Water and ethanol are miscible so we can modify the aqueous phase pH to acidic around pH=2 to destabilize the SIO2 particles. The destabilization of the silica particles may be obtained also by adding salt to the ethanol solution.

Oh I see the question is not clear. We don't want to flocculate the dispersion, it should remain stable. I'm looking for the reason why it flocculated to avoid it in the future.

OK, if there is no water traces in the ethanol used, then this organic solvent is considered is a dry solvent. Thus the silica particles are not ionized in the dried ethanol solvent, they flocculate. To avoid this in the future, you have to add a non-ionic dispersant agent to the silica ethanol dispersion. Such dispersant should be a copolymer for instance and it should be soluble in ethanol (i.e. a block of the copolymer should be soluble in the solvent, whereas the other block should be adsorbed to the silica particles).

alright, this does not explain why other batches were stable and only this one flocculated but at least an idea of too small surface charge. Is it known how the surface charge in water changes while stepwise replacing water for ethanol. Maybe you can recommend any literature on stability of colloids in polar organic media?

Pleases download the following papers on Researchgate :

1) Article: Adsorption and surface properties of silica with transformer insulating oils

Amane Jada, Abdelouahed Ait Chaou, Yves Bertrand, Olivier Moreau

Fuel. 01/2002;

2) Article: Use of pyrogenic silicas for petroleum oil polar fraction characterisation. Fourier transform infrared spectroscopy and microelectrophoresis studies

A Jada, A Ait Chaou

Fuel. 01/2002; 81(13):1669-1678

3) Article: Stability Calculations of TiO2 Nonaqueous Suspensions: Thickness of the Electrical Double Layer

B. Siffert, A. Jada, J. Eleli-Letsango

Journal of Colloid and Interface Science 10/1994; 167(2):281–286.

4) Article: Location of the Shear Plane in the Electric Double Layer in an Organic Medium

B. Siffert, A. Jada, J.Eleli Letsango

Journal of Colloid and Interface Science 03/1994; 163(2):327–333.

Best regards

A. Jada

Thanks a lot!

if it happens in only one batch I would be rather sceptical on the batch quality and not use it for further investigations.

That's correct, the results obtained from that batch are not much reliable. I just want to understand what could be the cause of such "misbehavior" because the chemical composition of particles is the same as in other batches.

I would compare pH and conductivity between batches to get more inside.

I had this idea as well but the samples are pretty small, only a few ml, so that standard electrodes for pH and conductivity become useless. Maybe I have to put a different emphasis in my question to generalize it. Under which conditions do the silica particles flocculate in ethanol in general? Let's assume that I would want to coagulate them deliberately or to modify them somehow before dispersion in ethanol, what would be the options except for increasing the ionic strength of the solvent?

Would think of cationic polyelectrolyte, multivalent cations, or for depletion flocculation polymer.