CHARACTERIZATION OF ULTRAFILTRATION REJEC TION PERFORMANCE BY MOLECULAR-SIZE

Rejection ability of an ultrafiltration membrane is expressed as molec ular weight cutoff (MWCO). However, in practical applications the obse rved MWCO is different from the nominal value and rejections of differ ent molecules having the same molecular weight are also different. The reason seems to be the rejection mechanism of an ultrafiltration memb rane, that is, the sieve effect based on molecular size and not molecu lar weight. In this study, Stokes radii of protein, dextran and PEG we re measured by the GPC molecular-weight calibration curve and ultracen trifuge 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 sam e molecular weight had different molecular sizes, and that rejection o f an ultrafiltration membrane was determined by molecular size, not by molecular weight, if there was no molecule-membrane interaction. Ther efore, the rejection property of an ultrafiltration membrane must be a nalyzed by the sieve effect by using molecular size, not-molecular wei ght.