Non-equilibrium surface reactions—a factor in determining steady state diffusion flux

Steady state diffusion flux through a membrane is described considering non-equilibrium sorption–desorption process at the interfaces. A dimensionless parameter α (similar to second Damköler number) is defined, which might serve as a measure for the deviation of the surface reactions from equilibrium. It is shown that α does not depend on the forward reaction rate constant. For α⪡1, the surface reactions might be considered in ‘near equilibrium’. For α comparable to or greater than unity, the permeability will appear dependent on the membrane thickness, and this dependence can be used to determine the surface reaction constants.