Originally Posted by Mandingo

What is in the waterless coolant? The structure of a molecule means everything in chemistry. Just bear with me. I live for this stuff and I'm bored at work. *takes a deep breath*

Ethylene glycol looks like this: HO-CH2-CH2-OH. And water looks like this: H-O-H (but with a bend)

Ethylene glycol is obviously the larger molecule. It has a carbon-to-carbon bond and interaction between its two -OH groups which limits bond rotation and vibration. For these reasons, the bonds contained within the ethylene glycol molecule are not as free to vibrate/rotate as the weaker bonds in the smaller H2O molecule.

Temperature is our measure of this molecular "vibration" (also called Brownian motion). So water (a very unique molecule actually) with its bent structure and multiple ways to vibrate and rotate does a better job of "picking up" Brownian motion from the metal in one area and transferring that vibrational kinetic energy to another metal surface (the radiator).

The trade-off of course, is that water has a lower boiling point than ethylene glycol. If you're still reading you are awesome. Ethylene glycol is larger than water and has more potential for intermolecular interaction with nearby ethylene glycol molecules (due to hydrogen bonding and London dispersion forces). This makes it much harder for EG to enter the gas phase since more heat energy would be required to break those bonds/interactions with nearby molecules. Water's intermolecular interactions, while still relatively strong, are not as strong as EG's. Therefore, water is more willing to enter the gas phase with less heat energy imparted to it.