Here's an addendum to the recent bit about ethylene glycol (the standard
antifreeze) vs. propylene glycol (the "environmentally friendly" antifreeze
and the main component in Sierra brand).
The freezing point of ethylene glycol is only 8 deg. F, while that of
propylene glycol is -74 deg. F (!) --- BUT THIS HAS VERY LITTLE TO DO WITH
EITHER'S ABILITY TO ACT AS AN ANTIFREEZE. The "antifreeze" action has more
to do with molecular weight and the property called freezing-point
depression. In this respect there is little difference between the two.
The only drawback to using propylene glycol is probably its lower stability
at higher temperatures. While ethylene glycol is quite stable at elevated
temperatures, propylene glycol tends to oxidize to form several organic
acids, including lactic acid and acetic acid. I would suppose that "Sierra
brand" antifreeze has some stabilizers to help overcome this tendancy, but
if I were to use it in my car I would probably plan on increasing the
flush-and-renew frequency.
The safety of propylene glycol is well known, since it is used as a solvent
in many pharmaceuticals (look on your cough syrup bottle) and as an
antifreeze in breweries and dairies.
OK, the only thing left is just a little more chemistry. If CHEM 101 was a
nightmare, exit now.
They are similar both chemically and physically, the only structural
difference being a -CH3 group on the propylene glycol. One would draw the
structures as:
Ethylene glycol Propylene glycol
H H
O H H O H
H-C-C-OH H-C-C-C-OH
H H H H H
Mol.wt. 62.07 76.09
B.P 198C 188C
M.P. -13C -59C
Lee M. Daniels - Laboratory for Molecular Structure and Bonding - Texas A&M
daniels@lmsbvx.tamu.edu | DANIELS@TAMLMSB.BITNET | (409) 845-3726
"Sometimes you're the windshield, sometimes you're the bug."
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