We previously discussed why oxygen molecules, O2, are smaller in volume than nitrogen molecules, N2. This is pertinent to understanding why O2 passes through your tires faster than N2 does.
It’s easy to understand how a gas molecule quickly passes through a volume of another gas. A gas is not dense; the molecules comprising any gas are spread widely. A gas molecule can also relatively quickly pass through a liquid. Although a liquid is denser than a gas, the molecules that comprise any liquid are held together by intermolecular interactions that rapidly form and break.
A gas molecule doesn’t pass through a solid substance as readily as through another gas or a liquid; a solid is clearly far denser than either one. Nevertheless, different solids have different molecular arrangements that are pertinent to gas permeation. Consider two distinct cases: steel and rubber.
Steel is an alloy (mixture) of iron, carbon, and other substances. It adopts body-centered cubic and face-centered cubic structures. Strong intermolecular forces help the atoms pack together tightly. The carbon atoms in steel further prevent random movement of the iron atoms.
Contrast the crystalline arrangement of steel with that of rubber. Tires (made of rubber) consist of large molecules (polymers) interwoven with each other; imagine a mass of chaotic yet tightly interlaced spaghetti noodles.
Due to random molecular motion within the interwoven polymer chains, temporary voids continually form, move, and disappear within the rubber matrix. Small substances such as gases can dissolve into rubber; diffuse into these transient, mobile voids; and exit the rubber, given enough time.
The chemical compositions of the polymers that comprise rubber tires greatly impact gas diffusion through and solubility within them, and thus how quickly gases permeate through the rubber. For example, researchers at Arlon have documented oxygen permeability through a wide range of rubbers:
- Zhang and A. Cloud. “The Permeability Characteristics of Silicone Rubber.” Proc. Global Advances in Materials and Process Engineering, SAMPE Fall Technical Conf., 2006, Nov. 6–9, Dallas, TX.
The smaller the gas molecule, the faster is the passage through your tires. To appreciate this fact from a numerical perspective, we must discuss equations of diffusion and solubility.