Hi,
I'm new to this type of measurement, from what I've read, the addition of salt is vital in aqueous measurements of Z.
However I have been measuring a silica micro-sphere suspension in pure ethanol, followed by the same in 1mM AgNO3 also in pure ethanol.
I wanted to show that the surface charge of the SiO2 attracted the Ag ions, however I'm not sure if this is the way to go.
Can zeta potential mean anything in ethanol without salt?
In the folded capillary cell in the Zetasizer ZS there’s typically enough conductivity in alcohols for mobility measurement. This should give you some indication of the magnitude of the charges. But be aware that you make assumptions in the conversion of mobility to zeta potential. Although there is no concept of pH in alcohols, ethyl alcohol, for example, will solubulize both HCl gas and NaOH. In terms of stability, then size and density constraints will be more important in thin continuous phases, such as alcohols, rather than any zeta potential consideration.
Alan,
Thanks very much, so I can infer that there is at least a change in charge from the mobility, not so much the Z?
I have a Z value of -24.6mV from the SiO2 in ethanol, and then 0.176mV when the AgNO3 is added.
Allan
I have requested the text from the author, I will check it out, thanks
@ Daragh. Remember that mobility is measured and zeta potential is inferred (so select the correct model from the Henry equation - either Huckel or Schmolukowski). Also quoting a ZP to better than +/- 10% or so is about the best you can do (so 0.176 mV is a bit too precise and optimistic!). Notwithstanding the previous you can gauge that the (negative) charge on your particle in EtOH has been reduced by adding Ag+ ions to the system. In EtOH the counterions are going to be bound strongly to the particle and there'll be very little ionic concentration in the continuous phase - so sharp drop in potential in a short length frame. Interpretation here can be interesting, but mobility is unambiguous.
@Alan F Rawle,
If the measurement is too precise, should I just report it as +/-10%?
So in your opinion, I have a case regarding the charge? just not to state categorically that the Z-pot is precise.
Also given that I have 1.5µm silica beads in ethanol I used the Huckel model, is this correct?
@ Daragh That's (+/- 10%) basically a good general rule. Look at what NIST provides on the certificate for the only primary mobility standard I know (SRM1980). This will represent a way best scenario. The secondary standard from Malvern Panalytical provides a +/- 10% tolerance. ZP can be affected by as little as 1 or 2 ppm of polyvalent ions so it's virtually impossible to get better precision here.
IMHO, yes, you can certainly say that the charge has shifted.
The model only changes the result by a maximum of 50%....
At 1.5 um would expect that you'd be getting some settling in EtOH. The field will hold this in suspension better than for a size measurement. But if you wanted a size measurement here (and I know you don't) I'd be looking at laser diffraction rather than DLS.
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