Facilitated transport mechanism in fixed site carrier membranes

Facilitated transport of gases, such as O₂, has been observed for fixed site carrier membranes. Carrier-impregnated ion-exchange membranes have also been used to produce facilitated transport of CO₂, H₂S and olefins when the carriers are not completely mobile. The actual mechanism for this facilitated transport has not been determined previously. The facilitated transport is provided by the exchange of solute between the free solute region and the bound complexing agent combined with gradients in the free solute and complexed carrier. The ratio of mobility for exchange between the regions is the dimensionless equilibrium constant of the reversible reaction. An expression is also derived for the relationship between the effective diffusion coefficient of the complex in facilitated transport, D_AB, and the diffusion coefficients for exchange between free and complexed solute, D_DH and D_HD. Since D_AB can be determined from experimental results, D_DH and D_HD can be calculated. The results also show the morphology dependence of D_AB. The diffusion coefficient of the free solute D_A also changes with the loading of the complexing agent in the film. This morphology dependence can explain why the gas permeability can decrease when the concentration of the complexing agent increases in the membrane.