Dear Kazem, if you would perform zeta-potential pH titrations, you should be able to see which functional groups are present by the change in zeta potential as a function of pH. The pKa of the surface groups can be determined by plotting electrophoretic mobility versus pH as shown in the picture below:

https://ars.els-cdn.com/content/image/1-s2.0-S0921883115002666-fx1_lrg.jpg

I hope this helps. Kind regards, Leo

Dear Kazem, if you would perform zeta-potential pH titrations, you should be able to see which functional groups are present by the change in zeta potential as a function of pH. The pKa of the surface groups can be determined by plotting electrophoretic mobility versus pH as shown in the picture below:

https://ars.els-cdn.com/content/image/1-s2.0-S0921883115002666-fx1_lrg.jpg

I hope this helps. Kind regards, Leo

Very simplified response to this question would be that if you have positively charged functional group, such as amines, the Zeta potential should be positive and for the negatively charged, like carboxyl, negative. The magnitude would depend, among other factors, on the amount of groups.

Again, this is very simplified response. You should always keep in mind, that there are many other factors influencing the Zeta potential, such as type of the solvent, its pH, type of particles evaluated, etc.

Keep in mind that zeta potential is not a measure of the surface charge of the particle. It is a measure of the electrical potential some distance away from the surface. Adsorbed species (e.g., ions) shield the charge. In some situations it is even possible to have a zeta potential of opposite sign to that of the charge at the surface of the particle.

Dear @Leo , @Aleksandra and @John, thank you all for your helpfull answers.