Now I am not a physicist or chemist but this article, "Nitrogen for Tires just hot air?" by Dave Toplikar in Sunday's Lawrence Journal World caught my attention. Apparently tire places are hawking nitrogen for tires rather than regular compressed air. One of Lawrence's tire places has recently installed equipment for filling tires with nitrogen at $5.00 a pop claiming that nitrogen filled tires hold their pressure three times longer than when filled with regular air. Tires filled with nitrogen also supposedly run cooler in the summer and since nitrogen is relatively inert, less corrosive to the innards of a tire than regular air.
Toplikar was rightfully skeptical of these claims because, recalling one of those facts that one should have learned if one was paying attention in school, or else watched the Discovery Channel, namely that "air" is about 78% nitrogen gas 21% oxygen gas with the balance being argon, water vapor, carbon dioxide methane and various other gasses. Since of the advantage of nitrogen supposedly rests with it being a bigger molecule, Toplikar asked the chair of KU's chemistry department about this and the chair gave an apt description of the relative sizes of a nitrogen molecule namely the difference is like a Chipotle burrito with lettuce vs one without lettuce. If there is any advantage to using pure nitrogen then is ought to rest with the reduction in corrosiveness but the chair was skeptical of that.
Molecular size is a bit tricky. As a quick comparison, we can use the covalent radius defined as 1/2 the distance between to identical covalently bonding nuclei. This is measured in picometers (= 1x 10-12 m). Nitrogen's covalent radius is 73pm so the length of a nitrogen molecule ought to be 4 X 75pm or 300 pm. A molecule of oxygen ought to be just a shade smaller 4 X 73pm or 292pm. So an oxygen molecule ought to be a little less than 3% smaller than a nitrogen molecule.
The companies that process nitrogen and make equipment for doing this tire filling are all for using nitrogen and there is even a Get Nitrogen Institute to promote nitrogen use including in tires. This site has a little link to "Deep Science" so let's see what is there. First is a "gas permeability table" (http://www.getnitrogen.org/pdf/GasPermTable.pdf). Now this gets the geek juices flowing. But the table makes absolutely no sense. It is basically a ranking of permeabilities with no units and no explanation of how the data were collected. Nitrogen gas and methane are both shown as having equally slow permeabilities and water vapor the worse, but beyond that there is no way to assess what these rankings mean. How much more rapid is the permeability of water vapor than nitrogen gas?
This "table" is typical of how promotional literature often times uses misleading graphics. Here is another promotion (http://www.matickchevy.com/Nitrogen.aspx) showing a huge apparent size difference between oxygen and nitrogen atoms when the real difference is a lot smaller.
Fortunately the site does provide some quantitative looking reports, for instance part of a thesis (http://www.getnitrogen.org/pdf/ubologna.pdf) on the use of nitrogen in tires suggesting from theoretical considerations that the permeability differences between nitrogen gas and oxygen that tires inflated with nitrogen gas should lose pressure at half the speed that tires filled with regular air.
So it's back to the web to look for articles relating molecular size to permeability through various sorts of films and found some interesting tidbits. For example at Stanford, scientists are attempting to use membrane separation to extract carbon dioxide based solely on molecular diameter which is the main determinant of diffusion if the pores in the membrane are at the nano scale, in line with the molecular sizes. (http://gcep.stanford.edu/research/factsheets/membrane_subnanoscale.html).
The idea is to use these membranes as a sieve to sort out the carbon dioxide produced by fossil fuels and the sequester it. There is a nice picture illustrating the concept. This suggests the tire rubber must have a complex of pores in the molecular size range of oxygen or smaller. Further it's likely that the nanoscale structure of these pores is makes the effect more like a series of sieves analogous to the sort of cascade of gas centrifuges used to sort out different isotopes of uranium by small differences in mass. So it is possible that small differences in molecular size lead to differences in permeabilities of a tire.
Getting back to tires, there apparently is benefit for freight truck tire longevity as described in this article (http://www.trucktestdigest.com/FeatureNitrogeninTires.htm). This article notes that a truck tire properly inflated might loose 2 pounds per square inch in three months as opposed to 2 pounds per square inch per month. But these are big semi tires. For space shuttle and other sorts of high performance tires maybe using nitrogen gas makes sense. These tires operate at much greater extreme of temperature and pressure than passenger car tires.
The car talk guys...and that's where I get my limited knowledge about cars have this to say about racing tires and nitrogen:
"One (i.e., Indy 500) racers use nitrogen in their tires because when you're traveling around an oval track at 200 mph, you want your tire pressure to be entirely predictable. They even "stagger" the tire pressure on those cars, making the outside tires a little fuller than the inside tires to keep the car turning inward. And at those speeds, an eighth of an inch in tire height can make a huge difference."
I suspect the predictability the car guys are referring to arises from the fact that regular air has water vapor in it so when the wheels cool, some of that vapor changes phase making the change in tire volume less regular.
Some of the advantage of nitrogen is also allegedly due to the fact that it is relatively inert compared to oxygen and one exhibit (http://www.getnitrogen.org/pdf/FordBaldwinResearchRaper.pdf) at the Get Nitrogen's "deep science" link attempts to address this. The study authors found that using nitrogen gas does indeed reduce the aging of tires and this seemed most pronounced for peel strength, the force required to separate the belts in the radial tires tested. But these tires were not run on passenger cars but 'cooked' for up to 12 weeks at 60 degrees centigrade. Indeed the study's authors note:
"Nonetheless, it is perhaps a fair assumption to say that there would be some improvement in tire durability if nitrogen was used as the inflation media, but it is too soon to speculate as to how much of an improvement it would be."
But does using nitrogen vs air in car tires make any difference for us as consumers? Probably not at $5.00 per tire when just periodic checking of pressure is sufficient, assuming of course we happen to have two quarters on us when we think about it to pay for the air. But this whole controversy illustrates how organizations can exploit a partial truth and through a little sleigh of hand can produce a new perceived need for the consumer.
By the way, in my searches I encountered a fascinating 19th century scientist who is not well known to non chemists, namely Josef Loschmidt who was the first scientist to estimate the size of molecules using the kinetic theory of gasses. There is a wonderful scientific biography for Loschmidt in Physics Today March 2001 which is online at http://www.physicstoday.org/pt/vol-54/iss-3/p45.html.
Balder, Afred and Leonard Parker (2001) Joseph Loschmidt, Physicist and Chemist. Physics Today Vol 54(3) p 45.