Freeze concentrating brandy

Yeah, above 30-40% alcohol you will not get it to freeze anymore at the temps you can work with. It will gel up instead of freeze.

I do not know the exact limit at which you can separate water from alcohol via freezing, but it is low, I would say that 40% is the absolute highest just off hand.

After much searching through the chemical libraries as a grad student, I did, at one time, find a phase diagram for water and ethanol. To get any ice to precipitate out from 40% ethanol, the temperature needs to be -40oC or colder. I don't remember the other points on the graph, but I also don't know if it will gel rather than crystallize- my few attempts at this didn't turn out very well.

Well lets see, I routinely precipitate nucleic acids out of solution with ethanol at -80 C...but I use 2 volumes of 70% ethanol, so the concentration is like 46% or so. There it does not freeze but it gels at that temp. And I know if you take 70% ethanol in water it will not freeze at all even if you put it in liquid nitrogen, it turns into gel there. I did have some freeze once in the -80 C...I think it was around 35%.

Of course, you are not goign to get that cold at hom in the freezer. Deep freezes run -20 C. even on a cold cold winter you will nto get down to -80C (unless you are in the far reaches of Alaska) that is about -112 F. There is actually an old story in the Alaskan Bootleggers Bible, one that is based on urban myth I guess about the dead Soldier (Miner). It goes that a soldier is bringing his commanding officer a case of booze during the dead of winter and swipes a biottle, hiding it in the snow. He later retrieves it and takes a swallow, but it is so cold it kills him on the spot because whisky will not freeze no matter how cold you get it.

Pure ethanol won't freeze- it simply gels, as you say. So any concentrated mixture of ethanol and water will gel rather than freeze when you put it in liquid nitrogen.

But I'm surprised that you don't see any water crystallizing out of a

46% ethanol-water mixture at -80oC. Is there a lot of other stuff in the solution?

Just some minor buffer salts...like in the low mM range. Sodium acetate I think.