Hi,

particles in nanopowder are aggregated due to attractive Van der Waals forces. Making a colloidal solution requires two steps: breaking the aggregates and stabilizing the dispersed particles by preventing re-aggregation.

Ultrasonication with high-energy probes is the standard dispersing procedure for aggregated nanoparticles in low-viscosity solvents.

Stability depends on the dispersing medium. Either you can use an electrostatic repulsion if the zeta potential is larger than +/- 10 mV, or you can use steric repulsion by adsorbing polymer chains in extended conformation. In either case, the type of dispersing solvent is important. Refer to our paper for more details.

Article Thermodynamic Parameters Controlling Nanoparticle Spatial Pa...

Abinash Panda You can't. You should never have dried the colloidal suspension. Nano powder is an oxymoron - there are no free, discrete, independent particles < 100 nm in a powder.

2 quotes from those much greater than I:

'I think dry nanotechnology is probably a dead-end' Rudy Rucker Transhumanity Magazine (August 2002)

‘If the particles are agglomerated and sub-micron it may be impossible to adequately disperse the particle… ‘The energy barrier to redispersion is greater if the particles have been dried. Therefore, the primary particles must remain dispersed in water...’ J H Adair, E. Suvaci, J Sindel, “Surface and Colloid Chemistry” Encyclopedia of materials: Science and Technology pp 8996 - 9006 Elsevier Science Ltd. 2001 ISBN 0-08-0431526

Take a look at: https://www.researchgate.net/post/How_to_make_Fe3O4_nanoparticles_in_powder

And this webinar (free registration required):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

Good luck dispersing aggregated particles with ultrasonication.

Dear Omar Gonzalez-Ortega, it depends on what you mean by "aggregated particles" because the term is used quite freely in different fields. This review sums it up nicely:

Article A Review of the Terms Agglomerate and Aggregate with a Recom...

We stick to the definition that aggregates are rigidly adjoining primary particles, held strongly by VdW forces, but otherwise retaining their individual character. Such particles could be separated by ultrasonication. Particles fused together by sintering cannot be dispersed by US that easily, but they are not considered aggregates according to this definition either.

Ultrasonication is one of the standard procedures in nanocomposite technology. Twin-screw extruding is another. In both, a strong shear field breaks the aggregates. Ultrasonication generates it locally through cavitation. High-power US probes are needed, not the US baths that are standardly equipped in most labs. The documented limit of this technique is around a few tens of nanometers, which is also a common range of many primary particles. In any case, ultrasonication is well-established in improving dispersion.

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Petr Lepcio Thank you for your comments and useful references. I've too have used the Gary Nichols paper many times to indicate the confusion in the literature with the terms agglomerate and aggregate. One of the reasons that ISO has recently used the term 'clusters' to generally refer to such entities of collections of particles. In common parlance (loose/van der Waals bound) agglomerates can be disrupted by sonication but chemically bound aggregates cannot. Anyway the use of sonication or high shear can always be tested in a practical manner (indeed always should be) and this is the basis of method development in laser diffraction (before, during, and after, sonication). See ASTM E3340-23 Standard Guide for Development of Laser Diffraction Particle Size Analysis Methods for Powder Materials. This follows the stabilization technique you inherently mention in your answer (wetting, separation, stabilization) for making stable dispersions of powders.

The usual danger with high-powered probes is that ultrasonic milling is being employed - both of the material and the probe. A simple experiment confirms this: take 18 Mohm-cm water and sonicate with a 600 or 800W probe. Measure the resistivity after 10 minutes. As well as considerable heating of the liquid the resistivity will decrease by 7 or 8 orders of magnitude (to 10 or 11 Mohm-cm). Chemically bound aggregates can only be separated by stress corrosion milling - getting the pH right to 'just' dissolve the chemical bonds. More information in various webinars (free registration required):

Ultrasound, cavitation, and the singing kettle

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W130619UltrasoundCavitationSingingKettle.html

Adhesion and cohesion

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W180725AdhesionCohesion

PST & BDAS - An acronym approach to laser diffraction method development

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190815PST

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

In terms of packing take a look at:

In pursuit of perfect packing

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W151021PerfectPacking.html

I'd additionally mention that measuring the SSA of the powder will provide an indication of its nano nature or not and this is the basis of one definition of nanomaterial (for unit density an SSA greater than 60 m2/g implies a particle size < 100 nm). Measurement and statement of the SSA of the material is a useful guide to quote.