There is no method to simply predicting the isoelectric point of a dispersion based on the additives because the additives may preferentially adsorb to the surface and alter the isoelectric point. You may find this post on "Isoelectric Points of Nanomaterials - Q&A" of interest: http://www.materials-talks.com/blog/2017/07/27/isoelectric-points-of-nanomaterials-qa/

With simple vial tests as we used to do in the 'old days' (1970's). See for example slides 46 & 47 in (open in IE):

Nov 11th, 2008. Dispersion and nanotechnology http://tinyurl.com/hpywsge

Note that the iep is a function of the pH. Your statement 'In this case how do we know the isoelectric point for adjusting the pH' makes little sense.

Hi Afza, once I made a experimental analysis comparing zeta potential, pH and viscosity of a few different TiO2 dispersions. The idea is that when the zeta potential is closer or equal to the isoeletric point, the viscosity achieves the maximum value.

If you make a "titration" of your suspension measuring viscosity and pH, you might be able to have a good reference on which pH would be the iop, altough it is not the most precise way of doing, for that you should use the Zeta potential instruments.

I've also used particle size analysis to do it. works fine as well for getting a reference. In this case you should observe a peak on the Particle size or the lowest obscuration value depending on the method.

I think this can be done with the help of a titrator that comes along with DLS analyzer.

BTW, a good rule is if the material is in powdered form, it's a laser diffraction measurement, not a DLS one.

There is no method to simply predicting the isoelectric point of a dispersion based on the additives because the additives may preferentially adsorb to the surface and alter the isoelectric point. You may find this post on "Isoelectric Points of Nanomaterials - Q&A" of interest: http://www.materials-talks.com/blog/2017/07/27/isoelectric-points-of-nanomaterials-qa/

I think (but I'm not sure) measuring the viscosity of the dispersion as a function of pH would work. The pH at which the viscosity of the dispersion is maximum should give you a guess of IEP. Similarly, measuring the diameter of particles in the dispersion as a function of pH should give you a guess of IEP too. Please correct me if I'm wrong.

'The pH at which the viscosity of the dispersion is maximum should give you a guess of IEP. ' You should take into account that this depends on particle concentration. Correct if particle concentration is high enough.

Hi Titus! Greetings!

you‘re right!!!

actually, for particles on the range of 0,5 microns, 50 to 60% without any additives was the one I had the experience where it was possible to see the viscosity differentiation.

maybe if your particles are smaller, or roughter, you should target something between 40 to 50% or less. It takes more liquid to “cover“ all the particles.

by the way!

Ulf is right...Use the water you'll use latter on... I observed a change in the IEP when there are more "charges" in the medium, making the eletric double layer thicker.

@Afza - I'm puzzled by your question.

Why do you have to make the dispersion at the IEP? If you are making a dispersion and you are using DLS, it's likely the instrument will do zeta measurement.

+1 for Alan's suggestion of simply looking at the suspension in a vial. I am a huge believer and preacher of the "just look at it" school - "macroscopic observation for microscopic understanding".

+1 for Alan's query about using DLS. Do you mean laser diffraction?

Irrespective of the technique, they both use dilute dispersions so the discussions around viscosity seem irrelevant since any non-Newtonian behavior won't occur. Unless, for some reason, you are preparing a slurry of your powder which you will then dilute.

It would be helpful if you could provide more information.