCO3) are soluble in water but do not dissolve in ethanol.
In this work, various amounts of sodium bicarbonate or ammonium dihydrogen phosphate, dissolved in water, were used to make NPs via the freeze-drying technique, which were then evaluated against NPs produced by freeze-drying.
Schematic diagram of lyophilization and lyophilization mechanisms for the formation and isolation of NaHCO3 nanoparticles. © Yu, Q., Wang, Y., Luo, J. and Yang, H. (2022).
Important Findings
To extract superfine powder and NPs from ice patterns in frozen particles, the proposed freeze-drying process provides higher efficiency and durability compared to the conventional freeze-drying approach.
Particles of aqueous mixtures of sodium bicarbonate and ammonium dihydrogen phosphate were rapidly frozen to produce sphere-shaped ice particles, which were then filled with NP and superfine NaHCO powder3 or NH4H2Purchase order4.
The frozen components were dispersed in ethanol for 5 minutes at 10°C using the freeze-dissolve procedure to separate the ice scaffold. The freeze-drying approach, on the other hand, required 1400 minutes to separate the ice scaffold via the sublimation process. In identical experimental settings, the dimensions of the final products generated by the freeze-drying approach were relatively small compared to those produced by the freeze-drying approach.
The freeze-dissolve approach reported in this study is approximately 100 times faster and consumes approximately 100 times less energy compared to the freeze-drying approach, without the need for a large facility or vacuum. As a result, the freeze-dissolve process is likely to be used on an industrial scale with less time, energy and footprint.
Reference
Yu, Q., Wang, Y., Luo, J. & Yang, H. (2022). Freezing method: a fast green technology for the production of nanoparticles and ultrafine powder. ACS Sustainable Chemistry & Engineering. Available at: https://doi.org/10.1021/acssuschemeng.2c02270
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