I try to measure the zeta potential of sol-gel solutions but I never get good results, each time I get a different value. My formulations are between 75-95% alcohol and I use a zetasizer.
Adrian Angulo-Ibáñez
You can’t measure the ZP of a solution. The ZP is holistic property of particles in a continuous phase. Hence you need particles (and not a gel) plus a continuous phase with ions present. The electrophoretic mobility is measured and converted to ZP. Thus you need conductivity in the continuous phase. Usually around 0.001M KCl is adequate. Check the instrument with the secondary transfer standard. There is often enough conductivity in ethanol to allow measurement in the standard capillary cell. Alternatively, the dip cell can be used. I can't see that you'd have decomposition on the electrodes as you would for a protein, say, (and would used the diffusion barrier method to overcome).
"Thus you need conductivity in the continuous phase."
This is true if the instrument uses a constant current source to generate the electric field. If the field is generated by application of a voltage then the conductivity requirement is not necessary. You can make a charged particle in a vacuum move in an electric field.
Why some instruments use constant current sources eludes me. There is no advantage. Some claim that applying a voltage can lead to variable current. Well, Ohm's law tells you that a constant current will lead to a variable field under the same situation. The raw voltage and current graphs that a number of commercial instruments offer show this.
The Zetasizer uses a constant current (mainly as a selling point) so, yes, the background electrolyte is necessary but that's rather inconvenient if you want to investigate very low ionic strengths.
Adrian Angulo-Ibáñez You say "sol-gel" so I assume you mean particles immobilized within a gel network. If the particles are able to move at all then gel electrophoresis is the way to measure the electrophoretic mobility and, hence, estimate zeta potential. (This is how it used to be done before the advent of light scattering methods).
Otherwise, override the instrument's voltage and frequency for the electric field and apply a higher voltage at lower frequencies (if possible). Use the monomodal method which is the PALS method. I don't know the limit for the Zetasizer, but other PALS instruments can measure particle oscillations of less than 1nm.
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