What are your nanoparticles protected with? I would assume whatever your protecting layer is soluble in will be similar to what your particles are soluble in.

Ethylene glycol

As alumina has a high IEP (around 9), it is best to dilute with a slightly acidic solution to ensure a high surface charge density. As a matter of fact, this only holds provided that the surface of the alumina was not modified, as this can have a large impact on the IEP.

If your nanoparticles have been functionalized or protected you should try the solvents that interact with the protecting agent through like dipole-dipole interaction or hydrogen bonds....etc., otherwise dispersion of non protected nanoparticles depend on particle size and on their affinity not forgetting the suitable pH value for stability issue

Dear Dr. Cademartori:

The decision of a proper fluid polar (H2O, acetone, ethanol…), no polar for colloidal dispersion of alumina ( aalfa-alumina) seems be connected with the application of the particle/nanoparticles of alumina. In fact, seems that a fraction of water is necessary despite, of selection of a no polar fluid at dispersion. This feature seems correlated to the development of a hydration shell with the function of improvement of interaction of surfactant agents. A priori, this fraction of water ha not a vinculum with to the colloid stabilizing phenomenon (also flocculating process), since any density of charge is created on the surface of the particle/nanoparticles and a steric structure does not exist to avoid the flocculation. Current understanding of this phenomenon involves adsorption of hydroxyl groups and its capability to form hydrogen bonds. As a whole, the alumina surface exhibits a hydrophilic character. Further stabilization is only reached when proper surfactant and/or co-surfactant covers in an integral way the particle/nanoparticle surface. I think that hydroxyl groups present at alcohol might simulate in a mimetic way the presence of hydroxyl groups of water in non aqueous system, but I can be wrong. Dispersion of alumina, at aqueous medium, with particle of nanometric size can be stabilized from several phenomena, as follow: electrostatic (tri-ammonium citrate, Tiron (Aromatic sulfonic acid))…, electrosteric (acetic acid, citric acid, Darvan C (ammonium polymethacrylic acid)… and steric (long chain molecules). Significant amount of nanopowder can be easily dispersed in an acid medium, without surfactant addition. A medium with weak acid character, a steric stabilization can be tried. Please, consider read the follow paper:

https://www.princeton.edu/~cml/assets/pdf/8871-04cesarano.pdf

I would like to comment that prior to start the colloid preparation is fundamental a deep cleaning of particle/nanoparticles surface reaching a surface absent of gas and vapor adsorbed. For this, a thermal treatment between 200 oC and 300 oC on vacuum or inert atmosphere can be adequate, cooling under constant vacuum or inert atmosphere to room temperature. Alumina is an amphoteric oxide with a slow kinect of surface covering. It is possible that complete cycle during two or more dozens of hours.

Kind regards

Marcos Nobre (M. A. L. Nobre)

Alumina particles dissolve in water. Therefore aqueous media maybe a bad choice for long term stability especially in the case of nanoparticles. Proper stabilization in nonaqueous media maybe a better choice.

Thank you for all the answers. My nanoparticles are not functionalized or protected.

Kind regards,

as your particles are not fictionalized by any groups. As stability of any dispersion depend on the zeta potential of the dispersion. By knowing the  exact IEP and adjusting the pH of aqueous solvent we may get stable dispersion. pH effect the zeta potential of particles. by making different dispersion in water with different pH and observing the stability , we can go for most stable dispersion. Solid loading also has a bigger effect on stability of dispersion so that also should be considered.