Properties of microfiltration membranes: The surface electrochemistry of anodic film membranes

Abstract

The surface electrochemical properties of capillary pore inorganic microfiltration membranes have been characterized by means of computerized measurements of rates of electroosmosis as a function of pH (reported as ζ potentials) and computerized surface pH titration. The membranes have modest values of ζ potential and high titratable surface charge. The incorporation of anions during the membrane manufacturing process has a substantial effect on their surface electrochemical properties. The electrokinetic and titration data have been quantitatively analyzed for a range of pH in 0.01 M NaCl solution using models based on electrical double-layer theory and accounting for the nature of the charge-determining groups. Membranes produced in phosphoric acid and membranes produced in oxalic acid have been studied. A description of the membrane surface in terms of a two-dimensional array of aluminum oxide and phosphate or oxalate groups, a site dissociation—site binding model, gave estimates of the ratio ( q ) of aluminum oxide/incorporated anion groups at the membrane surface and of the p K values of these groups. However, such a model gave a reasonable match of calculated and experimental surfae charge density/pH profiles only if physically unreasonable values for the thickness or dielectric constant of the inner part of the double layer were used. A much more satisfactory approach was to describe the membrane surface as a three-dimensional array (gel) of charged groups and consider that both protons and counterions could penetrate this gel region. Such a gel model allowed the a priori prediction of both surface charge density/pH and ζ potential/pH profiles and gave a good description of the membrane surface for both types of membrane. The model allowed estimation of the thickness of the gel layer, the specific adsorption energy of the counterions, the ratio q , and the p K values of charge-determining groups.