Hi, my name is David Garcia, and I’m the Characterization Services manager at nanoComposix.
Today, I’m going to be giving a brief overview of a cornerstone analytical technique for
colloidal systems: zeta potential measurement.
In a colloidal system, dispersed particles have two layers of oppositely-charged ions
on the surface, called the stern and double layers. The zeta potential is defined as the
voltage at the edge of the double layer. If two particles have high enough zeta potentials
of the same sign, they will not agglomerate because of those like charges repelling each
other. How does the measurement work?
A sample is loaded into a disposable folded capillary cell, which has conductive points
to receive an electric charge. Inside the instrument, a laser measures how fast the
particles are moving when they’re charged – the faster they move, the higher the absolute
value of their zeta potential. As long as the solvent is polarizable, the
zeta potential can be measured. Chloroform, THF, and short chain alcohols do require a
special zeta cell however. The capillary cells degrade over time, especially
in high-salt media, so it’s important to check them against a standard often. At nanoComposix,
we calibrate our cells every single day by using a Malvern-provided polystyrene latex standard.
One important use of zeta potential is that you can use it to predict the long-term stability
of particles. Zeta potentials of less than negative sixty millivolts or above sixty millivolts
will have excellent stability. Conversely, zeta potentials between negative ten and positive
ten millivolts are in danger of rapid agglomeration unless they’re sterically protected, such
as by polyethylene glycol ligands. Another important use of zeta potential is
that it can be used to indirectly determine if a surface change has occurred. For example,
if you take a citrate-capped nanoparticle and displace the citrate with polyethylene
glycol, you should expect to see the zeta potential go from highly negative to slightly
less negative.
Zeta potential numbers are meaningless unless the solvent and pH are also reported, as the
number can change dramatically because of those. For example, adding acid to a solution
will cause acidic protons to associate with the double layer, thus making the zeta potential
more positive. This leads to an important definition for
colloidal materials, the Isoelectric Point, or IEP. The IEP is simply the pH at which
that particular nanomaterial has a zeta potential of zero. Knowing where you are with respect
to a material’s IEP can let you assess stability and change solvent conditions to decrease
the chance of agglomeration. Salt also affects the zeta potential. More
salt in solution will compress the double layer, and enough salt will collapse it and
leave the particles vulnerable to agglomeration. To visualize the salt effect, observe what
happens when a citrate capped or polyethylene glycol, or PEG capped 50 nanometer silver
nanoparticle dispersion is added to a solution of saturated salt. The PEG polymer is a long,
covalently bound neutral ligand that we can expect to make little or no contribution to
Zeta Potential, but provides strong steric hindrance. In a salt solution, the PEG capped
particles remain stable indefinitely. In contrast, when citrate capped particles are added to
the salt solution, the double layer collapses, and with no molecules on the surface that
provide steric hindrance, the nanoparticles begin to agglomerate instantly.
And that concludes our segment on Zeta Potential measurements. In the next installment, we’ll
discuss how to prepare a sample in more detail, as well as some important things to consider
in order to obtain high quality data. For more information about zeta potential measurement
theory, sample preparation, or submitting your own sample for analysis, please visit
the Characterization Services website. You can also visit our new NCX University course
on Zeta Potential as well. Lastly, please feel free to email us at info@nanocomposix.com,
and thank you for watching!