Why does coolant volume contract when diluted?

If you mix 100 mL of water with 100 ml of water, at constant temperature and pressure the total volume will be 200 ml. Similarly, adding 100 ml of glycol to 100 ml of glycol will yield a final volume of 200 ml glycol. If both the water or glycol volumes are initially at an identical temperature, the mixture will be the same identical temperature. However, when volumes of water and glycol are mixed, the final volume is no longer the sum of the two initial volumes. The final mixture volume is typically 1 to 3% smaller than expected, depending on the type of glycol, the temperature and the mixing ratio. Besides this instantaneous volumetric contraction, mixing also generates an exothermic reaction, with heat being released into the environment. It turns out that the larger the volumetric contraction, the higher the heat release.

Water and glycol are very different components structurally, but what they have in common is that they both form hydrogen bonds. It is the rearrangement of these bonds when mixing the two that causes the contraction and heat dissipation.

There are two ways of looking at these complementary phenomena.

Thermodynamic level
The first approach is a thermodynamic one, in which the glycol/water mixture can be seen as fulfilling the basic laws of thermodynamics, namely maximum entropy (disorder) and conservation of energy. Maximum entropy is reached by breaking up the highly ordered, hydrogen bond-rich structure of pure water. As the water hydrogen bonds represent a high amount of chemical energy, and present in glycol/water mixtures at higher than average levels, heat is released and dissipated into the environment. The heat release of the water/glycol mixture results in mechanical work (pressure x volume change) by the atmosphere on the liquid volume, which forces it to contract. Overall energy neutrality of the glycol/water system must be guaranteed.

Molecular level
A second approach is to look at it at a molecular level. Glycol molecules, like water molecules, can form hydrogen bonds. However, in the case of glycol, two bonds can form between adjacent molecules, yielding a gauche configuration in which the vicinal (= neighbouring) groups are separated by a torsion angle. As a consequence, the repulsion between the pairs of glycol molecules in pure glycol is relatively strong. The addition of water enables the glycol molecules to form more spatially relaxed hydrogen bonds with the water molecules, creating intermolecular cavities into which the smaller water molecules can "fall". Rather like mixing fine sand and pebbles, mixing 1 litre of both will give you less than 2 litres because the fine grains of sand fit between the coarser pebbles.