You need conductivity in the continuous phase in order to measure mobility - pure water does not conduct well (think electrolysis where a drop of sulfuric acid is added to the water to ensure conductivity). Poor conductivity leads to field distortion (so-called polarization) where the field is uneven around the electrodes. Typically 0.001M NaCl or similar is used as a background to prevent these issues. ZP should never be measured in pure water. As well as electrode polarization, dilution of the ions and any surfactants almost certainly will cause destabilization of the system. Measure in the mother liquor - the same ionic background as the starting continuous phase.

You need conductivity in the continuous phase in order to measure mobility - pure water does not conduct well (think electrolysis where a drop of sulfuric acid is added to the water to ensure conductivity). Poor conductivity leads to field distortion (so-called polarization) where the field is uneven around the electrodes. Typically 0.001M NaCl or similar is used as a background to prevent these issues. ZP should never be measured in pure water. As well as electrode polarization, dilution of the ions and any surfactants almost certainly will cause destabilization of the system. Measure in the mother liquor - the same ionic background as the starting continuous phase.

Thank you for your profound answer! Do you know some reference, in which they show, what happens when the zeta potential is measured in pure water?

Hi David It's the other way around - all the (correct) literature from the 1930's shows measurement in backgrounds of a conductive nature such as NaNO3 to KCl. No 'scientist' would show in literature the probable random results from an attempted measurement in DI water. Remember too that you are not measuring zeta potential directly - it's inferred from the particle electrophoretic mobility (in a non-electroacoustic measurement).

Hi Alan,

I just wonder how much the results would fluctuate in case of pure water. How much of KCl would be sufficient? In our case, we use a solution of cationic surfactant that creates a certain affect on the surface. With the addition of salt this effect decreases. Would be 0.5mM enough to get reliable results?

We are especially interested in surface zeta potential measurements that is measured with the surface zeta potential cell for the Zetasizer from Malvern.

The 'results' are meaningless in DI water as a uniform field cannot be reliably defined or produced. An assessment of stability over time would be the route to determine whether the results are reliable or not and also whether they tie well to a Critical Quality Attribute or Product Performance Predictor.

The primary reason is to "collapse" the double layers. In pure water, the ionic strength is so low that electric double layers are thick enough to interact with each other and affect particle motion. Adding salt should definitely be done for particle sizing via DLS since the random diffusion can be significantly slowed down (electrohydrodynamic retardation).

Liquids do not have to be conductive. I've made measurements of 5nm CaCO3 particles in dodecane, hexane, benzene, toluene, CFCs and other non-polar liquids successfully using Phase Analysis Light Scattering. The electrophoretic mobilities were 10-12 to 10-11 m2s-1V-1.

See:

Miller et al, "The Determination of Very Small Electrophoretic Mobilities In Polar and Nonpolar Colloidal Dispersions Using Phase Analysis Light Scattering", Journal of Colloid and Interface Science, 143, 532 (1991)

Miller et al, Colloid. Surf. 66, 197 (1992)

Hello John Your comments are correct. My comments related to the measurement of mobility in systems with conventional electrodes in a capillary cell. This is the standard route for aqueous systems where the link between ZP and (colloidal) stability is more accepted.. You are correct that liquids do not need to be conductive in order that a (uniform) field can be applied across them. With a small gap and a high voltage (e.g. in a thin cell with ITO electrodes) a uniform field can be applied and particle mobility can be measured. We routinely do this with specific cells designed for the purpose.

I note that you correctly refer to mobilities (rather than ZP) in such systems - the conversion to a zeta potential (and interpretation thereof) is not trivial in such cases and indeed ZP probably has little relation to stability in such non-conductive systems - size and density probably being more important in keeping particles in suspension.

Another aspect is that the surface potential and hence zeta potential of constant charge colloids depends on the ionic strength. If you use DI water, the ionic strength is very low, but will depend on the quality of the DI water preparation procedure. On the other hand, if you add 1 or 10 mM of KCl, the ionic strength is fixed. This surely holds if you want to measure the zeta-potential as a function of pH. As an example, 0.1 and 1 mM HCl is needed to create pH 4 and 3, resp. If you add this to DI water, the ionic strength varies by a factor of 10. On the other hand, if you use 10 mM KCl as background electrolyte, the ionic strength varies much less, so that the measured value reflects mainly the pH effect and is much less affected by the ionic strength effect.

Dear all, may I ask what happens if I use a concentration for the background above the suggested value? I was using a NaCl concentration solution of 0.2 M. Could this lead to some distortion in the values measured?

You will have a very difficult time getting any meaningful result for a number of reasons:

1. The double layer around your particle will be very thin and, unless you have some mechanism for repulsion, your system will aggregate.

2. At that salt concentration there will be very high levels of polarization at the electrode and you will most likely run into the formation of gas bubbles due to chemical reactions at the electrodes. If you have gold electrodes then you'll get a strong chlorine smell...

Successful measurement at such concentrations has been the goal of many instrumentation companies. The have some success. If you want to see a zeta potential *distribution* then you are pretty much out of luck. If you want a mean value then an instrument that uses my phase analysis light scattering method may work. However, there are many hardware factors to take into account, too. Commercial instruments do things in unique ways so one instrument may give you something vaguely meaningful whereas another won't. But how will you know which is which :)

Thank you John for your answer. I am using a Zetasizer nono series, therefore I suppose that the concentration is too high.

I will reduce this for the next experiments. Thank you!

Alan Rawle works for Malvern and he should have a much better idea about the Nano and any real-world data that may be available.

@Pierluigi electrophoretic mobility measurements at high salt concentrations can certainly be tricky... Aside from a few settings to limit exposure time, it is also good idea to check whether your sample has not changed (especially for sensitive biological samples) by performing a DLS measurements before and after the ELS measurement.

http://www.materials-talks.com/blog/2015/11/24/diffusion-barrier-method-the-practical-details/

http://www.materials-talks.com/blog/2017/09/05/what-is-the-zeta-deviation/

http://www.materials-talks.com/blog/2017/01/12/electrophoretic-mobility-tips-for-proteins/

@David adding a little salt makes many of the assumptions taken in the interpretation of electrophoretic mobility more valid. With no salt, it msy still be possible to perform measurements, however the result interpretation may not be entirely correct. In practical terms, quite a few samples like latex beads or proteins may start to aggregate in pure water.

Dear all may i know how much does it need to add KCL to the solution for providing NP sample for ZP test ?(also we have NP solution that it's PH is around 7 or 8)