Hi,
Based on the literature, the zeta potential should reduce as one increases the concentration of electrolytes in the suspending medium. However, I did some DLS measurements and for 3um polystyrene particles suspended in DI, I found an average of -45 mV as the zeta potential. when I suspended them in 1mM KCl and did the measurement again, I found an average of -85 mV.
This trend does not make any sense to me. Can someone clarify what could be the explanation behind this observation? I repeated the experiment several times and also used a different cuvette but still got the same numbers.
Best
Amir
Measurements in DI water are problematic because there are so few ions in solution that no meaningful double layer forms and the concept of zeta potential becomes unclear.
The electrophoretic mobility (which is what the instrument measures) of polystyrene latex particles has been studied a lot by researchers decades ago. I recommend starting with this Google Scholar search:
https://scholar.google.com/scholar?as_q=polystyrene&as_epq=electrophoretic+mobility&as_oq=&as_eq=&as_occt=any&as_sauthors=&as_publication=&as_ylo=&as_yhi=&hl=en&as_sdt=0%2C34
In general, higher salt should lead to lower effective zeta, potentially with influence of specific ions https://www.materials-talks.com/zeta-potential-in-salt-solution-or-any-other-ions/
As indicated above, a certain amount of ions must be present for the correct determination of electrophoretic mobility/zeta potential.
For really low conductivity, you may also see inconsistent values https://www.materials-talks.com/zeta-consistency-top-5-signs-zetasizer/
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