The interior of a three-dimensional aerogel network contains an interconnected volume of gas-filled pore space. When various gas species (such as hydrogen, nitrogen, carbon dioxide, or organic vapors) traverse the open nanopores of an aerogel, their macroscopic diffusion behaviors deviate from gas flow through dense films or macro-scale channels. This diffusion and separation behavior—dictated by the spatial relationship between aerogel pore dimensions and gas kinetic diameters—is known as free volume diffusion and molecular sieving effects. Investigating the physics of gas molecule diffusion in confined nanopores clarifies gas transport kinetics in porous media.

In kinetic gas theory, gas transport regimes through porous media are quantified by the Knudsen Number, defined as the ratio of the molecular mean free path to the characteristic pore diameter. Within aerogels, because pore size distributions span several to tens of nanometers, this scale is smaller than or comparable to the mean free path of gas molecules at ambient conditions.

Consequently, the collision mechanism governing gas transport within aerogel pores shifts from intermolecular collisions to molecule-wall collisions. This boundary-dominated transport mode is termed Knudsen diffusion.

During confined Knudsen diffusion, lighter or smaller gas molecules (such as hydrogen or helium)—possessing higher thermal velocities and smaller collision cross-sections—penetrate the aerogel nanonetwork with reduced wall-collision resistance. Conversely, larger or polar molecules (such as carbon dioxide or organic vapors) experience stronger surface adsorption and frequent wall impacts, delaying their diffusion rates.

By compressing effective pore diameters via cross-linking or templating to match kinetic molecular diameters (e.g., below 0.5 nm), aerogel architectures exhibit molecular sieving effects—permitting small molecules to diffuse while blocking larger counterparts. Formulating micro-scale gas transport models coupling free volume theory and surface adsorption potential energy barriers enables prediction of diffusion coefficients and separation selectivities through aerogel topological networks.