Hindered protein diffusion in asymmetric ultrafiltration membranes with highly constricted pores

Although protein diffusion in constricted pores can play an important role in a variety of membrane processes, there are currently no quantitative data available on the effective protein diffusivity in the highly constricted irregular pores characteristic of asymmetric ultrafiltration membranes. We have obtained data for the hindered diffusion coefficient of bovine serum albumin in polyethersulfone ultrafiltration membranes with nominal molecular weight cut-offs ranging from 50,000 to 1,000,000 using a well-stirred diffusion cell. Results indicate that the highly porous membrane matrix provides nearly 25% of the overall resistance to diffusion in the larger molecular weight cut-off membranes. All measured hindered diffusion coefficients in the ultrathin skin were more than four orders of magnitude smaller than the free solution diffusivity. Protein adsorption in the 1,000,000 molecular weight cut-off membranes attains a full monolayer, thereby constricting the membrane pores and reducing the effective pore size. Comparison of the measured diffusivity with available hydrodynamic models, suitably extended to the asymmetric membranes, indicates that protein adsorption in the 1,000,000 molecular weight cut-off membrane reduces the diffusion coefficient by two orders of magnitude. In contrast, there is minimal adsorption in the very highly constricted pores of the 50,000 molecular weight cut-off membrane. Protein adsorption in the 100,000 and 300,000 molecular weight cut-off membranes attains only a partial monolayer, but this still causes close to a two order of magnitude reduction in diffusivity.