Green method to make nanoparticles and ultrafine powder

A new freeze-dissolving approach has been devised that offers greater efficiency and durability compared to the classic freeze-drying process to make superfine powder or nanoparticles.

Study: Freeze-dissolving method: a fast green technology for producing nanoparticles and ultrafine powder. Image Credit: petrmalinak/Shutterstock.com

In the study published in the journal ACS Sustainable Chemistry & Engineeringspherical ice particles were formed in an aqueous mixture of NH4huh2PO4 or NaHCO3 to produce their respective nanoparticles.

What is the freeze drying method?

Due to their significant specific areas and strong reactivity, nanomaterials and superfine powders are gaining popularity in areas such as sustainable and environmentally friendly applications.

Nanoparticles (NPs) and superfine powders are often produced using freeze-drying techniques. The initial stage of the freeze-drying technique is a cryogenic procedure that freezes target particles or molecules in an aqueous mixture.

In the aqueous mixture, water molecules rapidly solidify via the fast-freezing phase, creating a framework of crystallized ice. This step is also known as ice molding or freeze casting. The crystallized ice framework forces the targeted solutes or components to produce a nanoscale scaffolding architecture, resulting in substances with nanoscale or microscale pores.

The freezing phase defines the architecture of the scaffold and ice template, as well as the crystal architecture of the target substances in the ice templates or scaffolds, based on the freezing settings.

The second stage is a drying procedure that uses the process of sublimation to separate water as ice templates. The ice melts during the drying phase, but the targeted substances, particles or molecules remain in the ice. Frozen NPs or porous substances with identical architecture and characteristics can be retrieved from the ice.

Schematic diagram of experimental setup for the freeze dissolution method (top) and the freeze-drying method (bottom). © Yu, Q., Wang, Y., Luo, J., & Yang, H. (2022).

Limitations of freeze drying

Due to the cooler temperatures used in the drying phase, sublimation rates are slow and batch drying periods for common pharmacological items can last up to several days. The production rates of such batch-based technologies are limited by poor freeze drying rates and long cycle times.

Some drawbacks can be mitigated by purchasing a larger freeze dryer. Unfortunately, it takes much longer to achieve perfect vacuum settings and temperature and pressure are less consistent throughout the container, which can affect output quality. Due to the cold temperatures and the vacuum set-up, the drying phase takes a lot of energy.

How is the freeze-dissolving method better?

The initial stage of freeze dissolution is identical to that of freeze drying, that is, freeze molding to make ice containing the target components and build a target architecture for ice scaffolding.

The ice is then dissolved at a cold temperature, such as a sub-zero temperature, in an additional low-freezing solvent in the subsequent stage of the freeze-dissolving process. This additional solvent, such as ethanol, acts as an anti-solvent for the target components, yet exhibits water miscibility.

As a result, the ice scaffold will quickly dissipate in the additional solvent, leaving only the target solid state components in the mixture and preserving the architecture of the target components produced in the ice.

Fire fighting chemicals, baking soda, ammonium dihydrogen phosphate (NH4huh2PO4), and sodium bicarbonate (NaHCO3) are soluble in water, but do not dissolve in ethanol.

In this work, different amounts of sodium bicarbonate or ammonium dihydrogen phosphate, dissolved in water, were used to produce NPs via the freeze-dissolving technique, which were then evaluated against NPs produced by freeze-drying.

Schematic diagram of the freeze dissolution and freeze drying mechanisms for the formation and isolation of NaHCO3 nanoparticles. © Yu, Q., Wang, Y., Luo, J., & Yang, H. (2022).

Important findings

To extract superfine powder and NPs from ice templates in frozen particles, the proposed freeze-dissolution process offers greater efficiency and durability compared to the conventional freeze-drying approach.

Aqueous mixtures of sodium bicarbonate and ammonium dihydrogen phosphate were rapidly frozen to produce spherical ice particles, which were then filled with NPs and superfine powder of NaHCO3 or NH4huh2PO4

The frozen components were dispersed in ethanol at 10 °C for 5 min using the freeze-dissolving procedure to separate the ice scaffold. In contrast, the freeze-drying approach required 1400 minutes to separate the ice scaffold via the process of sublimation. In identical experimental settings, the dimensions of the final products generated by the freeze-dissolving approach were relatively small in contrast to those produced by the freeze-drying approach.

The freeze-dissolving approach described in this study is approximately 100 times faster and consumes approximately 100 times less energy compared to the freeze-drying method, without the need for a large facility or a vacuum. As a result, the freeze dissolution 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). Freeze-dissolving method: a fast green technology for producing nanoparticles and ultrafine powder. ACS Sustainable Chemistry & Engineering† Available at: https://doi.org/10.1021/acssusschemeng.2c02270

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