Soumia Cheddah What is your reason for wanting the zeta potential (ZP) of a material in an organic solvent? What is the link beween ZP and some critical quality attribute or product performance indicator? Do you have a relation between ZP and stability in such a system? 3 points:

• Density and size are probably more important for stabilization considerations
• As the conversion from mobility to ZP is only trivial in aqueous based systems, then how will you convert mobility to ZP?
• How will you get sufficient conductivity in the organic phase to avoid polarization of the field around the electrodes?

One question: which organic solvent(s)? Alcohols will be a lot easier to work with than aliphatic hydrocarbons, for example.

Thank you Alan F Rawle for your quick reply

Actually I am not an expert in interface and colloid science, but I am working with the assembly of colloidal crystals. Through experiments, the particles assemble differently in different solvents and I wanted to understand what kind of forces are behind that. usually electrostatic repulsion is the stabilizing force but my particles have been modified with hydrophobic moieties. Therefore, I am looking for a test to help me understand and interpret the results I obtained, so I thought of zeta potential to estimate the extent of electrostatic interaction.

To convert mobility to ZP, I found a paper who used Hückel approximation and their dispersion was in hexane.

The solvents I am using are of wide range of polarity going from aliphatic hydrocarbons to alcohols

Yes, you can. Specifically (emphasizing Alan F Rawle 's point), you can measure the electrophoretic mobility. Estimating zeta potential from that is a different matter and problematic in low dielectric constant media.

The technique used is phase analysis light scattering (PALS - see my profile). I developed it specifically to measuring mobilities in non-polar media. Many commercial instruments offer the technique but vary in their ability to perform well with highly non-polar samples.

I've used electrophoretic mobilities measurements of calcium carbonate particles in engine oil to infer mechanisms leading to the breakdown of the oil by aromatic hydrocarbons. Even though electrostatics may not have a stabilizing role in such systems, measuring the mobility can be a useful probe to understand the physical behavior (e.g., surfactant desorption).

Miller, John F., et al. "Electrophoretic studies of calcium carbonate particles dispersed in various hydrocarbon liquids." Colloids and surfaces 66.3 (1992): 197-202.

Soumia Cheddah Your answer is a good one, so you have done your research well. You will have great difficulty taking any measurement in hexane because of the volatility of the solvent leading to convection currents. Most measurements of mobility (and I caution you to use this measured parameter rather than ZP) are made with the Dip Cell (assuming you're using a Zetasizer). However, if your using ethanol (EtOH) or isopropylalcohol (IPA), then the standard disposable cells can be used as sufficient field can be generated in these polar liquids.

Your major issue is interpretation of the results as the counter ions are bound very tightly to the particle and thus the field/charge falls off dramatically from the particle surface. Again I repeat my question: if you have a ZP value how does that help you to gauge the stability of your system?

The ability to measure in volatile liquids depends a lot on the implementation of the technique. I successfully measured in hexane, toluene, benzene (!), dodecane, R113 (a refrigerant), dioxane, aviation fuel, ethanol etc. And water. There are three reasons for this:

1. Efficient cooling

2. Tracking the convection velocity to minimize its contribution to the phase response of the instrument. As far as I know, only Brookhaven implement this.

3. Building an instrument primarily for measuring mobility. Commercial instruments are optimized for sizing. Mobility measurement is almost an after thought.

Challenges with these also include keeping the liquids dry. Trace amounts of water (ppm) can influence the results and the behavior of the dispersion.

There is an ever-increasing body of research that shows that electrostatic phenomena can affect dispersion properties in organic media. Numerical treatment is difficult, though. Concepts such as the Huckel approximation may have some validity in, say, methanol, but are most likely meaningless in hydrocarbons.

Thank you for your answers John Francis Miller and Alan F Rawle

I understand from your answers that the measure of electrophoretic mobility is possible but its conversion to a ZP is meaningless.

I will check the work shared by John Francis Miller to see if I can implement that.

To answer the question of Alan F Rawle , I am not trying to measure ZP to see the stability of my system, but I only want to check the dispersions that are stable (visual inspection) because my particles form a colloidal crystal in only few of them (iridescence by visual inspection). According to many papers, the forces implicated in the stabilization of the crystal are repulsive electrostatic forces but I am not sure how to test in which dispersions they exist and how strong they are.

From the reply of John Francis Miller I can tell that electrostatic phenomena in organic media are not well-understood yet.

Soumia Cheddah For crystallization and interactions, you may wish to look at the second Virial coefficient, B22. This approach is found in protein chemistry a lot. I attach some application notes. You may wish to contact your local support in Shanghai (if this is where you are based). I assume you're using a Zetasizer but you haven't confirmed this yet.

Also a webinar:

Speed Your Way Through Protein Formulation Screening

https://www.brainshark.com/malvern/vu?pi=zGXzfTDwXz5chz0

Soumia Cheddah Back to the question. One measures mobility and one infers zeta potential (from the mobility and a bunch of assumptions).

Thank you Alan F Rawle for the valuable information

In a colloidal crystal, the particles are very close together - much closer than traditional theories for electrostatic interactions can model.

You shouldn't be concerned about how zeta potential is calculated. From what you have explained, what is important is to see if there is a link between your colloid crystal behavior and any information you can obtain via electrophoresis. Techniques that use light scattering require optically transparent samples which usually means dilute as far as particle concentration is concerned.

Electrostatic phenomena in non-polar dispersions are difficult to measure, interpret and control.

John Francis Miller your answers were very helpful. Please, I would like to know in which range of concentrations I should be preparing my dispersion of silica particles and if NanoBrook ZetaPALS will give me directly the values of mobility. Also, how can I keep my solvents dry ?

Soumia Cheddah Typically, you want of the order of 108 particles per milliliter. Start with a concentration such that, when placed in a 10mm cuvette, it is just too opaque to see through clearly. Do a series of two-fold dilutions until you can see through it clearly. Putting some printed paper behind the cuvette is a useful "trick". If you can read the print clearly then the sample should be dilute enough. Obviously, you don't want to diluted too much. Put samples from each concentration in the instrument and note the count rate. Use the concentration that best meets the instrument's preferred count rate. Note that if you have a very nonpolar liquid and appreciable charge then the particles may interact if the concentration is too high.

Regarding keeping the liquids dry, I've attached the relevant pages from my thesis that describe the procedure I used. (Yes, hand-drawn and typed!)

Soumia Cheddah

Note that if you have a system with a 90 degree light passage (square cuvette) through the sample then the concentration range is more limited. With a back-scatter mode then the path length is optimized for the count rate. A wider concentration and top size limit is the outcome.

So far I measured elecrophoretic mobility in three solvents (DMF, THF and acetone). The results in acetone vary too much from one measurement to another . Is this because of convection currents? (the temperature I set in the instrument was 25 degree C). Next, I am planning to repeat the measurements at low temperature

Soumia Cheddah Please share data from each liquid so I can compare them.