That’s a good idea, but there is one problem. When you use ionic solutes, you have to worry about two things: interactions with the radiator and residues formed around the seals. Most readily available ionic compounds will chemically interact with the wall of the radiator, corroding it or otherwise breaking it down. Although the freeze protection would be greater, the trade-off of shorter radiator lifetime is not worth it. Also, ionic compounds (which are all solids) tend to leave solid residues around the seals. This leads to leaks in the cooling system which, once again, is a poor trade-off for good freeze protection.