Pore-surface characterization of amphoteric charged membranes by means of zeta potential measurements

Weak amphoteric charged membranes were prepared by the radiation-induced graft copolymerization of the poly(ethylene glycol) (PEG) derivatives with pendant ionizable groups onto polyethylene (PE) porous membranes. In this study, two types of amphoteric charged membranes, which are the amphoteric ion-pair side chain (ASC) type and the mixed grafted chain (MGC) type, were prepared and then characterized by means of zeta (ζ)-potential measurements. The ζ-potential measurements were performed using streaming potential method over the pH range of 2–12. The ζ-potential/pH profiles for the amphoteric charged membranes showed their characteristic behavior. The pH dependence of the apparent surface charge density derived from the ζ-potential was compared with the calculated result from the theoretical model, site dissociation model (SDM), which was based on the dissociation of ionizable groups on the pore surface. Our analysis indicated that the interfacial charge of the ASC-type membrane was ascribed to the dipolar ion structure of cysteine residues. On the other hand, not only the charge groups but also the conformation of grafted polyelectrolyte chain affected the electrokinetic pore-surface charge of the MGC-type membrane.