I am working with brine-stable polystyrene latexes, which are colloidally stable at high electrolyte concentration and aggregate in distilled water. I will like to measure the zeta potential of the particles, and I am looking for advice about ideal solid contents concentration and conductivity of the media to perform the measurements. I have access to a Malvern Zeta sizer nano zs.
You need background conductivity for mobility of the particles and avoiding electrode (field) polarization. A concentration around 0.001 M NaCl is normally considered adequate and this could be increased to 0.1 M without issue.. At higher concentrations (say 1M), Joule heating may arise but this can be compensated by lowering the voltage (some broadening of the peak), reducing the measurement time, or the diffusion barrier technique. Ideal solids concentration will depend on the size of the material but remember that zeta potential (not mobility) refers to an infinite dilute system without particle-particle interactions.
Dr. Rawle, thank you very much for your -very quick- answer. That's is exactly what I was worried since I'd like to perform the measurements in a salt concentration range from 0.01 to 0.5M(k~40 mS/cm for the latter). I am still confused about how the diffusion barrier technique reduces Joule heating, though. I understand how It prevents contact of the material (organic in my case) with the electrodes to prevent degradation or aggregation but since the conductivity is the same I don't understand the decrease in Joule heating. Can you please clarify this?. From what I can gather in the Malvern's technical notes I am better off reducing the voltage and performing a Monomodal analysis instead of analyzing the complete distribution.
Hi Cristiam It's exactly as you say - the diffusion barrier method doesn't reduce Joule heating if the solution above the sample was the same ionic concentration/salt content as your actual material to be analyzed. However, if your material is contained in (for example) 1M NaCl you can add this to the bottom of the cell (thus saving valuable sample and preventing degradation of the sample if it's protein-like) and then use 0.001M NaCl to very carefully top up the cell. In that way the electrodes will see a more dilute NaCl and this will reduce the heating effect. The particles will, of course, still be in 1M NaCl and be exposed to the field (voltage/distance) which will be unchanged. Personally I prefer to try to reduce the time rather than the voltage to avoid peak broadening.
BTW, it's extremely unlikely to have a significant zeta potential distribution in polystyrene latex materials. I'd be expecting to see single sharp peaks in most cases.
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