My brother-in-law recently bought a new car with the intention of taking it with him into the Rocky Mountains in a few years. Worried about sub-zero temperatures, he looked into modifying his coolant/anti-freeze system and learned about a new trend in the automotive industry. An increasing number of suppliers have begun marketing propylene glycol as an alternative to the widely used ethylene glycol because propylene glycol has a lower freezing point than its two-carbon sibling. A savvy chemist should instinctively think “freezing-point-depression,” a staple of freshman general chemistry lectures. Mathematically, it’s identified by the following equation:

Wikipedia will inform you that freezing-point depression is a relation of the Clausius-Clapeyron equation and Raoult’s law. Personally, I have not found use for either of these equations, and I’m not about to BS my way through the derivatives (pchem was not my strong point as an undergrad). Anyhow, the above equation tells us that a pure compound’s freezing point will drop if you add another solvent to the solution. For example, water typically freezes at 0 oC and propylene glycol freezes at 59 oC. According to Sierra Anti-Freeze’s literature, a 2/3 mixture (by volume) of propylene glycol and water will start to crystallize at -4 oC, whereas a 3/2 mixture will begin crystallization at -54 oC. In fact, the change in water’s freezing point is much larger with propylene glycol than with ethylene glycol.

Ethylene glycol is an interesting compound. The majority of households know that anti-freeze (i.e. ethylene glycol) is highly toxic and that dogs like to drink it. Upon ingestion mammals digest ethylene glycol by oxidizing it to oxalic acid (among other compounds), which has been linked to kidney failure. Ethylene glycol purportedly tastes sweet to humans and dogs, which has prompted manufactuers to begin adding a bittering agent. C&EN covered the issue a couple years back and from what I’ve recently read only Maine, Arizona, New Mexico, California and Oregon require bittering additives by law; Wisconsin’s right behind them. For whatever it’s worth, the bittering agent (denatonium benzoate) is sold commercially as Bitrex or Aversion and is added to products such as rat poisons to prevent human consumption. By comparison, propylene glycol is generally regarded as a safe, non-toxic chemical.

If propylene glycol is safer and appears more advantageous than ethylene glycol, why does the auto industry use it? I couldn’t find an answer. I did learn that some automobile manufacturers will actually void your warranty if you do not use their “approved” anti-freeze (usually ethylene glycol). Perhaps this issue will be addressed by the Obama-appointed US car czar in January.

