I would like to make an alumina dispersion in absolute ethanol, with a alumina content about 20 volume %. The viscosity and agglomeration of the dispersion must be as low as possible. The alumina powder is fine (about 200 nm in size) and pure (99,99 %). Literature on water dispersion is extensive but articles on ethanol based dispersions are rare. Water must be avoided. What kind of additive should I try?
Dispersion of powders in ethanol occurs more easily than in water, so no additives of surfactants are needed. To effectively break down agglomerates, you will need a bead mill or ball mill with small aluminum oxide balls.
Marc Singlard How is it going? have you tried ultrasonic waves to enhance your dispersion?
At the moment it seems that dispersion with only alumina and ethanol leads to highly viscous sample (about 1000 mPa.s at 20 v.% alumina instead of 5-10 mPa.s for a well aqueous dispersed alumina). I tried US (sonotrode from sonics) but without any improvement. Next step is ball milling. I wonder if dispersion in ethanol without additive is not possible only at low concentration like 5 v.%.
Marc Singlard Sonication in volatile solvents is particularly poor. Most of the input energy is used to volatilize the solvent and in creating boiling/bubbles. Water is by far the best liquid to transfer ultrasound energy. Wetting is normally easy in ethanol for alumina, so I don't think an additive would help unless it was to stabilize any milled product. You might look for a 50 kDa PEG or PEI if you have substantial recombination of particles after or during the milling process. The comminution limit for alumina is about 2 microns though - see Table 3 in the attached.
For general stuff on wetting, separation, stabilization (the 3 steps in making a stable dispersion) see this webinar (free registration needed):
Dispersion and nanotechnology
https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat
What is your evidence the the alumina is ~ 200 nm in size? I suspect it's a collection of sub- and post-micron aggregates and agglomerates.
Alan F Rawle Thank you for your response. Size is measured by SEM and DLS and in good agreement with TDS. My dispersion in ethanol look like an aqueous dispersion without dispersant, i.e. yogourt like. It is not difficult to put alumina in ethanol but viscosity is very high.
Here are the experiments I carried out (basis of formulation: 20 v. alumina in ethanol, magnetic mixing):
- basic formulation --> high viscosity
- basic formulation + US dispersion --> same as above
- basic formulation + citric acid --> same as above
- basic formulation + citric acid + acetic acid --> same
- basic formulation + PVP (polyvinylpyrolidone) --> same
- basic formulation + ball milling --> same
- basic formulation + PEI (polyethylenimine) --> low viscosity (30 mPa.s)
It seems that, at least for the powder I used (in fact I tried with 2 powders: 200 nm 99.99% and 400 nm 99.8%), alumina needs a dispersing agent to produce low viscosity suspension at "high" (20 v.%) concentration in ethanol. Some improvements are needed in term of agglomeration but ball milling should be enough.
Marc Singlard Good, the PEI seemed to do the trick… The PEI is a steric stabilizer, not strictly a ‘dispersing agent’…
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