Anti-Freeze and the Automotive Industry

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. 

2Ethylene 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. 


  1. I don’t know why car companies would void your warranty by using it, but I imagine that the breakdown products of propylene glycol might mess up your engine- a bevy of organic, polymerizable acids and such. Apparently it’s more prone to oxidation than ethylene glycol in engines, too.

  2. Cars run at an optimal temperature. Cooler is not better. Too hot is bad, and too cold is bad too.

    People call it anti-freeze, and it is true it does that. But at the opposite end of the spectrum, you have freezing point depression as well as boiling point elevation! And this is the most important aspect of anti-freeze unless you live in a really cold climate, is not that it is an anti-freeze, but a coolant! It will not only lower the freezing point more, but raise the boiling point more as well. This essentially can play havoc on the cooling system. Many people think the radiator cap is just a cap, but it is one of the most vital parts of the cooling system (and in some new cars, it is no longer in the cap but in a seperate place in the radiator). The amount of coolant is usually controlled by pressure. The excess coolant/anti-freeze is pumped into a reservoir for when needed later (typically by a little valve in the cap). If your system has a higher boiling point than all these parts are designed around, it will have lower pressure. In short, you will have too much anti-freeze in your radiator most of the time. Your car will run cooler. Also, it will be able to remove more heat from the engine, as it has more “room” to increase temperature before it builds up to the appropriate pressure. That is not good for efficiency or the health of the car.

    A lot of effort is put into trying to maintain a certain temperature.

    It might seem like a good idea, just like most people think putting premium in their car is a good idea. It is not, unless your engine is designed for premium (which unless it is a high power engine it is not). You are just hurting the fuel efficiency of your car as well as the life. Do what your car manual says.

    This gets me into my philosophy of science and personal responsibility. Most people treat cars as toys. But if people were forced to understand the science behind them, the roads would be safer, they would be maintained better (assuming you can afford the cost of routine maintenance), last longer….and people would not be idiots…. But I can go on for pages.

    • Interesting point about maintaining a homeostatic temperature range.

      On a side note, I’ll mention that my other half’s car runs noticably smoother using mid-grade gasoline (c. 89 octane). My beast, however, doesn’t really care one way or another.

    • Your argument that an alternative anti-freeze may alter the operating temperature of the engine is flawed. No matter what chemical is used as a coolant, the Thermostat will determine the temperature at which the engine operates. A more efficient coolant will not alter the function of the Thermostat. After all, when the Thermostat is closed, the coolant is not circulating, and therefore, not cooling anything. No matter the freezing point of the coolant, the Thermostat controls the engine temperature.

      Suggesting that an alternative anti-freeze will result in various chenical changes and chemical breakdowns, is also a flawed argument. The manufacturer of such products, surely would have taken such complications into consideration. It is very doubtful that a manufacturing company would invite a liability such as you have suggested.

      Even though containing the coolant in a pressurized system does raise the boiling point of the coolant, the engine temperature is still controlled by the Thermostat. As long as the cooling system is capable of removing sufficient heat (and that is part of the reason for using pressure to raise the boiling point), the Thermostat will maintain control of engine temperature, in a properly functioning cooling system.

  3. When I was looking up cool things to do with liquid nitrogen (it was for a middle school demonstration), one of the suggestions was to make ethylene glycol ‘ice cubes.’ or anti-anti-freeze 🙂

    • No, with liquid nitrogen you make ice cubes of pure anyhdrous non-denatured ethanol. They sink to the bottom of your drink, keeping it cool. And when they do melt, it does not dilute the drink, it spikes it!

      ethylene glycol ice cubes….what a waste of liquid nitrogen!

  4. I just use water and hope for the best.

  5. I loves that p-chem. I was making an industrial amount of pink lemonade one time, from those frozen concentrates that come in cans at the supermarket. I needed a funnel and not having one handy I cut the bottom off a soda bottle and it did quite nicely. Well I had washed the soda bottle and it was still wet as I was using it. The pink lemonade mixture was mostly liquid even though I had pulled it out of the freezer and the water droplets on the outside of the funnel froze instantly as I ran the cold syrup through it.

  6. mr. shokouhi says:

    do you need meg.deg.teg?

  7. About Blog For more than 20 years AM has built a reputation for being the prime source of news, insight and best practice information for senior executives in the UK automotive retail sector. AM-online | Automotive Management | Automotive Industry News.

Leave a Reply

Your email address will not be published. Required fields are marked *