Antifreeze (the glycols) works to depress the freezing point below EITHER the freezing point of either constituent because of molecular shape--crystals of solid glycol or water can't form (the periodic structure required is interrupted)--technically, it vitrifies when the temperature is low enough (which looks like freezing). When the proportion is off the optimum, the freezing point increases, because crystals can form to a greater degree. Hence, why the lowest freezing point is with a mixture.
Boiling is the other way around, both propylene and ethylene glycol have higher boiling points than water, thus the mix boils (total vapor pressure of both constituents combined equals local ambient pressure) at a higher point, thus, the boiling point increases with increasing glycol fraction, and doesn't go down.
However, both the thermal conductivity and heat capacity are lower for the glycols than water (which is why race engines don't run antifreeze), thus, you should run the minimum quantity of antifreeze necessary to meet both the freezing and boiling limits to maximize heat transfer.
