Diffusion Through a Stationary, Nondiffusing Gas

DIFFUSION THROUGH A STATIONARY, NONDIFFUSING GAS

An important practical case arises when a gas A diffuses through a gas B, there being no overall transport of gas B. It arises, for example, when a vapor formed at a drying surface diffuses into a surrounding gas. At the liquid surface, the partial pressure of A is dictated by the temperature. For water, it would be 12.8 mmHg at 298 K. Some distance away, the partial pressure is lower and the concentration gradient causes diffusion of A away from the surface. Similarly, a concentration gradient for B must exist, the concentration being lowest at the surface. Diffusion of this component takes place toward the surface. There is, however, no overall transport of B so that diffusional movement must be balanced by bulk flow away from the surface. The total flow of A is, therefore, the diffusional flow of A plus the transfer of A associated with this bulk movement.