This is done for tyres on jets, I believe.
What is the reason?
One thing I heard that the nitrogen molecule is bigger than the oxygen molecule so that you get less leakage. But air is ~ 80% nitrogen already so the overall difference should be very small.
Most of the bigger shop do it and it's a good thing to do unless you get charged extra for it. The reason is that you want an inert gas. Ambient air is humid, nitrogen from the bottle isn't (well, technically it is but not in this scenario). You want to prevent reaction between the gas and the rubber / wheel. Nitrogen can not nurture a fire, ambient air can. The humidity in air can also freeze and then cause an imbalance in the tyre.
There are even ADs for jets requiring nitrogen after some accidents: Airbus nitrogen AD
PS: The case for nitrogen vs ambient air is even better for oleo struts as they are made from metal and thus there's a danger of corrosion.
ex Boeing manual Wheel & Tire Servicing:
In addition, Boeing has received reports of three confirmed cases and other suspected cases in which a wheel/tire assembly exploded when the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire. In one case, the tire became overheated as a result of a dragging brake, and the wheel/tire assembly exploded when it reached the auto-ignition temperature. In another case, a wheel/tire assembly explosion in the wheel well during flight was suspected in the catastrophic loss of one airplane. A similar explosion caused severe damage to two others.
As a result, the U.S. Federal Aviation Administration issued Airworthiness Directive 87-08-09 requiring that only nitrogen be used to inflate airplane tires on braked wheels. However, tires may be topped off with air in remote locations where nitrogen may not be available if the oxygen content in the tire does not exceed 5 percent by volume.
Auto ignition temperature is awfully high, too high to be a factor in my mind for light aircraft. I think for light aircraft, the argument to use nitrogen in tires would be strongest if it focused on the increased pressure rise during the landing roll that results from humidity in the air. Corrosion with tubed tires/wheels would not seem to be an issue, but I guess it could be for oleo struts.
Tire Rack has a web page on the subject, focused on cars but still informative.
My tires get filled from the air compressor in my hangar - like light aircraft tires have been forever. And my aircraft don't have serviceable oleo struts. Works for me.
One thing I heard that the nitrogen molecule is bigger than the oxygen molecule so that you get less leakage
Given that the atomic mass of Oxygen is 16 and Nitrogen is 14 I don't think that is correct....(had to google to confirm my high school chemistry!)
How many light GA tyre fires have we had - I suspect very few if any.
Here's a link that would appear to confirm that Nitrogen is the larger molecule, weight notwithstanding:
I wonder whether osmotic pressure has any influence - if so, you would expect pure oxygen to leak out faster than nitrogen. On the other hand, you would also expect nitrogen to leak into a tyre filled with oxygen...
In racing cars it is used instead of air as it doesn't hold moisture which significantly affects the expansion/contraction consistency of the tyre under thermal load.
On the other hand, you would also expect nitrogen to leak into a tyre filled with oxygen...
It would still be subject to the law of partial gas pressures, which raises an interesting question whether you can get transfer from say 15psi to say 50psi, of a gas not present in the 50psi container.
Can you still get a gas transfer up such a pressure gradient?
One possibly big difference between air and nitrogen is that air comes from the outside so holds water (and all kinds of other crap) whereas nitrogen is going to come out of a cylinder rented from a gas company which makes it cryogenically so there will be zero water. You can buy air in cylinders too (and it will be dry) but I cannot see the point. Most people then buy a single gas chosen for what they actually want.
I have to fill my tyres from a scuba cylinder, because I don't have access to an air line, so use a cylinder anyway.
I had some "funny business" recently with the local scuba shop which refused to refill that cylinder because it was just outside the 2 year test period, but for "ground use" you have 5 years. I asked them if the regs for ground use had changed; they just said I "could" use it for scuba so they won't fill it anymore (They used to fill up to 5 years previously). I said it is marked "ground use only" and they said "yes but you could put a sticker over that and go diving". They would refill it only if it had the DIN valve because, apparently, hey this is really clever, you cannot go scuba diving with a DIN valve. I asked what stops me buying my own compressor and just going diving; they didn't want to go into that and went off about blowing my legs off etc. So I have one cylinder which is within the 2 year test (and one could almost buy a new cylinder online for the cost of a pressure test - £100) and use a transfer hose to top off the little one for doing tyres, paint spraying, etc. Oh well; they will be going for ISO9000 next