[secretsunset29]

Hi! This is my first post here

I would like some advice on how to dry a powdered slurry back into fluffy, free flowing powder form with minimal to zero moisture content.

I'm working with borosilicate glass microspheres. A very very very fine powder. 

The powder gets washed in distilled water to separate out the microspheres that float and the ones that sink. We want the floating ones.

The slurry with floaters then is filtered and dried on filter paper / Buchner funnel by vacuum.

The end result is a very dense puck that's fairly brittle after drying overnight.

Instead, I'd like to have a non-clumped, free flowing, very very very very fine powder that acts the same as the starting material.

I've started adding a final wash step with some ethanol to increase the drying efficiency and help remove the water, but it's still not the same fine powder consistency as before.

I'm wondering if there's a salt or solvent that helps to kind of push apart the surface interactions of the borosilicate glass microspheres, as I think that's why they are sticking to each other (or perhaps I'm completely wrong). I think there's definitely some sort of interaction going on between the microspheres and whatever solvent it's in. For example, microspheres in distilled or deionized water acts different than microspheres in unfiltered tap water. The microspheres form a plug in the tap water, but in dH2O or dI, the plug is much more easily agitated and returns back into slurry form easily after shaking.

Any ideas or comments would be greatly appreciated! Thanks!

[Borek]

I wonder if it is at all possible - glass is soluble in water (a tiny bit, but it can be enough when you have it in a form with huge surface/mass ratio), so my bet is that is dissolves and then gets glued together on drying. Because of the huge surface area the effect can be impossible to avoid.

Can you use other solvent?

[secretsunset29]

I wonder if it is at all possible - glass is soluble in water (a tiny bit, but it can be enough when you have it in a form with huge surface/mass ratio), so my bet is that is dissolves and then gets glued together on drying. Because of the huge surface area the effect can be impossible to avoid.

Can you use other solvent?

I should've added in my original post that it is possible to get dry and fine powder.

So the usual process is actually we do the wash in 2.5% HCl (in dH2O) and then neutralize with appropriate amount of Mg(OH)2. And then just rinse with dH2O and finish with some ethanol. This combination of steps can produce amazing fluffy powder at the end when it's all dried overnight by vacuum. Something about the HCl and Mg(OH)2 allows it to be a nice powder.

I'm trying to figure out a way to achieve the same dry fine powder without having to use HCl. I haven't tried just Mg(OH)2, but for some reason I'm thinking that's not going to work.

I have tried just using ethanol solvent instead of distilled water, but the end result is relatively the same, brittle and not powdery puck.

I'll add that the microspheres in distilled water as a slurry has an alkaline pH of about 9.5. May be relevant?

[Arkcon]

You have good instincts.  You do need some sort of ionic substance, to give the glass surface a charge.  Problem is, using a regular salt will leave it behind when dried.  Like Borek: explained, the very large surface area is making the substance clump.

If dilute HCl works, you can stick with it.  And instead of neutralizing, just vacuum it off.  See if that gives you the powder consistency that you want.

[wildfyr]

What about using trimethyl methoxy silane in dry methanol or ethanol to passivate the surface so you don't get Oswald ripening. Because glass spheres with such high surface area may be clumping because of that phenomenon. An amino silane may be a good monolayer coating too, it will be partially charged causing the spheres to repel better than a short alkane.

[secretsunset29]

What about using trimethyl methoxy silane in dry methanol or ethanol to passivate the surface so you don't get Oswald ripening. Because glass spheres with such high surface area may be clumping because of that phenomenon. An amino silane may be a good monolayer coating too, it will be partially charged causing the spheres to repel better than a short alkane.

Thanks, I'll try the trimethylmethoxysilane in ethanol. Any suggestions as to what concentration? I'm assuming pretty low, maybe 1% v/v?

I also spent some time reading about the Ostwald ripening. Do you think it's worthwhile to try an anionic surfactant? I know that the microspheres have a negative zeta potential. But I need something that will remain attached to the microsphere surface after washing/drying.

[wildfyr]

You can try to make an educated guess on concentration by calculating the surface area and multiplying it by the silanol/nm² that glass has. It should get you in the ballpark. You definitely don't want to have way too much, even tho its a mono-reactive silane they have the tendency to olimgerize anyways.

As a total shot in the dark 1% sounds reasonable haha. A little base catalyst like triethylamine is good to add too.

I don't know about surfactants, but trying both an anionic and cationic at a few concentrations seems like a good thing to try.