We are trying to characterize various nanoparticles by light scattering (size and zetapotential). The particles are metal oxides mostly (Co3O4, ZnO, TiO2,Fe3O4,...) with sizes typically in the range around 20-50 nm by TEM.
The problem is that we have a significant aggregation, which can be reversed by ultrasonicating them. In lightscattering, we can see a signficant increase with time (->aggregation) in aq. solution (0.1-1% in water)
Does anybody have good suggestions for surfactants, which will prevent this aggregation?
You can use ultrasonic wave to reversed aggregation. You also can use the centrifugal machine. Or you can cover your particles by some kind of polimer similar to quantum dots synthesize process...
as I said, we ultrasonicated the samples and we could observe aggregation during the light scattering measurement.
Using "some kind of polimer " is one of the solutions, but would like to know what has been proven to work.
Dear Florian,
If these are truly metal oxides and not covered with an outer layer of organometallic material, then I suggest you try dispersants such as would be used for minerals equivalent to your specimens. So track the latex paint formulating industry or the mining industries or the dish washing industries.
Low molecular weight polyacrylic acid (say number average around 4000 g/mole or so) at pH's of about 9 might work nicely for some of these. Several materials (homo and copolymers) in the "Tamol" or Daxad trademark line might help, if you can find them. A few might also respond well to sodium hexametaphosphate, although it is not long term stable against degradation as a function of time.
Polymers like the polyacrylic acid may be targeted at about 1 to 10 mg per meters squared of expected surface area in the targeted primary particle. They may even work at lower levels.
Never ignore the benefits of pH control also. Try your materials at higher pH and also at lower pH, as they may be amphoteric.
Best regards,
Mike
Another thought, Florian
You could try them in isopropanol -- that sometimes is advantageous with materials like these. Assuming that you merely are trying to characterize them and not carry them forward as aqueous formulations into a material science project you are working on.
Mike
thank you very much Mike.
Yes, we only want to characterize size and zetapotential. The later use for the NP is in thin films, so we don't need to worry about the 1 mg or so for the sample.
Dear Florian,
I think I cannot contribute to the physicochemical aspect of the problem, but to the suppliers issue for polyacrylamide I would propose POLYSCIENCES in USA.
They are excellent professionals. We have purchased samples of Mw O(10^6), however. Possibly here http://www.polysciences.com/paa/ they have the information you need.
Best wishes to solve your problem!
Thomas
To make a stable dispersion requires three steps: wetting, separation, and stabilization.
For oxide particles mentioned here, both water and low MW alcohols as the liquid medium have no problem wetting them. But please be reminded that for particles that will react with or dissolve in water, ethanol or isopropanol is a better choice.
Separation of aggregated particles requires energy input. Ultrasonication is the routine method. Use an ultrasonic horn for the nanoparticles, not a much weaker ultrasonication bath.
Stabilization of separate particles requires some additives such as those mentioned by Mike Bender. More specifically, it depends on the surface chemistry of oxide particles (zeta potential curve), pH, and type of additives. For instance, TiO2 has a IEP around pH 4, so a cationic polyelectrolyte such as PEI is better than anionic anionic polyacrylates in the pH range 5-9 for TiO2. But generally, anionic polyelectrolytes are more widely used, as case like TiO2 is not frequently met. You may also get some anionic polyelectrolyte DARVAN 821A (molar mass around 3500-4500) and similar alternatives from Vanderbilt Minerals. And again as pointed out by Mike Bender, sodium hexametaphosphate may also work for the purpose. The discussion here is based on an aqueous medium. For stabilization of oxide particles in ethanol/isopropanol, maybe a nonionic polymer could be used. However, for this part, I do not have too much hands-on experience.
For zeta potential measurement, please be noted that when you "coat" those additive molecules onto oxide particles' surface, the measured zeta potential is no longer the true zeta potential of the oxide materials. Rather, it will contain a lot more contribution from the polymeric additive than from the particle themselves. For instance, IEP of calcined ZrO2 is around pH 6-7, but when its surface is coated with anionic polyacrylates, its IEP will shift to higher pH >9, which will be nearly identical to that of IEP of anionic polyacrylate used. There are journal articles around talking about this in details. Also, zeta potential usually implies an aqueous system. In the case of a non-aqueous liquid medium, I don't know how to interpret the result unless it is directly related to your specific applications. Dr. Renliang XU of Micrometritics had a journal paper on CARBON about the zeta potential measurement for carbon black in non-aqueous systems, just FYI.
If you see agglomeration upon sonication it means particles are not sufficiently stabilized.
Have you tried Triton X-35 (octal phenol ethics late)? It provides good wetting and stabilizes particles in dispersion very well.
You need surfactants to be adsorbing on the oxide surface. High water solubility of the surfactant will defeat the purpose. Try it and then confirm.
As the materials wet easily (your statement above) then you need a stabilizer to prevent recombination after separation. For inorganic oxides, a phosphates at the optimum concentration (too much is as bad as too little) are routinely used. Alternatively, a steric stabilizer such as a 50 KDa PEI or PEG. Take a look at (open in IE):
Nov 11th, 2008. Dispersion and nanotechnology
http://tinyurl.com/hpywsge
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