Pure ethanol is probably the best bet for the sorts of temperatures being considered, or a part ethanol/water mix. Ethanol being an easily managed room temperature liquid that has a freezing point of around -110C, and a thermal capacity about 60% that of water (from memory - could be wrong). Apparantly gets a little "slushy" below -60C or so.
If I were chasing liquid temps down to -50C, as I've often considered, ethanol would be my first choice. Avoid methanol though, which has dangeroud vapors.
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