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(This is an interdisciplinary question, between molecular chemistry, materials science & construction. I think Chemistry SE is the right fit, but please move it if needed.)

Silane/siloxane solutions are applied to porous building materials (e.g. brick or concrete) to repel water. Unlike a solid coating, these sealants are "breathable", allowing water vapor to exit instead of being trapped, which would cause slow structural damage. How does that work, molecularly?

I presume these compounds affect surface tension in some way, reducing the ability of liquid water to permeate, without completely sealing the pores, but I don't understand it.

Optional: also, how does this sealant affects the permeability of other liquids (polar vs non?) and other gases?

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According to How silicone water repellents work, "Because silicones have a very open molecular structure, water vapor trapped in the substrate can easily slip through. Water molecules, however, are too large to enter."

If I remember correctly, siloxanes' Si-O bonds are not intrinsically hydrophobic, but the long molecular chains with outward-facing hydrophobic methyl groups, and thermal molecular motion, prevent liquid water from seeping through, which presents less of a barrier to water vapor and other low-molecular-weight gases.

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    $\begingroup$ The info about methyl groups is helpful, but that quote is nonsensical. Liquid & vapor, water molecules are exactly the same size. $\endgroup$ – Foo Bar Mar 6 '15 at 22:24
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    $\begingroup$ Not true! Water molecules are not just H2O but are strongly held to adjacent molecules by hydrogen bonds; see www1.lsbu.ac.uk/water/water_hydrogen_bonding.html. There is electron delocalization, implying, in effect, a "supermolecule". The much more diffuse water vapor has "broken" much of that bonding. $\endgroup$ – DrMoishe Pippik Mar 8 '15 at 1:42
  • $\begingroup$ Aha. This comment is exactly what I needed to know. In chemistry classes last millennium, hydrogen bonds were always mentioned in passing, not important enough to merit an entire lesson. But actually they are. blogs.scientificamerican.com/lab-rat/2011/08/02/… $\endgroup$ – Foo Bar Mar 10 '15 at 20:38

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