I'm trying to increase the stability of my o/w emulsion droplets towards flocculation by increasing the magnitude of the zeta potential by increasing the amount of emulsifying agents during the emulsification procedure. However, when I then measure the zeta-potential (Malvern zetasizer) I get very large deviations, so my question is how can this be? How does the instrument even establish the zeta-potential in a sample with different particle sizes etc?
I dilute my samples with 10mM NaCl MQ until I obtain a derived count rate between 100k-500k kcps.
Daniel Laima
There are many things in your question I do not understand:
- The Zetasizer does not measure zeta potential - it measures movement (mobility) and can convert this to a zeta potential with appropriate assumptions
- Increasing the amount of an emulsifying agent isn't usually a direct route to improvement in stability (BTW, no emulsion is thermodynamically stable). However, increased emulsifier usually results in smaller particle size and more resistance/protection to dilution. Stability arises through charge or steric effects - and ZP is basically irrelevant for steric considerations as the slipping plane lies in the protective polymer 'coat'
- Dilution should be in the 'mother liquor' - the same ionic and additive make up as the original emulsion. It's likely/possible that dilution in 10 mM NaCl is destabilizing your system
- Generally speaking (and counter-intuitive, I know) the mobility of a distribution of particles is not related to the size (but to the charge) - the acceleration, under voltage, to the mobility speed is virtually instantaneous. There are no inertial effects
I suspect your issues lie in the dilution. However, what is the approximate size and concentration of your system? Have you contacted your local distributor? Find them on the MalvernPanalytical website. Have you ran the transfer standard to confirm instrument performance? Can you post the raw .dls an .zet files where there will be more specific, rather than general, advice we can offer.
Hi Daniel Laima , it might help if you have the raw data (or at least an excerpt of a few records) to take a look. A large zeta deviation could also happen when the mean zeta is close to zero. Here are three posts that might be of interest:
https://www.materials-talks.com/blog/2017/09/05/what-is-the-zeta-deviation/
https://www.materials-talks.com/blog/2015/09/24/zeta-deviation-larger-than-the-mean-how-can-that-be/
https://www.materials-talks.com/blog/2014/08/19/faq-how-can-i-submit-a-data-file-to-the-help-desk/
comment besides zeta potential - increasing surfactant concentration may lead to the point were surfactant micelles in excess cause depletion flocculation of the droplets.
Note that the assumptions used in the calculation to convert electrophoretic mobility to zeta potential do not apply to emulsions.
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