The automatic attenuation present in the Zetasizer Nano series facilitates an increased concentration range, in comparison to historical instruments, over which zeta potential measurements can be conducted. As a consequence, many samples can now be measured directly, with no dilution. However, the laser beam has to penetrate the sample for light scattered at a forward angle to be detected. Therefore in general, samples for zeta potential measurements need to be optically clear. If the concentration of the sample is too high, the laser beam will become attenuated by the particles reducing the scattered light that is being detected. Further discussion for the concentration requirements for zeta potential measurements on the Zetasizer Nano is available as a technical note from the Malvern website.
For samples requiring dilution, the way in which the dilution is performed is absolutely critical in determining the zeta potential value measured. For meaningful measurements, the dilution medium is crucially important. A measurement result given with no reference to the medium in which the material is dispersed is meaningless, in that the zeta potential is as dependent on the composition of the disperse phase as it is on the nature of the particle surface itself. It is difficult not to over emphasize this.
The aim of sample preparation is to preserve the existing state of the surface during the process of dilution. While the dispersant phase can sometimes be defined on paper, for many colloidal dispersions, the exact composition of the dispersant phase, e.g. the amount for “free” or unbound additives, is unknown. This problem can be avoided however, by filtering or centrifuging some clear liquid from the original sample and then using this supernatant to dilute the original concentrated sample. Using this approach, the equilibrium between surface and liquid are perfectly maintained.
If extraction of a supernatant is not possible, then one can many times just allow the sample to naturally sediment and then measure the fine particles left in the supernatant. Since the zeta potential is not a size dependent parameter, this approach will often avoid the hassles of spinning down the sample.
A final method for diluting zeta potential samples is to attempt to imitate the original medium as closely as possible. When using this approach, the medium should be matched according to:
Total ionic concentration of the system
Concentration of any additives (e.g. surfactants, polymers etc.) present