Dear all,
recently I have approached Zeta Potential measurements in order to calculate the Zero Point Charge for a dispersion of biochar derived particles. I have prepared several solution at different pH, and measured them after equilibration time (generally 4-5 days). I am not an expert, but what I know from tests in mass titration is that I expect a ZPC around pH 3. Unfortunately, by Zeta potential I cannot find anything, since the value is always negative.
There are some possible mistakes that I have made in order to adapt the experiment to my situation. Here the details.
I have used a background solution NaCl 0.2 M, with 1 % w/w sample. My dispersion is not perfectly colloidal. I have taken particles < 0.4 mm diameter, and left the bigger to precipitate previous experiment, obtaining a quite stable particle dispersion. I have done 3 measurements with 60 s equilibration time, in order to don' have too much perturbation due to more precipitation.
There are any mistake in the preparation? And, in case, how to interpret this result?
Hi Pierluigi,
May I ask what instrument/technique you are using to measure zeta potential? Laser Doppler Electrophoresis is a commonly used method for such measurements but is generally only usable for particle sizes up to 100um.
Hi Alexander,
I am using a Zetasizer nano series. The dispersion is put in disposable folded capillary. I am using the model Smoluchozski, that in this case is quite a big approximation
Hello Pierluigi, the size may be a little on the high side, we state ~100 microns as the upper limit. Can you show any data? What values do you measure? With a high ionic strength (i.e. >200mM) it may turn out that absolute values for your sample could be relatively small. You may also not have many data points below pH 3, which would make it more difficult to find the intercept.
http://www.materials-talks.com/blog/2015/11/24/diffusion-barrier-method-the-practical-details/
http://www.materials-talks.com/blog/2017/09/05/what-is-the-zeta-deviation/
Hello Pierluigi, Dr. Nobbmann has already provided the details. Your particle size is pretty large. Be careful with the apparent stability. Sometimes a viscous medium may show that the dispersion is stable yet microscopically, the particles are already agglomerated and (aggregated). Do optical microscopy of your suspensions and see how they look like under various pHs. Your ionic strength is too high as stated above.
Alternatively, search the literature for "pH drift method". Very simple method for characterization. It will give you similar information.
Thank you for your kind answer. Indeed, since I am not en expert, I had to follow some internal advice that I had. I will start reducing the ionic strength and try to further reduce the diameters (not easy at all, unfortunately). I will also try to have a look to agglomeration.
I have around -2 mV for pH < 3, and it decreases further at higher pH. It's for this that I cannot find the ZPC.
I will take a look also to pH drift lethid, thank you!
I have another question. After the solutions have been prepared I need to adjust the pH. Could be important to use an acid/basic solution with the same concentration of the background solution? I mean, if I have used a NaCl solution 0.001M as background, if I use a 0.1M HCl solution to change the pH it can be problematic for the ionic strength?
It may alter your overall ionic strength, and you can keep track of it. However, it should not suppress the overall values as much as 100mM would. {You may try a lower strength HCl for easier pH adjustment.} See attached graph as an example of the influence of buffer strength on alumina particles (and specific as well as non-specific ion absorption).
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