Hydrophobicity and hydrophillicity both depend on the surface energies
of the various phases. Liquids and gases maintain a stable surface
energy, but solids do not. With the passing of time, solids tend to
drastically change their surface energies. Especially at higher
temperatures.

Silicon monoxide oxidizes in air at room temperature. It won't take
long till the hydrophobic properties of the surface degrade, and not
only because of oxidation, but also and perhaps mostly because of the
formation of nasty monolayers and adsorbates. (in fact, it is very
likely that SiO becomes an amorphous mixture of Si + SiO2 over time. I
hear that is, however, stable near the surface of stars. :)  )

That's why I'd like to reiterate my question to the author of the
first post in this thread, if depositing some metallic oxide via ALD
would be acceptable for her application. That's about the only way I
can think of creating a thermally stable hydrophobic surface.


On 11/19/09, Gareth Jenkins  wrote:
> This is not really my area so I may be totally wrong here but I always
> thought the hydrophilicity of SiO2 was down to silanol groups on the
> surface. At high temperatures I thought the surface becomes dehydrated to
> form pure SiO which is naturally hydrophobic (I think?). Keeping it stable
> over time may just be a matter of heating it back up again or keeping it in
> a dry atmosphere.
>
>
> On Tue, Nov 17, 2009 at 10:06, Miyakawa, Natsuki
> wrote:
>
> > Dear all,
> >
> > Does anyone of you know some methods to turn the surface of thermally
> > oxidized Si hydrophobic (say contact angle > 70°)? The hydrophobisity should
> > be long-term stable (> years @ room temperature), thermally stable (> 1h @
> > 800-900°C), and chemically stable (esp. under alkaline conditions).
> >
> >
> > Thank you!
> >
> > Natsuki