Received this question via personal request, and think this could be a valid learning point for others:
"I conducted DLS test for determining Cellulose nanocrystal size that was supposed to be about 100-200 nm. But the result has two peaks, one at about 100nm and other on 16nm.
Is it due to rotational diffusion? If that is the reason, can I just eliminate that peak and claim that I have a monodisperse solution?
Thank you so much for your response in advance! "
If the nanocrystals are needle-like then the shape could potentially be the reason leading to two peaks. The 16nm peak is likely near the smaller dimension of the crystals, and the larger size is related to the larger dimension of the needle. The larger peak would normally be the hydrodynamic size of the ellipsoidal shape, and this would be smaller than the outside dimension.
For the distribution you described, the smaller dimension could be something like ~15nm and the larger ~100nm. {Make sure these distribution results are repeatable and not just a one-off.}
Due to the needle-like shape, the longer dimension is probably about 30% larger than the hydrodynamic size, see Perrin shape factor, http://www.materials-talks.com/blog/2014/11/17/tips-tricks-for-nanoparticle-characterization/#shape
Also, for specific data questions you could always attach the data [in dts file format for the Zetasizer] so that it is easier to actually see the correlation function and evaluate the size data quality. You can export the specific record into a new file by highlighting, then Ctrl-C and then going into the new file and Ctrl-V to paste the record there. http://www.materials-talks.com/blog/2014/08/19/faq-how-can-i-submit-a-data-file-to-the-help-desk/
There is no data in the question of how the cellulose nanocrystals were obtained. It is possible that small nanoparticles correspond to cellulose oligomers or their aggregates due to H-bonds or hydrophobic interaction. When swelling and dissolving in water, the role of hydrophobic interactions is great. The calculation of the potential mean force by the molecular dynamics method of short cellulose oligomers in water showed a mutual contribution to the intermolecular interaction of the hydrogen bond and hydrophobic interaction. This parameter is equal to 8.4 kJ / mol cellulose chain fragment. This value is much larger than the contribution from the H-bond only.
Alves L., Medronho B.F., Antunes F.E., Romano A., Miguel M.G., Lindman B. // Colloid Surf A. 2015. No. 483. - p. 257-263
Very good point Yuri Mirgorod . In order to narrow down if the small peak is from larger nanoparticles or from smaller fragments/oligomers, a centrifugation and filtration could help solve the issue: If the 10-15nm peak remains after filtration through 20nm pore size filters, then this peak is most likely real in the sense, that it could be from smaller oligomers. This might also be a good test for the situation described by Marc Kostag (there is also the possibility - for some nanoparticles - that a potential apparent peak at smaller size may be due to particle rotation, sometimes observed with for example colloidal gold)
This technique is used for spherical particles and two peaks is related to the width and length of CNC.
Agree with Danesh
I made some measurements some days ago and I obtained a narrow high intensity peak around 188 nm and a peak arround 16 nm too. I think it's normal, cause our particles aren't sphericals
Thank you for the information, Victória Fadel ! It might even be nice to add some data here, if you have the data file and are willing to share.
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