(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?