Effect of membrane structure and protein concentration on the osmotic reflection coefficient

Although there have been a number of important studies of the protein filtration reflection coefficient (or sieving coefficient) for asymmetric ultrafiltration membranes, there have been no corresponding studies of the protein osmotic reflection coefficient and its effect on solvent flux. We have obtained experimental data for the bovine serum albumin osmotic reflection coefficient for asymmetric polyethersulfone membranes with nominal molecular weight cut-offs from 30,000 to 1,000,000. The overall osmotic reflection coefficients for these membranes were smaller than the reflection coefficients of the skin layer due to the substantial diffusive resistance provided by the membrane support structure. The results for the skin layer were in relatively good agreement with available hydrodynamic models, suitably modified to account for the effects of protein adsorption and for the presence of a pore size distribution. The osmotic reflection coefficients decreased with increasing bulk protein concentration, with the concentration dependence somewhat larger than that predicted by available theoretical analyses. The osmotic reflection coefficients were in relatively good agreement with literature values for the corresponding filtration reflection coefficients, with the slightly smaller values for the osmotic reflection coefficients probably reflecting the variation in pore size within the ultrathin skin of these asymmetric membranes.