For intensity distributions refractive index is not relevant. If transferred to volume or number distributions particle refractive index might be relevant, but probably not for so small particles that you seem to have: http://www.materials-talks.com/blog/2014/08/05/faq-how-important-are-refractive-index-absorption-for-nanoparticles/

Optical properties of carbon depend on (see e.g. M. Bruna & S. Borini (2009) Optical constants of graphene layers in the visible range. Appl. Phys. Lett. 94, 031901, which covers optical behavior of graphene in the visible range, from the two-dimensional limit (single atomically thick graphene layer) to the bulk limit (graphite). It might therefore be too daring to use any single number in your PCS data analysis, although you might try.

To be on the safe side and to overcome the above mentioned obstacle / determine Delta n-value suitable for your purpose you could characterise one of your (ideally fairly monodisperse) preparations with PCS using several homogeneous solvents with known viscosity and different refractive indices (or several mixtures of two such solvents) for the purpose. You should then analyse all  results assuming the same average particle size and size distribution. This will allow you to extract a characteristic refractive index of your particles, which you should subsequently use  for your further measurements done in any solvent system.  (The proposed procedure relies on the fact that light scattering does not depend on absolute refractive index of the light scattering particles but rather on difference between such refractive index and the suspending medium refractive index.)

Good success lwith your measurements!

Dear Mr. Anderson & Dr. Cevc

Thank you very much for yours kind comments. These are helpful for me

Best Regards

for carbon black you need to use also imaginary part.

For a guide see http://www.tpdsci.com/tpc/RICarbon.php

some more

http://www.brookhaveninstruments.com/literature/pdf/BI-DCP/ParticleSizingUsingaDisc.pdf

it says 'Although there are minor variations (5) in the refractive index of carbon blacks, a suitable

average value for particle sizing is n = 1.84 - 0.85i .'

Hi  Dr. Sobisch

Thank you for your guidence

You may try to measure the refractive index of the colloid first using a refractometer.  Hope this helps.

We use Malvern Zetasizer for nanoparticle work. Malvern has brief technical information at the included link.   If all you actually need is comparisons between samples, then why not simply use the intensity results? Maybe you don't actually need volume or number distribution information. But in fact, for your small particles, Malvern says that even the volume distribution will not change if you use different RI values, which means you don't even need to know the real RI of the particles.

http://www.materials-talks.com/blog/2014/08/05/faq-how-important-are-refractive-index-absorption-for-nanoparticles/

Reading the blog post it turns out that they may speak about the particles where absorption is low (even if they do not state this explicitly and put in the table gold and silver with high k values). As far as I know, for carbon black there is a higher contribution of absorption for nanoparticles than for larger ones to extinction, therefore I assume that there should be an effect on scattered light intensity.

@Titus Your speculation that Malvern is speaking in general about low absorption particles is not consistent with the information in the blog post. As you noted, this is not explicitly stated. It is not implied, either. I think that you have made a misinterpretation of the point of the table. That table is merely, as titled, "Table of select material properties". Looking at the materials in the table, it is clear that these are merely a few materials that are very, very often encountered in the literature. And considering the small number of entries together with the fact that 1/3 of them have high k values, it seems quite unlikely that this table can be used to infer that the blog is addressing low k materials. Perhaps the reality is that studies of high k valued materials simply are not found in the literature?

I find it much more likely that these materials were included in the table either because Malvern has received many customer inquires about them, or they appeared in many of the results of a quick literature search. Perhaps the reality is that studies of high k valued materials simply are not found often in the literature?