If you have added a dry powder to a liquid medium you may find that no amount of sonication will break up aggregated material.

If your sample is stable through either electrostatic or steric stabilisation, then you may be able to measure the sample without any sonication.

What is your material and do you have an expected size range? It may also be worth carrying out a Stokes law calculation to determine whether the dynamics of the particles are dominated by sedimentation or diffusion. If the former is the case, then again sonication may not help.

Nov 11th, 2008. Dispersion and nanotechnology http://tinyurl.com/hpywsge

November 3rd, 2015 Adhesion and cohesion http://tinyurl.com/zwb2wlh

'If the particles are agglomerated and sub-micron it may be impossible to adequately disperse the particles……'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

To add to Alex's excellent answer, a good general rule is: if the material is in powdered form, it's a laser diffraction measurement. If the material is in suspension form it may be a DLS experiment. Time of sonication is irrelevant as can be seen from the following:

June 20th, 2013 Ultrasound, cavitation, and the singing kettle http://tinyurl.com/olueohz

Remember too that sonication alters the particle size distribution of the starting material - is this really what you want to do?

The attached protocol may be of some help. There is no set procedure to accomplish your task. You need to know what the material is, roughly what the primary particle size is, and, if possible, what the isoelectric point is, unless you use surfactants for dispersion. I disagree with the first response, in that it is entirely possible to disperse very small particles fully in suspension if sufficient energy is used, the solution chemistry is appropriate and the particles are not chemically fused together (i.e., aggregates vs. agglomerates). P25 TiO2 is a perfect example. It comes as a dry macroscopic powder. Low energy dispersion in acidic water will generate a broad distribution of agglomerates and aggregates from a few hundred nm up to several micrometers. Application of high intensity direct sonication, for an appropriate time, and under appropriate solution conditions can break agglomerates down to a primary aggregate size of about 70-80 nm. The primary crystallites are about 20 nm or smaller, but they are fused into small aggregates that are impossible to break up further under typical lab conditions. You will have to test your material to determine if you can reach a size small enough to measure by DLS (a micron or less, generally). Also, Nanotoxicology, 2013; 7(4):389 might be of use. Ping me for private share. Good luck.

I think Im using the wrong approach by adding the dried nano powder in appropriate solvent and adding dispersing agent/surfactant as well. The right approach for DLS analysis is to make the dispersion prior to drying of the nano powder. Is it so?

Hello Vince Thank you for adding useful information to the debate. Several points to think about:

• As you correctly state, the universal acceptance is that agglomerates are disrupted by sonication and aggregates are not
• We need to distinguish between dispersion/de-agglomeration and ultrasonic milling
• The keys in making a stable dispersion are wetting, separation, and stabilization with separation being the key or possibly difficult step
• The conditions can be deemed aggressive for some 'dispersion' procedures. For example, I have seen 800 W for 15 minutes to 'disperse' 20 mg of TiO2..... I would argue that this is milling the material. Indeed the collapse of a sonicating bubble is around 1011g and the temperatures generated around 5000K as shown in the earlier webinar above. Under inappropriate conditions, metal particles can be welded in such situations. Van der Waals attractive forces are particularly strong in the short scale and this is why aggregates have difficulty in being separated other than by techniques such as stress corrosion milling - dissolution of the weaker boundary between aggregates.
• The external conditions of pH and the zeta potential of the system are obviously very important as stress corrosion milling indicates. Ultrasound can encourage agglomeration from increase in collision rate and efficiency if the condtions are not ideal for separation and stabilization. Each material is different and the attached paper on comminution limit is useful to read. Solid-solid diffusion below 100 nm in dry powders favors aggregation but steric stabilizers can reduce this effect so that a powder can be separated on mixing with the appropriate liquid system. What is good for one system is obviously not applicable to another (other than general considerations such as pH change)
• Dispersion below 100 nm or so takes a huge amount of energy as shown in colloid mills and the like. Standard dispersion (or is it de-agglomeration) can take many tens of hours under appropriate conditions. Just look at the publications of Netzch, for example

@ Afza Shahid

'I think Im using the wrong approach by adding the dried nano powder in appropriate solvent and adding dispersing agent/surfactant as well. The right approach for DLS analysis is to make the dispersion prior to drying of the nano powder. Is it so?'

I agree with your comment and would add that the term 'nanopowder' is an oxymoron in most cases. However, measurement of the SSA if greater than 60 m2/cm3 will indicate a material that falls under the definition of nanomaterial (< 100 nm in at least one dimension)

Dear Afza and other respectful fellows!

I did an interesting analysis on this during my PhD study!

It will depend on particle, medium and dispersion equipment properties.

I was able to determine a few empirical correlations that might give useful insights in this case. It is an open source thesis and I added the articles that were published.

Article Study on required energy to deagglomerate pigmentary titaniu...

Article Wettability study for pigmentary titanium dioxide