Re: How to get SiO2 paticles from ethanol suspension ?

From: Uncle Al (UncleAl0_at_hate.spam.net)
Date: 10/15/04

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    Date: Fri, 15 Oct 2004 08:21:06 -0700

    Yung-Jen Lin wrote:
    >
    > I made silica particles from TEOS (tetra-ethyl orthosilicate), which
    > was dissolved in plenty of ethanol and gelled by adding ammonia water.
    > The silica particles are spherical with < ~ 0.3 micrometer in diameter
    > and could suspend in ethanol for a long time. Previously, I was
    > patient enough to wait for 2-3 weeks to let it settled to the bottom
    > of the beaker. Then, I syringed away the clear solution in the upper
    > part and dried the bottom part in an oven to get the solid particles.
    > Recently, I have a strong feeling that there must be a better way. So,
    > I try.
    > Centrifuging is certainly a good way. Unfortunately, the machine is
    > broken at this moment.
    > Filtration is also possible. But it is quite slow because the
    > particles are so small. The filter membrane needs to have small
    > openings.
    > Adjusting the pH by adding nitric acid in order to manipulate the
    > electrical double layer and coagulate. But this seems not woking,
    > maybe because of the low dielectric constant of ethanol.
    > The last method occurs to me is to add some kind of polymer or
    > surfactant. But I do not have any idea of which kind.
    >
    > Does anyone have similar experience? What is the best way to get those
    > silica spheres out of ethanol? Thanks.

    Can you rotovap the suspension?

    Depends on the volume being processed. Adding benzene (not in the
    US!) or toluene and azeotroping out water and ethanol with a
    Dean-Stark trap (with a stop***) could be nice. The much lower
    density and viscosity of the organic solvent will facilitate isolation
    by settling, centrifugation, or filtration. Benzene freeze-dries
    exceptionally well. Will your particulates come along for the ride in
    the vapor stream?

    Benzene is mildy leukemogenic with chronic exposure. Good lab hygiene
    is implicit.

    A second possiblity is to have a low density low dielectric constant
    solvent miscible with ethanol like ether or perhaps cyclohexane. That
    should dump out the denser microspheres. On any large scale you would
    have to recycle solvent. 

    -- 
Uncle Al
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 (Toxic URL! Unsafe for children and most mammals)
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