Protein adsorption in asymmetric ultrafiltration membranes with highly constricted pores

Although protein adsorption can have profound effects on membrane transport, accurate measurements of protein uptake in asymmetric ultrafiltration membranes with highly constricted pores are currently unavailable. We have evaluated bovine serum albumin adsorption of Filtron NOVA and OMEGA polyethersulfone membranes with molecular weight cutoffs ranging from 30,000 to 1,000,000 using both ¹²⁵I-radiolabeled albumin and direct measurement of unlabeled protein uptake by mass. Data indicate that the radiolabeled proteins proteins adsorb preferentially compared to the unlabeled protein in both the ultrathin skin and substructure, but not in the membrane matrix due to differences in polymer properties. Protein uptake was evaluated in each region of the membrane using data for the isolated matrix, the substructure and matrix in combination, and the full asymmetric membrane. Albumin uptake in the substructure and matrix attained monolayer levels in all of the membranes. Monolayer adsorption was also found in the ulthrathin skin of membranes with molecular weight cutoffs of 300,000 or greater, even though the pores in these membranes are only twice the size of an albumin molecule. Albumin adsorption in the ultrathin skin of the 100,000 molecular weight cutoff membrane was substantially reduced compared to monolayer levels, and protein uptake in the skin of the 50,000 molecular weight cutoff membrane was negligible. Measurements of the membrane hydraulic permeability before and after protein adsorption indicate that the effective pore radius in the larger molecular weight cutoff membranes was reduced by approximately 60 Å, which is in good agreement with calculations based on monolayer adsorption.