At first glance, it might seem that a mixture of water and ethylene glycol should freeze at a temperature between the freezing points of its components. After all, pure water turns into ice at 0 °C, and pure ethylene glycol crystallizes at approximately –13 °C. However, in practice, everything happens quite differently.
According to Raoult's laws, the crystallization temperature of a solution should be lower than the freezing temperature of the pure solvent. If a small amount of water is added to ethylene glycol, its freezing temperature will indeed drop slightly below –13 °C. But the most unusual result is achieved with a different ratio of components.
If water and ethylene glycol are mixed in approximately a 1:2 ratio, the solution can remain liquid down to –67 °C. This value is significantly lower than the freezing temperature of each substance individually.
Why the Mixture is More Frost-Resistant
The reason for such unusual behavior lies in the molecular structure of the solution.
Ethylene glycol molecules position themselves between water molecules and prevent the formation of a regular ice crystal lattice. At the same time, water molecules similarly disrupt the ordered arrangement of ethylene glycol molecules themselves.
As a result:
- neither water nor ethylene glycol can crystallize as easily as in their pure forms;
- the degree of molecular disorder significantly increases;
- the freezing temperature of the mixture becomes lower than the crystallization temperature of each component.
The maximum effect is achieved precisely at a 1:2 ratio of water to ethylene glycol, when the level of molecular disorder is highest.
What an Eutectic Mixture Is
A solution with the specified component ratio is called a eutectic.
At –67 °C, such a mixture transitions from a liquid to a solid state all at once, without separating into individual phases.
If the concentration of ethylene glycol differs from the eutectic, the freezing process proceeds quite differently.
For example, in a solution with an ethylene glycol content of about 50%, pure ice crystals begin to form at –35 °C. As they appear, the concentration of ethylene glycol in the remaining liquid increases, and its subsequent freezing temperature becomes even lower.
Instead of rapidly turning into a solid, the solution gradually becomes a thick mixture of liquid and ice crystals.
Why Antifreeze Doesn't Damage the Engine
The resulting mass differs significantly from ordinary ice.
It has several important features:
- does not increase in volume upon freezing;
- does not create destructive pressure on the engine block;
- gradually loses fluidity with further cooling.
However, in severe frost, such a mixture can become so thick that the cooling system pump can no longer circulate it through the engine's cooling jacket, hoses, and radiator.
The temperature at which the solution stops circulating normally is called the pour point.
Viscosity Also Matters Greatly
When choosing the composition of antifreeze, it is important to consider not only its crystallization temperature but also the change in viscosity.
Thus, at –13 °C, the viscosity of pure ethylene glycol is approximately 21 times higher than the viscosity of water at 0 °C.
Even a eutectic mixture remains significantly thicker – its viscosity is almost 9 times higher than that of water.
Therefore, in real operation, a different compromise option is used.
For most regions of Russia, where winter temperatures rarely drop below –40 °C, a 50% ethylene glycol solution is usually used.
This solution combines several advantages at once:
- the crystallization onset temperature is about –35 °C;
- the solution maintains acceptable fluidity;
- at –35 °C, its viscosity is only about 5 times higher than the viscosity of water at 0 °C;
- the cost of such antifreeze is lower than that of more concentrated formulations.
That is why mixtures with an ethylene glycol content of about 50% are most common in automotive cooling systems. They provide sufficient freeze protection for most climatic conditions while maintaining the characteristics necessary for the normal operation of the engine cooling system.