The Science Behind Antifreeze

October 1, 2015 | Sarah Tse

Ice crystals, frost
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Lowering the freezing point of water has numerous engineering and biological applications, and there are a few different ways to do it.

All systems that generate heat — from automobile engines to living organisms — need coolant fluids to absorb the heat and transfer it somewhere else. Water is a pretty effective coolant, but if i t freezes, it can expand enough to burst the rigid enclosure of an engine or electronic. To avoid icy explosions every time the temperature dips below freezing, we use antifreeze to change the water into a different chemical solution with a lower freezing point.

Antifreeze works because the freezing and boiling points of liquids are “colligative” properties. This means they depend on the concentrations of “solutes,” or dissolved substances, in the solution. A pure solution freezes because the lower temperatures cause the molecules to slow down. This allows the natural attractive forces between molecules to capture and bind them into rigid crystalline structures. But adding a different kind of molecule to the mix blocks those attractive forces and prevents the crystal structures from forming. The more solutes are added, the lower the temperature needs to drop before the solution can properly freeze.

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That’s why we sprinkle salt on roads and sidewalks to keep ice from forming in the winter. The salt and water mix into a solution that has a lower freezing point than water alone, so we don’t have to worry about ice until it gets much colder. But we can’t use salt as an antifreeze in mechanical cooling systems because of a few limiting factors.

In addition to dissolving in water — which salt does quite admirably — a useful antifreeze needs to remain chemically inert, meaning that it doesn’t interact with the surfaces of the system. That rules out salt, which corrodes metal. Antifreeze also has to be easy and safe to produce, and come with a high boiling point that will prevent the system from building up pressure. Ethylene glycol fulfills all those criteria. A 50% ethylene glycol solution freezes at -37 degrees Celsius (-34.6 °F) instead of 0 degrees Celsius (32 °F), which makes it pretty ideal for most engines.

Antifreeze in Nature

Since living cells are full of water, they’re also in danger of forming lethal ice crystals that can rupture the cell in subzero environments. But a few organisms have built-in mechanisms to avoid freezing to death. Some of them simply dissolve extra sugars and glycerol molecules in the fluids bathing their cellular structures, creating a sort of intracellular (meaning “inside the cell”) antifreeze.

But some organisms also make special “antifreeze proteins.” These proteins bind to the surfaces of very small ice crystals and prevent them from recrystallizing into larger, more lethal structures. Antifreeze proteins have been observed in bacteria, fungi, fish, plants, and insects.

Naturally, these antifreeze proteins can be used in a variety of medical and commercial applications. Researchers are testing their potential to enhance the preservation of transplant organs, prevent frostbite, and make fish and crops more resistant to cold temperatures. But the most important breakthrough in the application of antifreeze proteins may be the development of smoother ice creams that don’t form those annoyingly gritty ice crystals. After all, the importance of surviving hypothermia pales in comparison to enjoying a perfectly textured, frozen delight.

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