I have a question from @Pratima Adhikari that I will post on the open forum so all experts can contribute.
Hi Alan,
I hope you are fine and healthy. I have a question for you.
I am currently working with different concentrations of nanoparticles (nano-zerovalent iron and titanium dioxide) 0.002, 0.02, 0.2, 2, and 20 mg/l. I am planning to measure the zeta potential at different pH and find out the pH at zero point charge. I am not sure if I can just find the value of zero point charge of nanoparticles at concentration 20 mg/l or I have to find the point of zero point charge at all different concentrations(20 mg/l-0.002 mg/l) I think it's a lot of work to find the value of zero point charge at every concentrations. So, I was wondering if there is any simpler approach for this.
Please get back to me if you can.
Thank you in advance.
@ Pratima Adhikari
Hello Pratima
Thank you for your message.
I'm not sure how you're going to measure 'zeta potential'. The usual instruments measure electrophoretic or acoustic mobility and convert this to ZP with a bunch of assumptions. It's a pretty quick process to run through a titration, measuring the mobility at each pH point - and the quicker this is done, the better especially if you're around the point of zero charge where floculation and precipitation are most likely. You'd only need to hit 5 or 6 pH points to plot the mobility-pH plot and estimate the point of zero charge (not technically the same as the isoelectric point). In general terms there's a small parallel shift in the plots for concentration dependence. The distinguishing between mobility and ZP is an important one because at your higher concentrations you may get particle-particle interactions and thus hindered mobility. Do the experiments for all your 5 concentrations - I would expect it to be no more than an hour or so in total. Any mobility-pH experiment taking longer than about 5 or 10 minutes is too long, IMHO.
Good luck.
Best regards,
Alan
The question of electrophoretic mobility vs zeta potential is irrelevant. Zero mobility is zero zeta potential no matter what the model used to calculate zeta potential.
Do be aware that, strictly, the point of zero charge, cannot be measured electrokinetically. It would need to be done using, e.g., potentiometric titration. All an electrokinetic measurement will tell you is when the net electrokinetic potential is zero.
Just thought to add that it is sample dependent, whether different concentrations exhibit the same isoelectric point... You might find the section on specific and non-specific ion adsorption of interest:
https://www.materials-talks.com/blog/2018/03/06/zeta-potential-in-salt-solution-or-any-other-ions/
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