Knudsen diffusion occurs when the pore diameter is smaller than the mean-free path of the gas molecules. In diffusion terminology, this is most significant when the diameter is smaller than which length?

Knudsen diffusion occurs when the pore diameter is smaller than the mean-free path of the gas molecules. In diffusion terminology, this is most significant when the diameter is smaller than which length?

• A. The mean-free path
• B. The molecular diameter
• C. The pore length
• D. The diffusion distance

Answer: (A)

Knudsen diffusion arises when the pore diameter is so small that molecules collide with the pore walls far more often than with other gas molecules. In diffusion terminology, the critical length to compare with the pore size is the mean free path, the average distance a molecule travels between molecule–molecule collisions. When the pore diameter is smaller than the mean free path, wall collisions dominate and the diffusion behavior is governed by the pore size and molecular speed rather than intermolecular interactions. The molecular diameter is about the size of the molecule, the pore length is just the channel length, and diffusion distance is not a microscopic collision scale.

Knudsen diffusion arises when the pore diameter is so small that molecules collide with the pore walls far more often than with other gas molecules. In diffusion terminology, the critical length to compare with the pore size is the mean free path, the average distance a molecule travels between molecule–molecule collisions. When the pore diameter is smaller than the mean free path, wall collisions dominate and the diffusion behavior is governed by the pore size and molecular speed rather than intermolecular interactions. The molecular diameter is about the size of the molecule, the pore length is just the channel length, and diffusion distance is not a microscopic collision scale.