Dextran transport through asymmetric ultrafiltration membranes: Comparison with hydrodynamic models

Although dextran sieving is often used to evaluate the pore size characteristics of the ultrafiltration membranes, there is still considerable disagreement regarding the proper interpretation of these sieving measurements. We obtained experimental data for the transport of polydisperse dextrans through asymmetric polyethersulfone membranes of differing molecular weight cut-off in a stirred ultrafiltration device. Gel permeation chromatography is used to evaluate the actual sieving coefficient as a function of dextran molecular weight from data for the bulk and filtrate concentrations, with the bulk mass transport effects evaluated using a stagnant film model. There is no evidence of any dextran deformation even at fluxes as high as 10^-4 m/sec. The flux dependence of the actual sieving coefficients is in good agreement with membrane transport theory, allowing the evaluation of both the hindered diffusivities and asymptotic sieving coefficients as a function of dextran molecular weight. The results are shown to be in good agreement with available hydrodynamic models for spherical solutes in cylindrical pores with the effective solute to pore size ratio evaluated using a partitioning model which implicitly accounts for the membrane pore size distribution.