Rubbing Alcohol, or iso-propanol comes as you say in 91% or 70% depending on how much water it contains. Either way, water is attracted to alcohol like a nail is to a magnet. Naturally, if left un-capped, alcohol will absorb water out of the atmosphere.
It is an excellent cleaner/solvent for black powder use (but then so is Windex w/vinegar), and that contains alcohol too! I use 91% rubbing alcohol swab after using hot water and soap to clean my muzzle loader with a bore mop. It gets rid of the last vestige of water that may linger to cause rusting and doesn't leave any residue behind.