MPG & supercooling

The base fluid of an engine coolant typically consists of a mixture of water and a glycol. Two of the most commonly used glycols are ethylene and propylene glycol. When comparing the physical properties of ethylene and propylene glycol, and mixtures of both with water, several differences can be observed. One of the most peculiar of these differences is the one in the freezing behaviour of both compounds, and of highly concentrated mixtures with water. While ethylene glycol (EG), and EG-water mixtures, have a clearly defined freezing point, propylene glycol, and PG-water mixtures with over 60% PG, tend to “supercool”.

In order to understand this phenomenon, and its practical implications, we first have to look into the chemical background. Ethylene and propylene glycol, even though they are chemically very similar, will be impacted differently by a decrease in temperature. This difference in behaviour has a number of practical implications. In the case of ethylene glycol and its mixtures with water, a crystalline solid will be formed when the temperature is sufficiently low (i.e. when it freezes). In a crystalline solid, the individual molecules will be neatly arranged in a predefined structure with a long-range order. This means that, based on how molecules are arranged in one section of the crystal, one can perfectly predict how molecules are arranged in the rest of the crystal. As the temperature of ethylene glycol in its liquid form is lowered, from a certain point onwards, it will start to undergo a volumetric expansion until the liquid eventually becomes solid at a certain, well-defined temperature.

In the case of propylene glycol and mixtures with water containing more than 60% propylene glycol, amorphous solids will be formed. In amorphous solids, there is no long-range order. This means that it is not possible to make predictions about how molecules are organized.  As the temperature of propylene glycol is lowered, it will behave differently than ethylene glycol. When starting from liquid propylene glycol or concentrated mixtures with water, and gradually decreasing the temperature, the viscosity of the liquid with gradually start to increase, but no actual freeze point will be reached. Furthermore, it will not undergo the same volumetric expansion. This phenomenon is described as “supercooling”.