If you are willing to deal with potential galvanic corrosion, condensation, leaks, replacing fluid every now and then and so on I suppose you could use red or yellow radiator coolant from your local car service shop. It has all the properties you’ll want from a coolant liquid, but as others have already mentioned, it’s not really worth the hassle. Atleast if you’re not running something really power hungry, like GPU farm. And even with liquid you have the very same problems than air, you need the heat to go somewhere. So either have very long pipes (and pumps to match them) so you can have the radiator on a different room/outside or big fans to move the hot air away from radiator.