Originally Posted by
cyccommute
I dislike responding to a 12 year old thread but your statement kind of requires it. A heavier molecule doesn’t necessarily mean that it diffuses slower. There are other factors to consider like the solubility of the gas in the rubber of the tube. Think of it this way: If you made a container out of sugar, it would hold hexane forever. It would only old water for a little while. Hexane has a higher molecular weight and is much, much larger than water but it won’t dissolve the sugar.
Yup. If there were only diffusion, the size would play a pretty strong part - mostly as a function of molecular velocity and not due to a filtration-like effect. The diffusion rate is proportional to the inverse of the square root of molecular weight. But solubility/affinity plays a part in rubber tires. So if a molecule tends to "stick" to rubber more, it diffuses more slowly.
As an example you can make a chromatography column with powdered sugar in a glass tube, with glass wool plugs at both ends. Pour some hexane mixed with a little ether through the column to get it wet. The, take some great tree leaves and grind them up. Dissolve in the same hexane mix, with dry CaSO4 mixed in to absorb all water. Filter, and pour into the top of the column. You'll see green, yellow, and red bands develop due to the different affinity that each of the different compounds (chlorophyll, and different caretenoids and anthocyanins). The speed at which these bands move down the column is very much related to how each type of molecule "sticks" to the sugar. Size doesn't play as big a part.
In a porous substance like rubber, diffusion is modulated by this "stickiness".