I have modified my carbon nanoparticle surface with amino acids. Now I would have both amine and acid functionalities on the surface of my nanoparticle. The measured zeta potential value is about -17 mV. What it infers? May I conclude that the nanoparticle is primarily functionalized with carboxyl groups? And I also red that the nanoparticles having zeta potential more than 30 mV in magnitude are stable in solution state without aggregation or flocculation. Although the net charge of my nanoparticle is negative some of the positive charges should be there on the surface. The positive charges might be nullified due to the higher population of negative charges on the surface. Now what would be the aggregation behavior of my nanoparticles?
Dear Alexey Stepanov,
Thank you so much for the nice explanation. It fulfilled my needs.. :-)
Also, take into account that zeta potential depends on pH and salinity. If you play with pH, say by adding dilute HCI or NaOH, you will be able to go from pH=1 to almost pH=14, finding that at acidic pHs you will have more protonated amino groups and thus you will get a more positive zeta potential (up to a plateau). At some point, you will find a pH value at which your material is not charged (point of zero charge or isoelectric point, depending on having ions other than OH/H determining ions at the surface of the particles). Then as you keep increasing the pH, you will have negative zeta potential, which will be increasing (becoming more negative) with higher pH until you reach a plateau. At that point of course you will have a large contribution from deprotonated carboxylic acid groups (carboxylate, negative charge), whilst most amine groups will also be deprotonated (thus, without positive charge). So, if you have the chance and it may be of use to your research, it is a good exercise to vary the pH of your dispersions and measure the zeta potential as a function of pH.
Also, take into account that zeta potential depends on pH and salinity. If you play with pH, say by adding dilute HCI or NaOH, you will be able to go from pH=1 to almost pH=14, finding that at acidic pHs you will have more protonated amino groups and thus you will get a more positive zeta potential (up to a plateau). At some point, you will find a pH value at which your material is not charged (point of zero charge or isoelectric point, depending on having ions other than OH/H determining ions at the surface of the particles). Then as you keep increasing the pH, you will have negative zeta potential, which will be increasing (becoming more negative) with higher pH until you reach a plateau. At that point of course you will have a large contribution from deprotonated carboxylic acid groups (carboxylate, negative charge), whilst most amine groups will also be deprotonated (thus, without positive charge). So, if you have the chance and it may be of use to your research, it is a good exercise to vary the pH of your dispersions and measure the zeta potential as a function of pH.
Dear Margarita,
Thank you for the nice suggestion. The pH of my solution would be 5.5 to 6.5 (Demineralized water).
The "stronger than 30mV rule" was really based on particles stabilized by electrostatic repulsion. As indicated above, zeta potential is strongly dependent on the solution conditions (pH, ionic strength, additives,...). Many real samples may also be stabilized by steric stabilization, where excluded volume interactions can lead to stability even for particles that are not charged at all.
http://www.materials-talks.com/blog/2012/01/17/dispersion-stability-what-is-important/
The "stronger than 30mV rule" was really based on particles stabilized by electrostatic repulsion. As indicated above, zeta potential is strongly dependent on the solution conditions (pH, ionic strength, additives,...). Many real samples may also be stabilized by steric stabilization, where excluded volume interactions can lead to stability even for particles that are not charged at all.
http://www.materials-talks.com/blog/2012/01/17/dispersion-stability-what-is-important/
Zeta potential measurements should not be performed in demineralized water (as Ulf mentioned by the influence of the ionic strenght). Some reasons may be found within the reference (link, also from Malvern). You do not measure the net charge or surface charge of the particles you measure the zeta potential (Fig 7, which is dependent on the double layers). I suggest to keep the ionic strength large enough and constant and vary the pH to determine the pzc and representative non-influenced zeta potentials.
http://www3.nd.edu/~rroeder/ame60647/slides/zeta.pdf
Article Functionalized polymer colloids bearing primary amino groups
Dear Ramanan,
first of all you do not measure zeta potential. It is calculated from e.g. electrophoretic mobility applying a model. Mostly it is assumed that charges are localized in a plane. This does not hold the more you approach the nanosize range.
As already pointed out the degree of dissociation of the functional groups introduced depends on counter ion concentration. This is a dynamic equilibrium, which means the number of dissociated groups changes with pH and electrolyte.
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