In relation with production and stabilization of Nano-objects there is very often the talk about observing/ setting the right Zeta-potential. Also a lot of actual analytical instruments promote Zeta-potential determination together with e.g. size determination.
But let us have a closer look to the use of Zeta-potential today and its value.
In research of specific structure analysis Zeta-potential measurement is irreplaceable for:
- Finding reasons for surface loading resp. The structure of “double layers”.
- Detecting the adsorption equilibrium (and adsorption kinetic) of emulsifiers, dispersants, etc.
- Tracing dependency of interface characteristics on material properties of the disperse phase.
In contrary to this for industrial purposes the Zeta-potential is merely used for:
- Research on flocculation mechanisms.
- Predicting stability of emulsions and suspensions.
- Behavior forecasting of multiple component dispersions resp. of adsorption characteristic of fine particles passing through porous media.
- Determination of optimum amounts of stabilizing additives resp. flocculants.
- Tracing the change of interface characteristics during milling additives/ dispersing / emulsifying aiming for a standardized addition of additives.
These two different tasks also need different accuracy in Zeta-potential measurement.
Research needs as accurate as possible measurement of the Zeta-potential. Most industrial tasks are just interested in stability/instability determination.
This determines the specific needs of instruments for research or stability control.
Zeta-potential is, as we have seen in my last blog: “Sample preparation for particle size determination.” , highly affected by the concentration of surrounding ions that are forming the electrical double layers. This means that correct Zeta-potential measurement needs to be done always in original concentration or at least iso-ionic dilution. As concentration of industrial nano products are mostly very high, only instruments based on acoustic principle can be used. All optical instruments need dilution to make the light beam pass and to avoid inhibiting amounts of multiple scattering.
This is the extremely limiting background for measuring Zeta-potential in very high diluted systems by means of extended light scattering instruments. Only if the dilution is done in accurate iso-ionic manner such a measurement would be acceptably correct but dilution means reduction of statistics which is critical as I had pointed out in my blog: “Particle size measuring in the nanometer range.”
In general can be stated, that for reasonable Zeta-potential measurement in research, only acoustic measuring principles should be applied because only those can work in original concentration with high accuracy. Light scattering instruments cannot accurately enough determine Zeta-potential as high dilution has to be applied.
The mere determination of stability does not need Zeta-potential measurement and in no case a non accurate one. The observation of changes of the amplitude of correlation function within a sequence of repeated PCS/PCCS measurements is able to indicate stability/instability extremely sensitively because slightest growth of particle size is resulting in an amplified increase of scattered light intensity. The reason for this is, that with a growth of particle size by times 10 the scattered light arises by 106 in the Reyleigh area and at least still 102 in the MIE regime. This means that the value of instrument combination of PCS/PCCS size determination and Zeta-potential is mainly promotional only. Accurate PCS/PCCS measurement can do as well and if Zeta-potential is really important acoustic principle instruments should be preferred.