Srinivas Banala CeO2 has a density of ~ 7.2 g/cm3, so it'll be very difficult to suspend micron sized particles of such a material in pure water. Carry out a Stokes' Law calculation to determine what sized particles of this density could remain in free suspension. I suspect it'll be around 150 nm. The largest CeO2 stable suspension I've seen had a mean particle size of ~ 130 nm. You may need to mill the particles in your system. I'm not sure what you mean by 'assist floating'. If you carry out an experiment similar to froth flotation then your particles will be attached to air bubbles and floating (thus not in suspension). If you mean in free suspension, then you can increase the concentration of the particles as this increases the mixed density of the system and thus reduces settling. A 10 wt% of CeO2 in water has an effective density of (0.1*7.22)+ (0.9 + 1) or around 1.6 g/cm3 and (again) Stokes' Law will indicate increased (hindered) settling times. Polymeric coatings aren't likely to reduce the density of the CeO2 by much as you'll only be able to incorporate a monolayer or so. This isn't much in relation to a particle of size around a micron.

You talk of 'stop sticking to the other surface'. This would be the function of surfactant. Try a liquid washing up detergent at around 0.5 - 1 wt%. This should decrease any sticking tendency. Or go for a sophisticated solution such as Tween, Daxad, or Triton X-100 again in the 0.5 - 1% range. More details on dispersion of nanomaterials in this webinar (free registration required):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

The Wurster fluid bed coating process enables film or melt coating application on particles; thus, particle density could be altered to that of water with application of a suitable low-density coating. Unfortunately, the process is generally applicable to core particles near 50 µm and larger. Regardless, the amount of coating needed to reduce your particle to that of water is excessive. For example, reducing a 7.2 g/cc particle with a 0.9 g/cc wax would require a particle weight composition at 1.587% CeO2 and 98.413% wax coating. Any deviation from this composition would result in particle density shifts up or down and particle drift up or down in water.

If your application allows multiple CeO2 particles within a wax particle, this becomes more feasible with the application of spray chilling technology. With a proper weight ratio of CeO2 particles and wax, the molten suspension could potentially be spray chilled off a spinning disk or through a nozzle. If the weight ratio remains homogenious in the particles, the density should match that of water.

To address any agglomeration tendencies, addition of a surfactant, as noted above, should help through introduction repulsive charges on all particles or surface modifications that mitigate agglomeration and improve wetting properties.

Charles Frey A very good addition to the discussion. The question does ask for water soluble coatings, but use of a surfactant could help disperse hydrophobic materials. Indeed I’ve been able to disperse Al flake in white sprits (Stoddard solution) to enable measurements in water (rather than more expensive alcohols or white spirit themselves) by spatulating with neat surfactant.

We often have requests for water-soluble coatings even when incompatible with the design. For example, a water-soluble coating on particles in an aqueous suspension where the coating must be shelf stable for an extended time - there is nothing to prevent coating dissolution once the coated particles are added to the water even if an insoluble coating is added to protect it. All film coatings have some degree of permeability, which allows water penetration to the water-soluble layer.

If particles in this case are being added to water, why would a water-soluble coating be needed; the coating will just dissolve when they are added. I assume it is needed for other purposes such as particle suspendability or to mitigate agglomeration.

There are also weak acid and weak base polymers that have pH dependent solubility depending on there protonated state that offer pH controllable water solubility.

Dear Alan, dear Charles,

Thanks for your answers.. I should have posted more details to make the problem clear, so I am sharing some more details now here.

the micrometer sized CeO2 particles were planned/used for Glass-polishing; the commerical CeO2 polishing materials have an undisclosed coating that makes those particles float in water (for polishing process, it is usual to make avg. density of 1.01 to 1.02 g/cm3), and the coating somehow is important that keeps the particles in water and stops them 'sticking' to polishing disks which are of metal oxides.

So we explored some polyacrylate polymer, with over 10% w/w, we can make particles to float, however these particles tend to stick to the polishing disk surface compare..

So we are looking for a suitable coating, that can allow the polishing process to smoothly.

Thanks for your replies agian,

with best regards

Here are a few coating options you could consider:

Srinivas Banala It may be that your particles are not actually 'floating' due to density reasons but are simply not wetting in the liquid (and thus would resist the water on top due to surface tension considerations). What happens if you do a beaker experiment with a small amount of the material ('floating' on the surface) and you add a little washing up liquid to the system? Stabilization of ceria is normally with the optimum amount of phosphate once the particles have been dispersed in the liquid. For further information, see this webinar (free registration required):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

I agree, it may be a "wetting" mechanism that maintains surface floating. Otherwise, it points to a density consideration such as air entrainment within the particles.

Another potential option might be a guest-host complex. This would depend on the ability to insert the CeO2 into a porous host particle. Host materials that come to mind include a commercially available highly porous starch that is capable of absorbing liquids at up to 50% or more of its weight. Silica materials with similar capacities are available, but I expect the silica structure might be a problem. Cyclodextrins also come to mind. If CeO2 is suspended in water or other suitable vehicle and added to the absorbent host to a level within its absorption capacity (where the particles remain free-flowing) and the CeO2 particles are small enough to be carried into the structures, the hydrophilic surface of the host would likely provide good wetting properties. Depending on loading, it should offer a more favorable specific gravity.