화학공학소재연구정보센터
Journal of Membrane Science, Vol.108, No.1-2, 143-159, 1995
Protein Retention with Modified and Unmodified Inorganic Ultrafiltration Membranes - Model of Ionic-Strength Controlled Retention
The variation of retention of a single protein, either positively-charged (lysozyme) or negatively-charged (bovine serum albumin, BSA) in a buffer solution at pH 7 was investigated by varying the ionic strength with unmodified or chemically-modified zirconia UF membranes. Chemical modification was obtained by coating polyvinylimidazole the amine group of which reacted further with bisepoxiranes in order to have, both, a partly quaternized amine group and a pH-stable network on the membrane surface. Zeta potentials of zirconia-based materials with a positive polymer coating was dependent on the nature of the electrolyte. For elucidating the mechanism of protein retention, an HPLC study with similar coatings was performed. Salt-promoted interactions between protein and the cross-linker used with the polymer were shown at high salt content in the phosphate buffer and displacement of these interactions was achieved with acetonitrile. Retention of lysozyme or BSA is well-fitted with a model of ionic strength controlled retention (ISCR) based on a three term equation corresponding roughly to size effect, electrostatic and salt-promoted interactions the physical meaning of which is discussed. At low ionic strength, when protein retention is far higher than expected on size retention, the free protein acts as a co-ion of the fouled membrane and electrostatic interactions are accounted for with the leading retention term depending on l(-1/2) according to the model. Protein retention with unmodified hydrophilic membranes is close to size retention with increasing ionic strength. However, protein retention is U-shaped with a minimum larger than size retention when ionic strength induces salt-promoted interactions between protein and the polymer coating on the membrane surface. Finally, it is pointed out that phosphate buffer allows lysozyme retention to be close to (or lower than) the size retention at low ionic strength as modified membranes are presumably negatively-charged. The application of the model involving attractive forces between protein and modified surface rather than repulsive forces is under investigation.