Because of the opacity of the MWNFs, zeta potential could not be directly measured using the dynamic light scattering (DLS) method. Therefore, First centrifuge the test sample then clearer solution of 1.0 mL drawn from near the liquid surface should mix with 20 μL of the original sample for use as a test sample to determine .

Ref. Article Evaluating Stability of Aqueous Multiwalled Carbon Nanotube ...

http://iopscience.iop.org/article/10.1088/1361-6528/aa52b4/meta

read above article to clear your doubts.

Dear Parisa Zare

In dark solution you wont get the correct results. so dilute the solution as clear as possible (mean while measure the dilution factor), then do DLS measurement and finally multiply the result with the dilution factor so you will get the exact result of your concentration.

Note: the suspended MWCNT nanoparticles in DI water should not settle.

Dear Arun,

Thanks for your kind answer.

What about the zeta potential, you only talked about DLS measurement (particle size), is it the same way possible for measuring zeta potential? (I mean first diluting and then multiplying the zeta potential by the dilution factor, is it correct?)

You need to get light in and out of the system for a light scattering electrophoretic measurement. This is not true for an electroacoustic method for measuring movement, but I have limited experience of that technique. Remember you're measuring a mobility and using an equation to convert that mobility to an inferred zeta potential If the system is crowded then you have hindered motion and the zeta potential will be incorrectly inferred. Your last statement above in parentheses is not correct.

Dear Alan,

What do you recommend for measuring zeta potential of a dark nano suspension such as 0.15 wt.% MWCNT in DI water?

Allow your suspension to settle a little (using the supernatant as your sample) and dilute in the ’mother liquor’ - the same continuous phase that the particles are in. You can recover this mother liquor by centrifugation, filtering, or a knowledge of the components.

Dear Parisa Zare

DLS-nano zeta analyser will give you directly the zeta potential value. In general, the zeta potential value will be given in + & - values. The zeta potential value as to be greater thaan 30 in both + & -, which gives the solution you are analysing is stable disperse solution.

@ Arun. It is not correct to state that the zeta potential is measured directly in a DLS-zeta instrument. It is inferred from the electrophoretic mobility which is the parameter that is actually measured.

@ Alan, I agree with you sir. what I mean here is the instrument coupled with the software will give us the zeta value directly as O/P.

@ Arun .....based on assumptions..... (which may or may not be valid)