Measuring zeta potential - a specific measurement problem

This is a recording of a webinar I gave on How To Measure Zeta Potential More Confidently.

I show you how you can maximize the confidence in your measurements - and their interpretation - needed for robust decision making.

TRANSCRIPT

Question: I have tried measuring the zeta potential using 4 different ionic strength solution (0.1M, 0.3M, 0.5M and 1M) with graphene oxide particles (size approx. 5 micrometer). I am getting same zeta potential (value around ZERO). How do you interpret this? Also, I have tried using different pH solutions of 4, 7 and 13, which means 12 samples in total. I get ZERO value in each case.

There’s a few things that come to mind.

First, five microns is very large. I don’t know the density of graphene oxide but I suspect these particles must sediment very quickly. So, it’s possible they are sedimenting out of the laser beam before the measurement starts. There’s always a few seconds delay while the various shutters and filters are adjusted.

Also, the high ionic strength – it could be that the instrument is refusing to attempt the measurement. Some check the current and, if it is too high due to too much salt, will simply not attempt a measurement.

Getting a value around zero could be valid but it depends what you mean by “around zero”. Instruments such as the Zetasizer Nano ZS provide very useful reports with a wealth of information about the measurement. Lack of particles due to sedimenting will show up as a very low “count rate”. Look for noisy phase functions or erratic voltage and current plots.

It can be helpful to put a sample in a cuvette and shine a laser pointer through it. You should see a clean beam though, with such large particles, it may look “broken up”. But if you have a sedimentation problem, you’ll see the beam eventually disappear (if you leave the cuvette standing). You may also have too many particles. You need to be able to see through it (as I described earlier).