Characterization of an Amphoteric-Charged Layer Grafted to the Pore Surface of a Porous Membrane

Porous membranes having amphoteric charge groups were prepared by heterogeneous graft polymerization from aqueous solution containing acrylic acid and ( N, N -dimethylamino)propyl acrylamide. The charging properties of the amphoteric charge groups grafted to the pore surface were investigated by ζ potentials obtained from streaming potential measurements. Theoretical development in terms of the amphoteric pore surface was attempted by introducing a site dissociation model of pH-dependent ζ potentials. The theoretical model is based on the assumption that the surface charge arises only from the protonation and deprotonation of the charged surface groups exposed to electrolyte solution. Good fits between experimental results and theoretical equations were attained, and thereby the isoelectric point, dissociation constant, apparent surface site density, and acid-to-base ratio of the amphoteric pore surface were determined. Depending on the variation in the feed monomer ratio for graft polymerization, these parameters correspondingly changed. If we assumed that the acid-to-base ratio of the amphoteric pore surface corresponds to that in the whole membrane, the net charge density of the membrane quantitatively obtained from the potentiometric titration gave the respective charge densities of acidic and basic groups using the acid-to-base ratio determined from the site dissociation model. Here we proposed an approach to characterize the amphoteric-charged pore surface by the pH-dependent ζ potential and its theoretical modeling.