I measured the Zeta potential of a micro-emulsion sample, separately diluted by DI water and by 10 mM KCl solution. After 6 times of measurements, the Zeta potential results were as following:
DI Water diluted: -9.0; 37.2; -45.0; 12.0; -67.7; 42.1 (Average: -5.1; S.D: 44.35)
10 mM KCl diluted: -14.0; -8.4; -26.0; 28.5; 78.3; 56.4 (Average: 19.1; S.D: 42.12)
In this case:
- Which of the two is closer to the true Zeta potential of the sample (I know that 10 mM KCl is usually used as a diluent for Zeta potential measurement)?
- What do I need to do if I want to get smaller deviation of Zeta potential for this sample?
Duy Khang Nguyen Vu DI water is about the worst diluent for small (nano) systems as it dilutes or washes out the surfactants and stabilizers and thus usually destabilizes the system. The correct diluent is the 'mother liquor' - the same continuous phase as present in your original stable, more concentrated system. This can be obtained by centrifuging or knowledge of the chemical make-up of this phase.
Milliomolar salt concentration is used only as an indicator of when conductivity will be adequate in the system. Pure DI water has low conductivity.
Note that zeta potential is inferred from mobility measurements and is a holistic property of the system. It will depend on the pH and concentration of species. Thus you should be careful in asking for 'true zeta potential' as this will depend on the surface chemistry of the particle and the external conditions.
Even given the vagaries of zeta potential measurement, I would think that the variability in both your sets of measurements is not acceptable and would look to make a dilution in the mother liquor as indicated above. You may have destabilized the system just as much with milliomolar KCl.
Generally, never use DI water for zeta potential measurements or particle size via light scattering.
For zeta potential, you want a well-defined double layer, so some electrolyte is required (even 0.1mM is enough but don't exceed 10mM otherwise you may start getting problems with electrode polarization and/or destabilize your sample).
For sizing, you need to reduce the thickness of the double layer so that the double layers of neighboring particles do not interact with each other. This can slow down the diffusion and give an artificially high size (electrohydrodynamic retardation).
You need to be aware of how the electrophoretic mobility (the measured property) is converted to an estimated zeta potential. There are two limiting extremes - small diameter, low electrolyte (Huckel limit) and large diameter, high electrolyte (Smoluchowski limit). If you fall in the middle, you need to apply the Henry equation.
"DI Water diluted: -9.0; 37.2; -45.0; 12.0; -67.7; 42.1 (Average: -5.1; S.D: 44.35)
10 mM KCl diluted: -14.0; -8.4; -26.0; 28.5; 78.3; 56.4 (Average: 19.1; S.D: 42.12)"
Your results are so variable, I suspect either something is wrong with your instrument or your sample preparation. The standard deviations in both cases are nearly twice a large as the difference in the averages. Something isn't good.
Duy Khang Nguyen Vu Verify the instrument with the transfer standard to eliminate it from the 3M variable list (Machine, Method, Material).
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