Characterization of Ultrafiltration Rejection Performance by Molecular Size

Abstract

Rejection ability of an ultrafiltration membrane is expressed as molecular weight cutoff (MWCO). However, in practical applications the observed MWCO is different from the nominal value and rejections of different molecules having the same molecular weight are also different. The reason seems to be the rejection mechanism of an ultrafiltration membrane, that is, the sieve effect based on molecular size and not molecular weight. In this study, Stokes radii of protein, dextran and PEG were measured by the GPC molecular-weight calibration curve and ultracentrifuge method, and the relationship between molecular weight and size was determined. Dextran, PEG, PVP and proteins were ultrafiltered by three types of membranes. It was obvious that molecules having the same molecular weight had different molecular sizes, and that rejection of an ultrafiltration membrane was determined by molecular size, not by molecular weight, if there was no molecule-membrane interaction. Therefore, the rejection property of an ultrafiltration membrane must be analyzed by the sieve effect by using molecular size, not molecular weight.